ALBERT

Description: We determined the genetic structure of Acartia tonsa using sequences of the mtCOI gene obtained from 58 specimens collected on the Brazilian coast. Variations in the sequences discriminated three distinct lineages, with genetic distances of 6–16%. The genetic structuring observed in the Brazilian A. tonsa populations indicates that they are heterogeneous.

Description: Using fast repetition rate (FRR) fluorometry, we succeeded in measuring the chlorophyll a (Chl a ) content and photo-physiological state of symbiotic algae within individual cells of host foraminifers. We found that the Chl a content and photo-physiological state of the symbiotic algae varied greatly over the life cycle of their host. The results of this study indicate the potential benefits of using FRR fluorometry to reveal host–algal symbiont relationships in the ocean.

Description: Sex ratio affects the growth rates of wild populations; so, for a better understanding of population dynamics, it is crucial to know the mechanisms that determine it. We analyzed seasonal variation in sex ratio of the copepod Arctodiaptomus salinus . It was in general male-skewed and affected by temperature. Our results suggest that investing in the "cheaper sex" might be a good strategy of this population to increase population growth rate.

Description: We had the opportunity to evaluate the utility of the chromophyte xanthophyll cycle pigments (XCP), diadinoxanthin (DDX) and diatoxanthin (DTX), for assessing upper ocean mixing processes at Station ALOHA (22.75°N, 158°W) during August–September 2012. De-epoxidation state (DES, [DTX]/[DDX + DTX]) kinetic rate experiments were performed that quantified the conversion of DDX to DTX in the light. DES values ranged from 0.13 to 0.51 (weight:weight), and yielded a first-order rate constant of 0.16 ± 0.02 min –1 . In addition, a time-series of chromophyte pigments was collected from the sea-surface in order to quantify the time-dependent rate of change in the ratio of XCP to chlorophyll a (XCP/CHLA). In situ XCP/CHLA values were measured by rapid collection of cells in the mixed layer using a diaphragm pumping system and the xanthophyll cycle activity inhibitor dithiothreitol. Turbulent transfer velocity (TTV) was estimated as the quotient of the XCP/CHLA photoadaptation rate and the XCP/CHLA gradient in the mixed layer. TTVs ranged from 0.3 to 0.5 cm s –1 , and agreed to within a factor of ~2 of the Lagrangian float-derived estimate of vertical mixing at a comparable friction velocity. Coincident measurements of dissolved gases suggest that XCP dynamics may be useful for the interpretation of trace gases species distributions, such as hydrogen and carbon monoxide, in the upper ocean.

Description: Conservation groups have called for protective measures in the Sargasso Sea, a region characterized by unique planktonic seaweed communities. To better understand population connectivity and facilitate effective conservation efforts, we assessed slender Sargassum shrimp ( Latreutes fucorum ) 16S rDNA variation along a 3093-km transect. F ST analysis and an analysis of molecular variance indicate a highly dispersive population, suggesting that a network of protected areas may be necessary to protect the Sargassum ecosystem.

Description: Blooms of Dinophysis ovum and Mesodinium spp . have been observed in the Gulf of Mexico since 2007 using the Imaging FlowCytobot technology. Bloom dynamics of these two organisms in conjunction with ancillary environmental data for a 5-year period were analyzed to identify the conditions necessary for bloom initiation or presence with the goal of predicting future blooms of D. ovum . Using time-series analysis, we observed a positive time-lagged correlation between the two organisms in each year when both were present, which suggests that the presence of Mesodinium may be useful as a leading indicator for a D. ovum bloom. Although in some cases D. ovum and Mesodinium co-occurred, no strong predator–prey relationship was observed. We identified a narrow range of temperature and salinity that could be necessary for bloom initiation of D. ovum and Mesodinium in the Gulf of Mexico. Analysis of images over the time series revealed a wide range in the size of Mesodinium cells, which suggests that species other than M. rubrum may be present in the Gulf of Mexico. Based on the occurrence of a D. ovum bloom preceded by low abundances of Mesodinium , we suggest that D. ovum is able to utilize ciliates other than M. rubrum as prey. Our observations indicate that environmental conditions, as well as Mesodinium abundance and species composition, can affect initiation, presence or abundance of D. ovum and thus may help in the prediction of future blooms.

Description: We investigated the spatial and inter-annual variations in planktonic protist community taxonomic composition with respect to size and trophic structure in the West Spitsbergen waters during the 2007, 2009 and 2010 summers. The protist communities were studied along a longitudinal transect from Kongsfjorden through the shelf to open sea waters. Our investigation was conducted on the Atlantic water inflow, which varied between the years. We observed a weakening trend in the northward advection of warm, Atlantic water in the Eastern branch of the Fram Strait, which was confirmed by long-term satellite-derived sea surface temperature (SST) measurements. The changing hydrological regime affected the summer protist taxonomic composition, abundance and biomass. These communities were numerically dominated by nanoplanktonic phototrophs, primarily cryptophytes. The microplanktonic heterotrophs (ciliates and dinoflagellates) were the major contributors to biomass, especially during the warmest year, 2007. Compared with previous investigations conducted in colder years, we observed a lower relative contribution of diatoms to the total protist biomass. Our results indicate that differences in the strength of Atlantic water inflow may have consequences for the composition of the plankton community in Kongsfjorden and the adjacent Eastern Fram Strait.

Description: Substantial variations are reported for egg production and hatching rates of copepods exposed to elevated carbon dioxide concentrations (pCO 2 ). One possible explanation, as found in other marine taxa, is that prior parental exposure to elevated pCO 2 (and/or decreased pH) affects reproductive performance. Previous studies have adopted two distinct approaches, either (1) expose male and female copepoda to the test pCO 2 /pH scenarios, or (2) solely expose egg-laying females to the tests. Although the former approach is more realistic, the majority of studies have used the latter approach. Here, we investigated the variation in egg production and hatching success of Acartia tonsa between these two experimental designs, across five different pCO 2 concentrations (385–6000 µatm pCO 2 ). In addition, to determine the effect of pCO 2 on the hatching success with no prior parental exposure, eggs produced and fertilized under ambient conditions were also exposed to these pCO 2 scenarios. Significant variations were found between experimental designs, with approach (1) resulting in higher impacts; here 〉20% difference was seen in hatching success between experiments at 1000 µatm pCO 2 scenarios (2100 year scenario), and 〉85% at 6000 µatm pCO 2 . This study highlights the potential to misrepresent the reproductive response of a species to elevated pCO 2 dependent on parental exposure.

Description: In species exhibiting intersexual conflict over mating, the outcome of female–male sexual encounters may depend on the phenotypic value of traits of either sex that are important for precopulatory behaviors. To explore this prediction, we investigated the relationship between female morphology and male mating success using mate choice experiments in the diaptomid copepod, Eudiaptomus graciloides (Lilljeborg), where intersexual conflict seems to cause an accumulation of spermatophores among small, easily accessible females. Three randomly selected receptive females were combined with an adult male in small glass vessels containing filtered lake water. After an introduction period of 24 h, copepods were preserved for subsequent morphometric analyses. We examined whether measurements of the different body parts of the males, and the mated and the unmated females correlated with the female mating status/male mating success. The results suggest that mated females had relatively longer furca than unmated females, confirming observations from earlier field studies. Moreover, mated females had a smaller body size and a longer genital double somite compared with unmated females. The results of the experiment, that smaller easily captured females are those first mated, are consistent with predications generated by a model of a sexual conflict. Based on the behavioral sequence and the mechanics of copulation in this system, we discuss the influence of such traits on the observed outcome of mating interactions.

Description: Ocean iron fertilization experiments enable the quantitative study of processes shaping the structure and functioning of pelagic ecosystems following perturbation under in situ conditions. EIFEX was conducted within a stationary eddy adjacent to the Antarctic Polar Front over 38 days in February/March 2004 and induced a massive diatom bloom. Here, we present the responses in abundance and biomass of all identifiable protozooplankton taxa (heterotrophic protists ranging from 2 to 500 µm) during the bloom. Acantharia, dinoflagellates and ciliates together contributed 〉90% of protozooplankton biomass in the upper 100 m throughout the experiment with heterotrophic nanoflagellates, nassellaria, spumellaria, phaeodaria, foraminifera and the taxopodidean Sticholonche zanclea providing the remainder. Total protozooplankton biomass increased slightly from 1.0 to 1.3 g C m –2 within the fertilized patch and remained at 0.7+0.04 g C m –2 outside it. However, distinct trends in population build-up or decline were observed within the dominant taxa in each group. In general, smaller less-defended groups such as aloricate ciliates and athecate dinoflagellates declined, whereas the biomass of large, spiny and armoured groups, in particular acantharia, large tintinnids and thecate dinoflagellates increased inside the patch. We attribute the higher accumulation rates of defended taxa to selective, heavy grazing pressure by the large stocks of copepods. Of the defended taxa, acantharia had the lowest mortality rates and the highest biomass. Large stocks of tintinnid loricae in the deep water column identify this group as a relevant contributor to deep organic carbon export. Highest accumulation rates (0.11 day –1 ) were recorded in S. zanclea .

Description: Estimating growth and production rates of mesozooplankton, and copepods in particular, is important in describing flows of material and energy though pelagic systems. Over the past 30 years, the Moult Rate (MR) method has been used to estimate juvenile copepod growth rates in ~40 papers. Yet the MR method has been shown to have serious flaws. Here we re-examine the results from the majority of published MR method studies and re-estimate growth rates using the modified Moult Rate (MMR) method, which ascribes changes in mass to the appropriate time period over which it was accrued. The MR method has typically over-estimated growth rates (on 80% of occasions) for life stages where the subsequent stage is actively moulting; the median and mean MR values are 138 and 164%, respectively, of the corrected MMR values. We were unable to correct the original data for life stages that are followed by a non-moulting stage, e.g. copepodite stage 5 to adult. We performed experiments with Calanus pacificus to estimate growth of stage C5 using an alternative method. We found that the error size and sign varied between mass type (i.e. DW, C and N). Recommendations for practical future assessments of growth in copepods are made.

Description: Stable isotope ratios for six size fractions of zooplankton (80 to 〉2000 μm) were analyzed in the Gulf of Lion in spring 2010 and winter 2011. Environmental and plankton community variables were also recorded. The originality of this study is the use of a Lagrangian transport modeling system to determine the origin of the water masses and the assessment of the proportion of detritus in the plankton samples. The highest 15 N values were observed in the 1000–2000 μm fraction in January and in the 500–1000 μm fraction in May. The largest size class (〉2000 μm), dominated by salps, had lower 15 N values owing to the low isotopic signatures of these organisms. The history of the water masses resulted in two main patterns with different isotopic signatures: the water masses which resided on the shelf and the waters masses carried onto the shelf from off-shelf region by the Northern Current. The 13 C values varied strongly between January and May, mainly owing to changes in hydrographic conditions. The 15 N values, plankton size structure and zooplankton feeding activity varied depending on the season, revealing differences in the seasonal trophic structure of the plankton communities. The trophic structure was characterized in January by a high chlorophyl a (Chl- a ) concentration, a population dominated by small organisms and herbivores, and in May by patchy Chl- a distribution, higher particulate organic matter concentration, a population dominated by large size organisms and an increase in the number of omnivores.

Description: Light and nutrient availability are assumed to largely control the dynamics and trophic efficiency of marine planktonic food webs, and are expected to be influenced by climate change (i.e. changes in stratification due to global warming). During an 8-day mesocosm experiment, we investigated the propagation of light energy up to the copepod trophic level in ultra-oligotrophic Eastern Mediterranean waters. Using mesocosms with light treatments corresponding to ca. 40% (L1) and 4.5% (L4) of the incident light intensity (simulating light conditions at 10 and 50–60 m depths, respectively), we assessed the feeding behaviour of the dominant copepod Clausocalanus furcatus every second day. We hypothesized that increased availability of copepod food at higher light levels would result in increased copepod ingestion rates and selectivity. Our results showed that copepod prey (microbial biomass〉5 μm) had only a small increase in the L1 and daily rations (DRs) of C. furcatus were similar among treatments (usually 〈15% of body carbon). Nevertheless, microplankton was clearly selected in L1, having double the contribution in the DR of C. furcatus compared with L4. A more balanced selective feeding pattern was observed in L4, with nanoplankton also being important in the diet. An increase in the copepod stock in the L1 compared with L4, observed at the end of the experiment, is likely associated with the light-mediated effects on C. furcatus feeding behaviour. We suggest that the importance of light availability on the efficiency of the marine planktonic food web should be further considered by climate change predictive models.

Description: Eurytemora affinis is the dominant copepod in the St Lawrence estuarine transition zone (ETZ). The distribution patterns of two co-occurring genetically divergent clades of E. affinis were analyzed. The Atlantic (A) and the North-Atlantic (NA) clade occurrences were spatially and temporally variable. Three and five distinctive habitats were found in June and August 2010, respectively. The habitats in the upper and the middle ETZ were predominantly defined by SPM characteristics such as particulate organic matter, chlorophyll a and protists while, in the lower ETZ, water mass properties such as nutrients, salinity and temperature were responsible for habitat differentiation. In June, the NA clade occurred throughout all habitats whereas, in August, it was mostly restricted to the middle and lower ETZ. The A clade was absent in June and was found in similar densities in the upper and middle ETZ in August. Associated with these habitats, distinct zooplankton assemblages were found, dominated numerically by at least one of the two clades , except in the most downstream habitat where Acartia spp. occurred in higher densities. Highest densities of the NA clade coincided with high food quantity and quality in the middle ETZ, whereas the A clade was equally abundant in the upper and the middle ETZ. Thus, we suggest that distribution patterns of the E. affinis clades were probably controlled by food source conditions and ‘clade-specific’ physiological tolerances, resulting in either spatial or temporal segregation or in coexistence of both clades in some of the habitats along the ETZ.

Description: Diel vertical migration (DVM) of zooplankton is a global phenomenon, characteristic of both marine and limnic environments. At high latitudes, patterns of DVM have been documented, but rather little knowledge exists regarding which species perform this ecologically important behaviour. Also, in the Arctic, the vertically migrating components of the zooplankton community are usually regarded as a single sound scattering layer (SSL) performing synchronized patterns of migration directly controlled by ambient light. Here, we present evidence for hitherto unknown complexity of Arctic marine systems, where zooplankton form multiple aggregations through the water column seen via acoustics as distinct SSLs. We show that while the initiation of DVM during the autumnal equinox is light mediated, the vertical positioning of the migrants during day is linked more to the thermal characteristics of water masses than to irradiance. During night, phytoplankton biomass is shown to be the most important factor determining the vertical positioning of all migrating taxa. Further, we develop a novel way of representing acoustic data in the form of a Sound Image (SI) that enables a direct comparison of the relative importance of each potential scatterer based upon the theoretical contribution of their backscatter. Based on our comparison of locations with contrasting hydrography, we conclude that a continued warming of the Arctic is likely to result in more complex ecotones across the Arctic marine system.

Description: Anti-grazer responses of phytoplankton have been widely documented to vary with external abiotic factors (e.g. nutrient status), suggesting an environment-dependence of phytoplankton–zooplankton dynamics. Here, we examined the trophic interaction between the cyanobacteria Coleofasciculus chthonoplastes and the cladoceran Moina micrura under three nutrient levels from oligotrophic to hypereutrophic conditions (0.3, 3.0 and 100.0 μM $$\hbox{ P }{{\hbox{ O }}_{4}}^{3-};$$ molar ratio $$\hbox{ N }{{\hbox{ O }}_{3}}^{-}:\hbox{ P }{{\hbox{ O }}_{4}}^{3-}=16:1$$ ) to determine the effect of nutrients on the stability of a phytoplankton–zooplankton system. In the absence of M. micrura , cell density and filament (single-filament) length of C. chthonoplastes were lower at the low nutrient level compared with those at the medium and high nutrient levels. When M. micrura was present, cell density, filament length, colony proportion and size (the mean number of cells per colony) of C. chthonoplastes were highest under the medium nutrient level, corresponding to the lowest amplitude of density fluctuations in M. micrura . Regression analyses revealed that low cell densities of C. chthonoplastes (due to its limited ability to form colonies and long filaments and hence a high consumption rate of M. micrura ) at low and high nutrient levels limited the growth of M. micrura adult females, leading to a sharp decline in embryo length, clutch size and population density. In general, our results suggest that the stability of phytoplankton–zooplankton systems varies with nutrients because of the nutrient-dependent morphology in phytoplankton, and imply that extremely low or high nutrient load may destabilize phytoplankton–zooplankton systems.

Description: The bloom-forming dinoflagellate Karenia brevis produces a suite of allelopathic compounds that inhibit the growth of several phytoplankton competitors in laboratory experiments. However, it is less clear how allelopathy affects competition in the field, including whether allelopathic compounds impact K. brevis bloom dynamics. We investigated the extent to which phytoplankton species typically found offshore in the Gulf of Mexico, where K. brevis blooms initiate, are sensitive to K. brevis allelopathy. Natural assemblages of offshore phytoplankton dominated by diatoms were largely resistant to K. brevis allelopathy, even experiencing slight stimulation of growth from exposure to K. brevis exudates. When tested in pair-wise laboratory experiments, four diatom species often found offshore in the Gulf of Mexico exhibited varying degrees of resistance to K. brevis similar to that observed with field assemblages, although some competitors displayed a trend toward growth inhibition. In concurrent pair-wise laboratory experiments, four diatom species whose nearshore habitats frequently experience dense K. brevis blooms also demonstrated a range of responses to K. brevis allelopathy, from strong sensitivity to resistance. Overall, Gulf of Mexico phytoplankton that co-occur with K. brevis blooms in both nearshore and offshore environments respond variably to K. brevis allelopathy.

Description: Heterocyst frequencies in Baltic Sea Aphanizomenon sp. were similar along a strong nutrient gradient from the discharge point of a sewage treatment plant at the head of the Himmerfjärden bay to the open sea. Filaments lacked heterocysts in winter and for over a month after the spring bloom had depleted combined nitrogen in the surface layer. Heterocyst-free filaments in spring contained granulate structures that decreased in abundance simultaneously as colony nitrogen content decreased, but 15 N remained unchanged, indicative of storage of fixed nitrogen in overwintering Aphanizomenon sp. filaments. Heterocyst formation was initiated when water temperature was sufficient to form a shallow seasonal pycnocline that allowed filaments to be exposed to enough light to initiate growth and a subsequent intracellular shortage of nitrogen. During the growth season, heterocyst frequency varied significantly with maximum values in early summer (May), lower values in mid-August that coincided with maximum temperatures and an increase in late summer. Heterocyst frequencies decreased with increased temperatures, suggesting a more efficiently functioning nitrogenase enzyme. Based on data from three seasons, filament C:P ratios did not correlate with heterocyst frequencies, neither did reduced heterocyst frequencies coincide with high dissolved inorganic nitrogen concentrations. Increased heterocyst frequencies, however, resulted in decreased C:N ratios, probably as more heterocysts likely increase nitrogen fixation.

Description: Planktonic organisms are exposed to turbulent water motion that affects the fundamental activities of swimming and feeding. The goal of this study was to measure the influence of realistic turbulence levels on (i) swimming behavior and (ii) fluid interactions during feeding by the lobate ctenophore, Mnemiopsis leidyi , a highly successful suspension-feeding predator. A laboratory turbulence generator produced turbulence ( = 0.5–1.4 x 10 –6 W kg –1 ) representative of a field site in Woods Hole, MA, USA. Compared with still water, M. leidyi avoided regions in the experimental vessel where turbulence was greatest ( = 1.1–1.4 x 10 –6 W kg –1 ) by increasing its swimming speeds and accelerations. Both laboratory and in situ particle image velocimetry data demonstrated that feeding currents of M. leidyi were eroded by ambient fluid motions. Despite this, the overall flux to the feeding structures remained constant due to higher swimming speeds in turbulent conditions. Instantaneous shear deformation rates produced by background turbulence were higher than those produced by ctenophore feeding currents and frequently exceeded the published escape thresholds of copepod prey. Feeding current erosion and fluid mechanical signal noise within turbulent flows affect the mechanics of predator–prey interactions during suspension feeding by the ctenophore M. leidyi .

Description: Temporal changes in the in situ germination flux of cysts and the abundance of vegetative cells of the toxic dinoflagellate Alexandrium catenella were investigated in Ago Bay, central Japan from July 2003 to December 2004. The in situ germination flux (cells m –2 day –1 ) was measured using ‘plankton emergence trap/chambers (PET chambers)’. Germination of the cysts in the sediments occurred continuously during the study, ranging from 52 to 1753 cells m –2 day –1 , with no temporal trend. This germination pattern appeared to be promoted by a short mandatory dormancy period for newly formed cysts coupled with a broad temperature window for germination. For the vegetative populations, high abundances (〉10 5 cells m –2 ) were recorded in the water column from spring to summer and from autumn to early winter. The size of the vegetative populations did not correlate with the cyst germination flux but rather larger vegetative populations were often observed when the water temperature was around 20°C, indicating that bloom development was mainly regulated by the temperature. Nonetheless, the continuous germination pattern of A. catenella is advantageous enabling the germinated cells to immediately exploit favorable bloom conditions.

Description: The ctenophore Mnemiopsis leidyi A. Agassiz (1865) is not generally believed to exhibit diel feeding. As a result, the majority of studies documenting feeding of M. leidyi have been performed during the day, which may underestimate or overestimate daily clearance and ingestion rates if feeding changes over the diel cycle. Here, diel feeding by M. leidyi was examined during seven separate 24-h periods using gut content analysis. The total prey abundance and number of prey consumed per ctenophore did not differ between day and night; however, the percent of ctenophores with empty guts was higher during the day. These data show that, although fewer ctenophores consumed prey during the day, those that did consumed a larger number than at night. Additionally, the composition of the prey assemblage and the diet of M. leidyi did not differ between day and night; however, the composition of the prey assemblage differed from that observed in ctenophore guts, indicating selective feeding. Despite the lack of an overall difference in prey abundance and diet composition, isopods, cumaceans, decapod shrimp larvae and mantis shrimp larvae were observed in ctenophore guts only at night. These observations emphasize the importance of both day and night sampling, especially in ecosystems where prey availability changes significantly over the diel cycle.

Description: A northward shift in distribution of the subarctic copepod, Calanus finmarchicus is associated with climate-forced warming. We conducted experiments to measure hatching success (HS) of C. finmarchicus eggs at temperatures ranging from 6 to 26°C. HS is not significantly decreased by exposure to temperatures up to 19°C. HS dropped to 〈25% by 22°C; no eggs hatched at higher temperatures. These results have ecological significance for the Gulf of Maine, at the southern margin of the species' range.

Description: Generalized additive models (GAMs) were used to test the hypothesis that changes in physical and biological environmental conditions affected by current climatic warming would negatively impact the euphausiid populations in the North Atlantic. Two zooplankton time series were used, one collected by the Marine Research Institute (MRI) on a transect south of Iceland during spring (1990–2011) and the other by the Continuous Plankton Recorder (CPR) survey (1958–2007) in the oceanic waters south of Iceland covering all months. Due to limitations of the sampling gears used, the results mainly reflect the variations of the early stages of euphausiids. On a spatial scale, results reveal a general decline of euphausiid abundance from the east coast of Greenland to the Faroe Islands. On a temporal scale, euphausiid numbers decreased in most CPR areas from 1958 to 2007. Conversely, an increase was observed in numbers of larvae during spring 1990–2011 for the shelf south of Iceland. Single variable-based GAMs indicated that phytoplankton biomass was generally the main environmental factor regulating euphausiid abundance. Multiple variable-based GAMs showed that phytoplankton biomass was the strongest predictor of euphausiid abundance in the west, whereas in the east temperature appears to be most important. In addition, the onset of the spring bloom also affected the long-term changes in euphausiid abundance. For the oceanic areas, it is concluded that a weakened temporal synchrony between the development of young euphausiids and the phytoplankton bloom influenced by recent climate warming may have led to the observed decrease in euphausiid populations.

Description: We used nitrogen stable isotopes to study the regulation of nitrogen fixation by filamentous cyanobacteria. Nitrogen fixation was found to be almost insensitive to combined nitrogen, along a gradient from the Himmerfjärden sewage treatment plant discharge to the open sea. We found similarly low cyanobacterial (mostly Aphanizomenon sp.) 15 N-values at all stations, despite significant differences along the bay in both total nitrogen concentrations in water and 15 N in seston (particles 〈10 µm), the latter used as a proxy for algae growing on combined nitrogen alone. Only late in the productive season, when cyanobacterial biomass was declining or already low, did elevated 15 N suggest uptake of combined nitrogen. However, this coincided with an increase in the contribution of Dolichospermum spp. to overall diazotrophic biomass and may indicate uptake of combined nitrogen by this species. These results indicate that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. This study was part of a whole ecosystem experiment analyzing the effects of nitrogen removal in a sewage treatment plant discharging to the Himmerfjärden Bay, northern Baltic Sea Proper.

Description: Our current understanding of zooplankton interactions with surrounding fluid is limited, partially because traditional methods of particle image velocimetry (PIV) become impractical at scales less than a few millimeters and microscope-based systems restrict motions and can incur "wall effects". We present a new approach to small-scale PIV imaging and our results demonstrate the ability to observe detailed kinematics simultaneously with fluid motion at small scales around free-swimming zooplankton. This can improve our understanding of animal–fluid interactions at small spatial scales and low Reynolds number.

Description: During copulation bouts, Euterpina acutifrons males grasp females and cling for extended periods as a form of mate guarding. The present study assessed the effects of such mate guarding on the swimming behaviour of grasped females using three-dimensional video analysis techniques. Mate-carrying female (CF) E. acutifrons spent significantly more time swimming at slower speeds than non-mate-carrying females (NCF). In addition, CFs did not attain the maximum swimming speeds attained by their NCF counterparts. Carrying females, however, displayed more complex swimming trajectories and were observed swimming in tight helical loops. We postulate that CFs were attempting to dislodge guarding males with this behaviour as a form of mate-guarding interference. Furthermore, given that mating pairs are more conspicuous and slower than non-mating individuals, we suggest that mate guarding is associated with increased predation risk.

Description: The aim of this work is to evaluate whether interference competition between predators plays an important role in a rotifer predator–prey system. Our results indicated that higher predator density leads to depression of predation rates even if sufficient prey was offered. Crowded culture volume strengthens competition. In addition, this competition might be time-dependent. Our contour plots show that the behavior of the predators changed with time spent foraging.

Description: An increasing number of studies show the importance of chemical interactions in the aquatic environment. Our understanding of the role of chemical cues and signals in larger crustaceans has advanced in the last decades. However, for copepods, the most abundant metazoan zooplankton and essential for the functioning of the marine food web, much is still unknown. We synthesize current knowledge about chemical ecology of copepods including foraging, survival and reproduction. We also compile information on the sensory apparatus and new analytical approaches that may facilitate the identification of signal molecules. The review illustrates the importance of chemical interactions in many aspects of copepod ecology and identifies gaps in our knowledge, such as the lack of identified infochemicals and electrophysiological studies to confirm the function of sensory structures. We suggest approaches that are likely to further our understanding of the role of chemical interactions in the pelagic ecosystem.

Description: The mechanisms by which phytoplankton cope with stressors in the marine environment are neither fully characterized nor understood. As viruses are the most abundant entities in the global ocean and represent a strong top-down regulator of phytoplankton abundance and diversity, we sought to characterize the cellular response of two marine haptophytes to virus infection in order to gain more knowledge about the nature and diversity of microalgal responses to this chronic biotic stressor. We infected laboratory cultures of the haptophytes Haptolina ericina and Phaeocystis pouchetii with CeV-01B or PpV-01B dsDNA viruses, respectively, and assessed the extent to which host cellular responses resemble programmed cell death (PCD) through the activation of diagnostic molecular and biochemical markers. Pronounced DNA fragmentation and activation of cysteine aspartate-specific proteases (caspases) were only detected in virus-infected cultures of these phytoplankton. Inhibition of host caspase activity by addition of the pan-caspase inhibitor z-VAD-fmk did not impair virus production in either host–virus system, differentiating it from the Emiliania huxleyi -Coccolithovirus model of haptophyte–virus interactions. Nonetheless, our findings point to a general conservation of PCD-like activation during virus infection in ecologically diverse haptophytes, with the subtle heterogeneity of cell death biochemical responses possibly exerting differential regulation on phytoplankton abundance and diversity.

Description: Daphnia lumholtzi invaded North American lakes and reservoirs. It forms prominent spines, which are inducible by fish kairomones. We tested the two hypotheses that the long spines protect against vertebrate (fishes) and invertebrate ( Chaoborus ) predators. Our experiments revealed that adults with longer spines survived significantly better under fish predation and juveniles with longer spines were additionally better protected under Chaoborus predation. Our study is the first direct proof that long spines effectively protect against both predators. Our results support the idea that the anti-predator devices may represent an important functional mechanism for the invasion success of D. lumholtzi .

Description: The fatty acid (FA) composition of Pseudodiaptomus hessei , one of the most abundant copepod species in southern African estuaries and lakes, reveals potential food selectivity and the importance of flagellate prey in their diets. Differences in FA profiles between males and females were recorded; however, none of these patterns matched the reproductive activities of the copepod. Our findings stress the importance of improving our knowledge of FA biosynthesis pathways to employ fatty acids as trophic markers in feeding studies of zooplankton in temperate environments.

Description: Deliberate fertilization of a patch of water west of Haida Gwaii, British Columbia, with iron sulphate and oxide occurred in summer 2012 and triggered a phytoplankton bloom strongly visible in satellite imagery in late August and detectable through September 2012. Routine sampling by the Continuous Plankton Recorder Survey from commercial ships occurred in the vicinity of the fertilized patch between April and October that year. Comparisons with samples from the same region in the years 2000–2011 showed that phytoplankton and microzooplankton abundance indices were the lowest recorded over the time series in the autumn of 2012, while crustacean zooplankton were higher than average, and often higher than previously recorded in the autumn. Possible other contributory factors are discussed but this evidence suggests that the iron-induced bloom could have caused an increase in zooplankton that in turn exerted a heavy grazing pressure on the large phytoplankton and microzooplankton by the autumn of 2012.

Description: Thermohaline fronts are present globally and have been identified as biodiversity hotspots, encouraging enhanced primary productivity and attracting dense aggregations of higher trophic level organisms. The Celtic Boundary Front (CBF) sits in an important zone of economic activity yet no contemporary studies describing the meso-zooplankton community exist for this region. High-frequency sampling during June 2009 revealed three distinct areas on the mixed (Irish Sea) and stratified (Celtic Sea; Celtic Deep) sides of the front, each exhibiting distinct physical and biological characteristics. Low zooplankton abundance was found above the Celtic Deep, despite apparent optimal phytoplankton conditions, conflicting with observations made at other frontal locations. Although zooplankton assemblages were generally distinct within areas, the copepod Acartia clausii was ubiquitous across all three areas and accounted for almost 60% of all counted individuals. The long-term variation of zooplankton during the seasonal front development was examined using samples from the Continuous Plankton Recorder. The results of this analysis suggest that low abundance is a recurring feature over the Celtic Deep. Possible explanations for the observed patterns may be top-down effects on zooplankton from pelagic fish; alternatively, these patterns may be a consequence of the strong density and stratification gradients in the Celtic Deep.

Description: During the summers of 2002–2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) (freshwater estuary) to 19 km offshore in Lake Michigan (LM) (a Great Lake). Despite considerable inter-year variability, the average rates of gross production (GP), respiration (R) and net production (NP) across ML (604 ± 58, 222 ± 22 and 381 ± 52 µg C L –1 day –1 , respectively) decreased steeply in the furthest offshore LM site (22 ± 3, 55 ± 17 and –33 ± 15 µg C L –1 day –1 , respectively). Along this land-to-lake gradient, GP decreased by 96 ± 1%, whereas R only decreased by 75 ± 9%, variably influencing the carbon balance along this coastal zone. All ML sites were consistently net autotrophic (mean GP:R = 2.7), while the furthest offshore LM site was net heterotrophic (mean GP:R = 0.4). Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore. Reactive and dynamic estuarine coastal zones everywhere may contribute similarly to regional and global carbon cycles.

Description: Environmental conditions during the larval phase (food concentration and temperature) impact recruitment success of marine bivalves by affecting growth and survival. We analysed the seasonal match between environmental conditions and larval presence of six coastal bivalve species over eight consecutive years (2006–2013) in the western Wadden Sea, taking species-specific physiological responses to these conditions into account. The larval occurrence of four species overlapped with their optimum in environmental conditions as calculated following the Dynamic Energy Budget theory: the occurrence of larvae of Cerastoderma edule, Mya arenaria and Mytilus edulis matched with their optimum food conditions, while larvae of Crassostrea gigas were mainly found during the temperature optimum of this species. Larvae of Ensis directus and Macoma balthica , however, only occurred months after their most favourable food conditions. The first group probably uses the energy immediately after uptake for the production and release of gametes (‘income breeders’), whereas the second group accumulates and stores energy reserves for later reproduction (‘capital breeders’). Larvae of C. gigas and M. balthica were observed much earlier in the year than expected from previous occurrences and from formerly know threshold temperatures for spawning. These apparent changes in timing of spawning suggest that Pacific oysters and Baltic tellins have recently acclimated to new environmental conditions.

Description: The zooplankton community changes that may occur with increases in concentration of allochthonous dissolved organic carbon (DOC) are not well understood in unconstrained lowland rivers. We examined in mesocoms over 8 days the short-term responses of river zooplankton to amendments of DOC from two different sources, glucose and leachates of a common riparian tree ( Eucalyptus camaldulensis ; river red gum) both alone and with inorganic nutrients added. DOC additions with and without nutrients increased heterotrophic respiration and led to significant increases in bacterial biomass. These responses varied between glucose and leachate addition. In treatments with added DOC, zooplankton density significantly increased relative to controls. Some zooplankton genera only responded to the leachate as a DOC source, and community structures significantly varied between the control and the glucose and leachate amendments. Zooplankton are particularly important in lowland river systems as they are key organisms for the transfer of carbon to higher trophic levels and this study indicates that allochthonous DOC has the potential to be an important basal resource to lowland river food webs. This may be particularly important in lowland sections of unconstrained flood plain rivers during and immediately following floods when allochthonous DOC is more available.

Description: Our goal was to determine (i) the origin of fecal pellet-associated prokaryotes of calanoid copepods and doliolid gonozooids, (ii) their rate of colonization on such pellets during the first 20 h after they are released and (iii) to what extent different food organisms affect prokaryotic colonization of doliolid pellets. Our studies were conducted with individual pellets of both copepods and doliolids to obtain information on the variability of the factors influencing prokaryotic colonization of zooplankton pellets. Previous findings imply that the prokaryotes in fecal pellets of calanoids including the calanoid Eucalanus pileatus originate from the ingestion of food. Our results on just-released pellets of Dolioletta gegenbauri lead to the assumption that prokaryotes associated with doliolid pellets most likely originate from ingestion processes. Increases of prokaryotic abundance of doliolid pellets surpassed those of copepod pellets during the first 18–22 h after their release. Our results on the amounts of prokaryotes on/in doliolid pellets imply that feeding rates on prokaryotes by large gonozooids of D. gegenbauri are rather low.

Description: Basin-scale carbon recycling estimates were obtained by combining high-resolution data on zooplankton taxonomic and functional composition with species-specific respiration rates. Datasets were collected in the Atlantic and Indian sectors of the Southern Ocean during four cruises covering spring, mid-summer, autumn and late winter between 1993 and 1998. Carbon recycling by Antarctic meso- and macrozooplankton represented a significant (44–62%) fraction of the primary production, which is at the higher end of previous estimates. Assessment based on detailed community structure appeared to be more realistic than previous estimates and showed that carbon dioxide recycling at the global scale is not a mere function of temperature and abundance of zooplankton. Both species and functional diversity influence current estimates at the community level through trophic type and developmental stage composition. In addition, a regional spatial heterogeneity linked to hydrodynamic features (frontal zones) is also important. The Southern Ocean zooplankton community respiration is assessed to be ~0.6 GtC year –1 .

Description: We studied the development of autotrophic picophytoplankton and heterotrophic bacterioplankton during the transition from winter ice cover to open water under natural and manipulated mixing conditions in eutrophic Lake Vesijärvi. During the melting of the snow and ice cover, a convection layer developed which eventually met the chemocline at the interface between the oxic and anoxic water masses. However, in the years with mechanically enhanced mixing, the whole water column remained well oxygenated and the deepening of penetrative convection was facilitated. Stochastic variations in weather, primarily the thickness of the snow cover, likely determined the timing of picophytoplankton growth. Mechanical mixing slightly increased the biomass of picophytoplankton, especially the eukaryotic fraction, probably because eukaryotic picophytoplankton coped well with changing light conditions. Bacterial biomass was not notably affected, although their mean cell size was significantly smaller in the oxic deep water than in anoxic conditions during the natural mixing regime. Our results show that, when snow melts and solar radiation increasingly penetrates into lake water, picophytoplankton biomass and numbers already start to increase weeks before ice-break and may reach abundances typical of early summer under the ice. A similar development of bacterioplankton suggests a trophic relationship between heterotrophic and autotrophic producers.

Description: The holoplanktonic jellyfish Pelagia noctiluca is renowned for periods of high abundance, causing considerable problems to tourism and aquaculture. Little is understood about the drivers of its periodic presence and absence or how it survives unfavourable periods. Studying the effect of starvation, we evaluated the main metabolic expenses (reproduction, respiration and excretion) during those periods. P. noctiluca could shrink in size, losing up to 85% of their mass (6.6–7.1% loss day –1 ), while continuing to release eggs quasi-daily over a 28-day period. Egg production was proportional to size (mean 759 eggs day –1 at 6 cm bell diameter), with up to 19 526 eggs released in a single spawn, thereby providing huge potential for population growth despite undergoing starvation. Small food rations decreased the rate of shrinking to 3.1% day –1 , prolonging life (49 days), potentially enhancing the chances of encountering more prey and regrowing. Metabolism increased with wet mass (allometric exponent: 0.93 for respiration, 0.82 for ammonium), however reproduction was the greatest carbon expenditure for individuals larger than 9 cm bell diameter. Temperature (9–29°C) also significantly increased both respiration and, to a greater extent, excretion ( Q 10 = 2.25 and 4.76). Consequentially a warming ocean may negatively affect survival rates unless prey abundance balances the increased metabolic demands.

Description: We evaluate the potential for decapod larvae to develop under temporally limited daily access to food across a broad thermal range. Larvae of the European lobster Homarus gammarus and velvet swimming crab Necora puber were successfully reared to juvenile and megalopa stages, respectively, at four temperatures (range 15–24°C) under both permanent (24 h) and limited (4 h) daily access to prey. Survival was high in both species with no consistent effect of food limitation. Larvae developed faster with increasing temperature. Model selection indicated that limited access to food in N. puber slowed development from zoea III onwards by 10–25% depending on stage and temperature. Food limitation slowed development of H. gammarus in zoea III and stage IV by 6–30%, with significant interactions of food x temperature. Dry mass, carbon and nitrogen content of H. gammarus all decreased with temperature and were lower under limited access to food. Decapod larvae of the tested species show a high potential to survive and develop with a temporally patchy food resource, although a longer development and smaller body mass at the time of settlement may lead to fitness costs.

Description: Brachionus variabilis , a facultative epibiont on cladocerans, has a pronounced morphological response to Asplanchna . Adult amictic females of the Asplanchna -induced morph, compared with those of the basic or non-induced morph, had a 13% larger lorica, 150% longer posterior spines and 30–40% longer anterior spines. Similar responses occur in many other brachionid rotifers as an effective defense against Asplanchna predation, but the response of B. variabilis is surprising. The large size and epizoic habit of B. variabilis should provide good protection against this predator. Also, a smaller epizoic congener ( Brachionus rubens ) readily ingested by Asplanchna has no morphological response to it. Brachionus variabilis also has a pronounced behavioral response to Asplanchna . Without Asplanchna , most individuals are actively swimming, with only a small proportion of them attached to surfaces. With Asplanchna , the proportion of attached individuals is much higher. Significant attachment responses in both young and adult individuals occurred shortly (~5 h) after exposure to live Asplanchna (three experiments) or to its kairomone (five experiments), but not to a Daphnia kairomone. The Asplanchna -enhanced propensity for attachment to surfaces, possibly including cladoceran hosts, may be an additional defense against this predator. This is the first report of a predator kairomone inducing a behavioral shift in a rotifer.

Description: The 0- to 1000-m vertical distribution patterns of micronekton and macrozooplankton were determined in the Costa Rica Dome region (9° N; 90° W) of the Eastern Tropical North Pacific in October–November 2007 and December 2008–January 2009. The area has a pronounced oxygen minimum zone (OMZ) that impacts the distribution of both migrating and mesopelagic species. Sampling was conducted at a relatively fine scale (mesopelagic depth intervals as small as 25 m) within ecologically relevant strata to assess how this hydrographic environment influenced the structure of these groups. Zooplankton were collected in vertically stratified Multiple Opening/Closing Net and Environmental Sensing System tows during the day and the night, and abundances were analyzed in association with variations in oxygen concentration, temperature and depth. Each vertical stratum of the water column was a unique ecological zone with a specific community makeup. The upper and midwater column (0–550 m) was strongly influenced by diel vertical migration patterns, particularly the daytime descent of euphausiids and myctophid fish into the core of the OMZ. Distinctly different communities occurred below the OMZ core. The lower oxycline (LO) (~500–700 m depth range) was dominated by Cyclothone spp. fish as well as a diverse population of other taxa, often aggregated into a discrete layer (25 m thick), particularly in 2008. In the suboxycline (〉700 m depth range), the community shifted to typical bathypelagic taxa. These finer scale vertical patterns provided new insight into the ecological structure of OMZs, revealing the unique layering at the LO and differential impacts on diel vertical migrators.

Description: Production of diapausing eggs in many zooplankton species occurs after partial switching from parthenogenesis to sexual reproduction. Storage effect theory predicts the stable long-term coexistence of competitors investing in diapausing stages, but it does not address the effect of such investment on short-term coexistence. The freshwater rotifer Brachionus calyciflorus and the cladoceran Moina micrura were used in this study to test experimentally if diapause could promote coexistence of active individuals under inter-specific competition. Our results show that B. calyciflorus was more affected by inter-specific competition and exhibited a greater and earlier production of resting eggs compared with M. micrura . Costs of diapause investment in Moina were only observed in competition. This was translated into reduced probability of B. calyciflorus of being outcompeted by M. micrura . We show that investment in diapause might promote local and short-term coexistence of competing zooplankton species. Our study strongly suggests that the ecological effects of diapause encompass a wider range of temporal scales, from weekly to decadal, and levels of organization, from individuals to communities, than previously thought.

Description: Recent changes in Lake Michigan's lower trophic levels were hypothesized to have influenced the diet of omnivorous Mysis diluviana . In this study, the stomach contents of Mysis were examined from juvenile and adults collected monthly (April–October) from a 110-m bottom depth site to describe their seasonal diet in Lake Michigan during 2010. Diatoms were the most common prey item ingested, followed by calanoid copepods, and chrysophytes. Dreissenid veligers were documented in mysid diets for the first time in the Great Lakes, and Cercopagis pengoi were not only consumed but even preferred by adults in summer. Diet proportions by weight were dominated by calanoids, although diets showed a marked shift toward cladocerans in autumn. Juvenile and adult Mysis selected primarily for cladoceran prey but also selected for some calanoid copepod taxa. Comparing available Mysis diet data from 1985 to 2010 indicated generally fewer cladocerans and rotifers per gut and less consistent differences in copepods and Peridinium consumed. The seasonal composition of phyto- and zooplankton prey documented herein should be useful to those seeking to understand the trophic role of Mysis in offshore food webs, but caution should be expressed when generalizing similarities in Mysis diets across other lakes because Lake Michigan's population seems relatively more herbivorous.

Description: The western rock lobster, Panulirus cygnus , provides Australia's most valuable wild caught fishery but, in recent years, there has been a dramatic decline in settlement of the post-larval phase into their natal coastal habitat. One hypothesis for this decline was that the oceanographic conditions no longer favour the survival, feeding and growth of the larval (phyllosoma) phase. To explore this, the oceanography and corresponding zooplankton prey field along five latitudinal transects in the south-eastern Indian Ocean were quantified during July 2010. Leeuwin Current Water (LCW) and Sub-Tropical Surface Water (STSW) were distinguished and a prominent front at ~30°S characterized by strong eastward flow separated them. Although zooplankton abundance increased towards the north, the prey field was unevenly distributed with patches of higher prey concentration associated mainly with LCW. Chaetognaths were the most abundant prey item (means: 17.2 and 4.1 m –3 in LCW and STSW, respectively) and were positively correlated with chlorophyll a in both water masses. Panulirus cygnus phyllosoma had a highly patchy distribution but, despite lower prey concentrations, were more abundant in STSW than LCW, particularly south of the front. Our results suggest that LC meso-scale features with strong fronts may be implicated in phyllosoma aggregations and shoreward transport of late-stage larvae and that this warrants further investigation.

Description: Concurrent physical, chemical and biological observations across the Kuroshio Front collected in October 2009 provide a detailed view of the relationship between the physical environment and the phytoplankton community. Depth profiles were taken at stations ~9 km apart along five 70 km transects. With a combination of flow cytometry, microscopy and high-pressure liquid chromatography pigment analysis, we characterized the phytoplankton community structure across the front. The observed phytoplankton community fell into two distinct assemblages, largely separated by the front, but which also reflected patterns in the distribution of Kuroshio and Oyashio water masses shaped by mesoscale lateral mixing. Phytoplankton biomass was elevated where there was a positive vertical flux of nitrate towards the surface, and the frontal circulation drove a lateral transport of nutrients southwards into the subtropical gyre. The observations showed that the phytoplankton respond to forcing on several scales: the phytoplankton community across the front was shaped by a combination of the large scale biogeography of the region, mesoscale mixing of populations and finer scale modification of the light and nutrient environment.

Description: Filtration by adult Euphausia pacifica was measured before and during the upwelling season, using both "disappearance of chlorophyll" and "disappearance of cells" techniques. Results show that feeding rates and selectivity varied with food assemblages. Filtration rate ( F ) was best modeled by the Ivlev function: the average F on total Chl- a was 92 mL euphausiid –1 h –1 , and 119 mL euphausiid –1 h –1 on microscopy cell counts. F averaged 36 for the 〈5 µm size fraction of Chl- a , 94 for the 5–20 µm fraction and 107 mL euphausiid –1 h –1 for the 〉20 µm fraction. The average F values were 155 and 163 mL euphausiid –1 h –1 for chain-diatoms and single diatoms, respectively, and115 and 137 mL euphausiid –1 h –1 for the 〈40 µm and 〉40 µm ciliates, respectively. Ingestion rates based on total Chl- a and size fractions, total cell counts and ciliates were significantly correlated using Hollings' models ( P 〈 0.01). Maximum daily ration was 23% body C day –1 when a high food concentration (700 µC L –1 ) was available, but over the carbon range of 50–200 µg C L –1 , daily ration averaged 4% body C day –1 . Diatoms were consumed almost exclusively during blooms associated with summer upwelling events; larger types of ciliates and dinoflagellates were fed upon preferentially compared with their smaller counterparts.

Description: Despite the acknowledged importance of small copepods of the genus Oithona in marine pelagic ecosystems, there is little information about their ecological role, potential food resources and egg production rates (EPR) in tropical environments. In the present study, feeding and EPR of adult females of two species of Oithona were determined in two different tropical marine food webs in North Queensland, Australia, during the 2011 austral autumn. Oithona attenuata was studied in the waters of the Great Barrier Reef lagoon, and Oithona dissimilis was studied in a mangrove area. Oithona spp. ingested dinoflagellates and ciliates preferentially to other prey items of the nano- and microplankton assemblage. Oithona spp. clearance rates on dinoflagellates and ciliates ranged from 3.7 to 10.4 mL female –1 day –1 , and from 4.3 to 18.1 mL female –1 day –1 , respectively. The daily body carbon ingested per female was 〈1% when feeding on dinoflagellates, and varied from 1 to 10% when feeding on ciliates. Our results suggest that Oithona spp. feed on small flagellates (5–20 µm), although the contribution of carbon to the diet was low (2.5–3.2% body carbon). Egg production and weight-specific EPR ranged from 0.22 to 3.34 eggs female –1 day –1 , and 0.2–4.5% day –1 respectively. The ingestion rates measured in all the feeding experiments were too low to sustain metabolic and egg production costs, indicating that other food resources, not considered in this study, might contribute significantly to the diet of Oithona spp . in tropical environments.

Description: The zonal (~15°W–40°W along 26°N–29°N) and meridional (~30°N–30°S along 28°W–29°W) variability of 15 N of suspended particles and zooplankton (〉40 µm) was studied to assess the influence of nitrogen fixation in the isotopic budget of the tropical and subtropical Atlantic ocean. Two cruises were conducted in October–November 2007 and April–May 2008 comprising a zonal and meridional transect each. In the region between 30°N and 15°N, the concurrently measured nitrogen fixation was insufficient to explain the consistent patch of suspended particles with 15 N 〈 2 and points to a significant contribution of atmospheric deposition of light nitrogen to the isotopic budget. The equatorial region (15°N–10°S) is subject to intense nitrogen fixation, which, according to a two-end-member mixing model, may explain 40–60% of the observed 15 N in suspended particles and 3–30% in zooplankton. In the South region between 10°S and 30°S, low values (〈4) were measured in suspended particles and zooplankton during 2008. The values of 15 N of suspended particles suggest that nitrogen fixation, which is usually low (〈10 µmol N m –2 day –1 ), may represent 50–60% of phytoplankton nitrogen in this region. Hence, diazotrophy in the South Atlantic may be more important than previously thought.

Description: To establish a metabolic state along a north–south transect in Antarctic waters, we approached community respiration (CR) from a combined perspective based on the metabolic theory of ecology (MTE) and the size-scaling of the whole planktonic community. A detailed analysis of a summer integrated multi-trophic normalized biomass size spectra (NBSS), from heterotrophic bacteria to zooplankton, was carried out. To acquire individual size data, different techniques were combined: flow cytometry for smaller fractions (〈20 µm of equivalent spherical diameter), FlowCAM for larger nano- and microplankton and scanning and image analysis for the zooplankton fractions. The distribution of the NBSS was linear at all stations ( R 2 values: 0.87–0.93) but dome-shape features appeared related to phytoplankton cell distribution which are responsible for a large fraction of microbial respiration. Generally, the region showed an autotrophic budget south of the archipelago due to gross primary production (GPP) values up to 2804 mg C m –2 day –1 , where salps could significantly contribute to the carbon export flux. Contrastingly, higher CR rates (〉1000 mg C m –2 day –1 ) were found at the northern stations due to a higher phytoplankton respiration activity associated with increasing sea water temperatures and a higher presence of heterotrophic organisms (microheterotrophs, chaetognaths and copepods) resulting in a net heterotrophic metabolic state (GPP/CR 〈 1).

Description: We investigated the seasonal vertical migration of seven dominant arctic copepod species from October 2007 to July 2008 in Amundsen Gulf. The large herbivore Calanus hyperboreus resided in the deep Atlantic Layer from December to mid-April, rapidly invaded the surface layer at the onset of the phytoplankton bloom in early May, and started its descent to overwintering depth in July. C. glacialis overwintered at shallower depths than C. hyperboreus , moved into the surface layer in early April as ice algae bloomed, and remained in the subsurface chlorophyll maximum (SCM) until late July. The small omnivore Oithona similis slowly rose into the Polar-Mixed Layer from February to April, reaching the ice–water interface in early April as ice algae developed. The very small Triconia borealis associated with C. hyperboreus in the Atlantic Layer from October to April. The mesopelagic omnivores Metridia longa and Microcalanus pygmaeus could be characterized as mid-depth interceptors, feeding on microzooplankton in winter, the upward flux of C. hyperboreus eggs in late winter-early spring, and on the rain of microalgae in spring-summer. The epipelagic Pseudocalanus spp. distributed in a narrow band centered around 50 m from October to April, then descended under the SCM from May to July. Copepods generally avoided the lens of warm (2–8°C) surface water in summer, but readily migrated across the –1.7°C to +0.3°C gradient from the Polar-Mixed Layer to the Atlantic Layer. The different patterns of seasonal migrations dictated the different temperature regimes experienced over the year by each species and copepodite stage.

Description: Copepods play central roles in pelagic food webs linking primary production to higher trophic levels. Biomarkers (lipids, stable isotopes) provide modern approaches to study dietary preferences and trophic interactions. A cluster analysis based on the fatty acid (FA) and fatty alcohol compositions of calanoid copepods (copepodids C4/C5 and adult stages) from the southeastern tropical Atlantic identified five distinct groups according to lipid composition and storage strategy, coinciding with differences in vertical distribution from the surface to 1800 m depth. Most epipelagic species were characterized by low lipid levels (~10% of dry mass, DM), low quantities of wax esters (WE) and low 15 N ratios indicating low trophic positions. In contrast, surface-dwelling Rhincalanus cornutus had higher lipid levels (〉29% DM) and stored WE (〉90% of total lipid), whereas vertically migrating Pleuromamma species did not store WE. Most mesopelagic copepods belonged to another cluster, defined by high lipid level (max. 47% DM), high amounts of the FA 18:1( n -9) and high 15 N ratios 〉9 indicating carnivorous feeding at a higher trophic level. Diapausing Calanoides carinatus (copepodids C5), collected at great depth, formed a separate cluster with low 15 N ratios and high amounts of herbivory markers. The latter were apparently accumulated during active feeding on phytoplankton in surface waters and transferred to the deep sea during ontogenetic vertical migrations. In conclusion, these tropical calanoid copepod species from the surface to the deep sea have adopted diverse feeding strategies and occupy a wide range of ecological niches, affecting energy flux and carbon cycling in the tropical Atlantic.

Description: An upward looking 75 kHz ADCP (Acoustic Doppler Current Profiler) was moored at 100 m above the bottom in 912 m water depth in the central-north Alboran Sea (W-Mediterranean Sea). The ADCP sampled current and acoustic echo-amplitude at a rate of once per minute for 9 days, thereby revealing particular internal wave–zooplankton interactions that varied with depth. The ADCP's echo-amplitude, a measure of zooplankton abundance, showed a dominant diurnal periodicity due to vertical migration, and occasionally intense reflections manifesting high-frequency interfacial internal waves close to the buoyancy frequency. These waves reached excursions of up to about 90 m crest-trough, around 250 m. They were only visible in acoustic data during daytime when the zooplankton gathered near these thin interfacial layers. Around 500 m where the vertical density stratification was 10 times smaller than at 250 m, larger amplitude internal waves were observed, measuring up to about 150 m crest-trough. These waves had the lowest internal wave frequency, close to the inertial frequency. Below 600 m, the stratification was so weak that large (~100 m) vertical convection in the direction of the earth rotational vector dominated over internal gravity waves. At these depths, diurnal vertical migrations were not observed, which suggests that zooplankton either avoid or become dispersed by waters with large vertical convection. Light limitation is not expected to be a key factor in this case, as vertical migration has been observed deeper than 1000 m in other areas where stratification is greater.

Description: We examined large volumes of seawater under the microscope to explore the limits of phytoplankton diversity in a highly dynamic coastal ecosystem. Our analysis showed that conventional sample volumes severely underestimate the species richness of these phytoplankton communities. The number of species observed doubled after a 10-fold increase in sample volume, implying that estimates of phytoplankton species richness depend critically on sampling effort. The volume of sample needed to detect 90% of the species varied between 0.25 and 1 L depending on the concentration of phytoplankton biomass.

Description: While top–down cascades from fish to phytoplankton have been a core topic in limnology for the past four decades, it has attracted far less interest in marine ecology. This is partly for historical reasons, since lake studies have been motivated by the ability to regulate algal blooms induced by cultural eutrophication, while marine studies have been more oriented towards fish yield and thus bottom–up processes. Also freshwaters are closed ecosystems with lower diversity and complexity, making models and predictions on trophic levels and interactions comparatively simpler. Here, we compare some key properties of freshwater and marine top–down cascades and argue that despite some striking differences, the large number of freshwater studies may pose valuable insights also for marine systems. Moreover, we argue that there is an urgent need for more focus on top–down cascades in marine systems that address how top predators or fishing may propagate through the food web and impact autotrophic biomass, production, C-sequestration and thus ultimately the global carbon cycle and climate.

Description: The ecology of the chaetognath community and its relation to varying environmental factors were studied in the eastern half of the northern Indian Ocean. Analysis of data from two major oceanographic programmes performed over four decades apart suggests a pronounced shift in the community composition and a poleward migration of chaetognaths. This migration has led to an aggregation in the north, as the Eurasian land mass prevents further movement beyond 30°N.

Description: The hypothesis that the joint effects of predation and cyanobacteria favor the performance of small-bodied cladocerans relative to large-bodied species was tested using three competition experiments in a two-factor full-factorial design. The large-bodied Daphnia pulex outcompeted the medium-bodied Simocephalus vetulus when fed a green alga Chlorella pyrenoidosa . However, the presence of the toxic cyanobacterium Microcystis aeruginosa and the coelenterate predator Hydra oligactis reversed the competitive outcome. Their joint effect was significant and contributed to the strongest dominance of S. vetulus . Similar dominance shifts were also observed in the competition between D. pulex and the small-bodied Bosmina longirostris , and in the competition between S. vetulus and B. longirostris . Life table and predation experiments showed that the order of sensitivity of three cladoceran species to Microcystis and to Hydra was the same as the order of competitive superiority in the absence of cyanobacteria. Therefore, cyanobacteria, predation and their joint effects are important drivers for competitive outcomes among cladocerans, potentially creating new opportunities for small-bodied species to dominate in a community. The present study underscores the importance of considering the combined effects of dietary palatability and predation on competitive dominance among herbivores, when predicting zooplankton responses to frequent cyanobacterial blooms.

Description: We used bulk stable isotope analysis (SIA) to examine diel feeding activity in two vertically migrating krill species, Thysanoessa gregaria and Euphausia similis, off eastern Australia. SIA indicated feeding by both species above the thermocline at night, potentially on both particulate organic matter and microzooplankton. Our results support the use of SIA to investigate vertical migration and feeding in zooplankton.

Description: Although theoretical mechanisms permitting phytoplankton co-existence have been extensively examined, few empirical field tests exist. Competition theory predicts greater diversity when species occupy heterogeneous habitats creating spatial niches. In a whole-lake experiment, we deepened thermoclines in two of three lake basins to examine alterations to the degree to which major algal groups showed spatial overlap ( SO ), in the vertical water column, and the consequences for diversity. Deeper thermoclines were expected to lead to less SO if species selected different depth positions in the water column in the absence of a thermal barrier, but to lead to greater SO if increased entrainment of phytoplankton in the mixed layer occurred. Increased diversity was expected to accompany less SO . Phytoplankton SO was determined using spectral group depth profiles from a FluoroProbe, and community diversity was estimated using high-performance liquid chromatography-estimated pigment diversity on daily samples taken over a 3-week focal period during the stratified summer period. SO declined and deep chlorophyll maxima were thicker when thermoclines were deepened. Unexpectedly, increases in SO preceded increased diversity by 5–7 days in all basins. This response likely arose from positive growth by most species within the unmanipulated deep chlorophyll maximum. The surprising result of our study, although supported by some theory, stresses the importance of large-scale field tests to validate hypotheses generated by resource-ratio theory.

Description: The term "population synchrony" refers to the phenomenon of synchronous fluctuations of populations. The strength of this phenomenon may indicate the nature of the synchronizing mechanisms. In this study, we evaluated the relative importance of the Moran effect and dispersal on the levels of population synchrony in a zooplankton assemblage. We monitored the density of 36 taxa (27 genera, Bdelloidea, cyclopoid nauplii, calanoid nauplii, cyclopoid copepodites, calanoid copepodites and four broad taxonomic groups) at 7 sites through 19 months (from May 2004 through November 2009). For each taxon, we estimated the regional levels of synchrony as the mean Spearman correlation coefficient for all pairwise combinations of sites. These values were significant for 33 of the 36 taxa analyzed. Variations in synchrony were uncorrelated with hydrological or geographical distances. Environmental synchrony was unrelated to distance. The low relationship between synchrony (both environmental and population) and hydrological distance indicates the importance of the Moran effect. Nevertheless, climatic effects were not the main synchronizing mechanism. We suggest that damming was the main synchronizing force in this system.

Description: Lake Taihu, a large eutrophic freshwater lake in China, has suffered severe Microcystis blooms since the mid-1980s. In situ nutrient enrichment experiments were conducted to investigate the spatial and seasonal patterns of size-fractionated phytoplankton growth in Lake Taihu. Growth rates were significantly higher in western Taihu than in Meiliang Bay, except in May and September. In western Taihu, phytoplankton growth was only slightly limited by phosphorus. In Meiliang Bay, phytoplankton growth was phosphorus limited during May–July, whereas it was nitrogen limited during August–September. Switching between phosphorus and nitrogen limitation closely followed the seasonal fluctuations in levels of these nutrients. Phytoplankton in the large-size fraction tended to dominate the phytoplankton communities during the Microcystis blooms, although the growth rate of the small-size fraction was significantly higher than that of the large-size fraction. Our results demonstrate that nitrogen and phosphorus levels control the phytoplankton growth patterns in Lake Taihu, and that colonial Microcystis may be critical to maintain blooms under nutrient-limited conditions.

Description: Zooplankton vertical distribution data and velocity estimates from a hydrodynamic model were employed to determine zooplankton daily exchange during four cruises conducted from March to June 2010 in Rivers Inlet, a fjord in central British Columbia, Canada. Zooplankton transport rates varied temporally, being fastest in March when water velocities were highest. The active vertical movement of the zooplankton interacted with the vertically sheared flow field to influence zooplankton advection. Surface dwelling plankton such as Acartia longiremis and larvaceans experienced the highest advection losses of –0.4 and –0.5 day –1 , respectively. In contrast, maximum advection losses of Paraeuchaeta elongata , a deeper dwelling copepod, were an order of magnitude lower, at 0.04 day –1 . Transport rates varied by stage, extent of diel vertical migration and timing of ontogenetic migration. Advection rates were lower than literature-derived egg production rates, but comparable to literature-derived mortality rates. We suggest that advection may be a significant driver of population dynamics in years with similar rates of population growth. We also stress the importance of determining advection rates to obtain accurate estimates of vital rates.

Description: Oxygen optodes were used to compare respiration rates of newly caught and laboratory-incubated Mnemiopsis leidyi ctenophores. No significant difference was found between the treatments. Overall, the respiration rate was significantly correlated with temperature (8.5–30°C) and organism weight (0.41–19.4 g wet weight, 6–42 mm oral–aboral length), together explaining 87% of the variation. Greater individual variation was observed 〉25°C. Q 10 measured at near-ambient temperatures (2.49) and over the entire range (2.57) was lower than or comparable with previous estimates for this species. Mnemiopsis leidyi from a French lagoon have the metabolic capacity to survive year round in the ambient temperature range of the western Mediterranean.

Description: Cyanobacteria may reduce Daphnia fitness through their toxicity, low nutritional value or poor manageability. In this study, the relative importance of the low nutritional value and poor manageability of cyanobacteria on Daphnia life history were determined. To separate these two aspects of cyanobacteria impact, short or long filaments of Cylindrospermopsis raciborskii were given as food to five clones of Daphnia magna either alone or together with the green alga Scenedesmus obliquus . Feeding with short cyanobacterial filaments caused reductions in growth rate (〈45%) and fecundity (〈60%) in some clones. The deleterious effects caused by cyanobacterial poor nutritional quality could be completely overcome by supplementation with high-quality food. Long cyanobacterial filaments caused greater reductions in growth rate (〈65%) and fecundity (〈80%) in the most affected clones, and the addition of high-quality food only partially diminished the effects of poor filament manageability. In Daphnia fed with long filaments, there was no correlation between body size at maturity and fecundity, and the maximum fecundity was reached by individuals of intermediate size. Decreased body size may, thus, represent an adaptive mechanism against interference with food collection caused by long filaments. The pattern of responses to different food regimes was similar in all Daphnia clones, but they differed in the absolute values of the life history parameters affected. Differences in the fecundity of Daphnia caused by the presence of cyanobacteria may affect clonal composition of cladoceran assemblages, thus, cyanobacteria can shape microevolutionary changes in Daphnia populations.

Description: We analyzed the species composition of pelagic chaetognaths in the Indo-Pacific region as reported in published papers to demonstrate geographic variation in their species diversity and its relationships with environmental variables. The resulting latitudinal pattern in species richness (SR) was asymmetric to the equator, with a peak at 35°N in the Kuroshio area and a gradual decrease to a minimum at ~40°S. Shannon's diversity index ( H ') showed a similar pattern, except that it plateaued between 20°N and 35°S. Regression analyses and structural equation models (SEMs) showed that temperature was the most important explanatory variable, both for SR and H '. The SEM also demonstrated that chlorophyll a concentration and mixed-layer depth were the second most effective variables for SR and H' , respectively. On the basis of these results, models were constructed to estimate geographical variation in SR and H' . The estimated SR was high in coastal waters in tropical to subtropical areas, while H' was high at midlatitudes in the open ocean. The present data highlight the importance of the coastal waters of Southeast Asia and the Indian Ocean, but also indicate that a gap still exists in this area, which necessitates further data mining as well as basic research.

Description: Using Landsat and Moderate-resolution Imaging Spectroradiometer (MODIS) satellite data, cyanobacteria bloom initiation dates over two decades (1987–2011) in Lake Taihu showed three distinct trends. Initial blooms occurred later each year between 1987 and 1997 and then generally earlier until 2007, when the earliest and most extensive blooms occurred. After 2007, bloom initiation dates occurred later each year. Climate and catchment control over bloom dynamics was observed, in particular winter temperature minima and nutrient ratios.

Description: Increases in ultraviolet radiation (UVR) and CO 2 affect phytoplankton growth and mortality in a variety of different ways. However, in situ responses of natural phytoplankton communities to climate change, as well as its effects on phytoplankton annual cycles, are still largely unknown. Although temperature and UVR have been increasing in temperate latitudes during winter, this season is still particularly neglected in climate change studies, being considered a non-active season regarding phytoplankton growth and production. Additionally, coastal lagoons are highly productive ecosystems and very vulnerable to climate change. This study aims, therefore, to evaluate the short-term effects of increased UVR and CO 2 on the composition and growth of winter phytoplankton assemblages in a temperate coastal lagoon. During winter 2012, microcosm experiments were used to evaluate the isolated and combined effects of UVR and CO 2 , under ambient and high CO 2 treatments, exposed to ambient UV levels and photosynthetically active radiation (PAR), or to PAR only. Phytoplankton composition, abundance, biomass and photosynthetic parameters were evaluated during the experiments. Significant changes were observed in the growth of specific phytoplankton groups, leading to changes in community composition. The cyanobacterium Synechococcus was dominant at the beginning of the experiment, but it was negatively affected by UVR and CO 2 . Diatoms clearly benefited from high CO 2 and UVR, particularly Thalassiosira . Despite the changes observed in specific phytoplankton groups, growth and production of the whole phytoplankton community did not show significant responses to UVR and/or CO 2 .

Description: An important requirement for continuous cultures is a homogeneous distribution of resources and microorganisms, often achieved by rigorous mixing. Many dinoflagellate species are known to be vulnerable to turbulence. Here, we present a newly developed continuous culture system based on gentle mixing in which the two dinoflagellate species Scrippsiella trochoidea and Alexandrium tamarense , with different turbulence sensitivities, grew well under steady state conditions. We also show that the continuous culture system can be applied at low nutrient conditions and low population densities.

Description: Population density changes the intensity of mate choice in numerous animal taxa. We used video recordings to test whether such changes can also be observed in the copepod Acartia tonsa during short-term exposures to densities (16–640 ind L –1 ) for which their egg production is known to be affected. The ratio of observed mating attempts to theoretical encounters was constant among treatments. This shows that these densities do not affect mate choice in A. tonsa .

Description: Daphnia responds to low availability of carbon (food quantity) or limiting concentrations of nutrients relative to carbon (C) in excess (food quality) by respectively saving or discharging C via different pathways. We investigated which kind of food limitation leads to a faster regulation in Daphnia C budgets, and whether the pre-assimilative C pathways, ingestion and faeces egestion and the post-assimilative C pathways, excretion and respiration, are regulated concurrently. Daphnia magna were exposed to dietary shifts in different food quantities or qualities; food quality was varied in terms of the essential component, cholesterol. After acclimation to the new diet ranging from 0 to 96 h, C budgets were measured by a radiotracer technique. Dietary shifts in quantity and quality caused Daphnia to quickly adjust their C budgets within 6 h, but different C pathways were affected. A shift to low food quantity reduced Daphnia respiration indicating C retention. In contrast, sudden low quality food caused increased faeces egestion to discharge excess C. Furthermore, we observed a delayed increase in excretion but no change in respiration within the time frame studied. Such time-shifted responses appear to be an appropriate means to keep the costs of physiological adjustments relatively low, which in turn would benefit Daphnia performance.

Description: Factors regulating the biomass and flux of organic carbon through heterotrophic bacterioplankton were examined monthly in the southern Bay of Biscay during 2006 and 2007. Temperature and resource supply were able to explain significantly changes in bacterial biomass (539–1389 mg C m –2 ) and production (5.4–93 mg C m –2 day –1 ), although at different periods of the year. A switch between the relative strength of both factors in the spring–summer transition seems to be a general feature in temperate coastal waters.

Description: The identification of salp species by morphological features alone can be challenging. This study combined morphological and molecular approaches to characterize different salp reproductive forms (oozooids and blastozooids) collected from the remote Tristan da Cunha archipelago. Novel cox1 barcoding primers developed in this study proved successful for the identification of Thalia salp species and generated much needed barcoding data for this taxonomic group.

Description: We define the biogeographic status quo of zooplankton communities sampled during the summer of 2008 in the sub-Arctic Pacific and western Arctic regions and examine spatial patterns of stable isotopes in the context of regional oceanography. Eight zooplankton assemblages were identified and corresponded to these regions: Gulf of Alaska, Bering Sea Slope, St-Lawrence Island, Western Bering Strait, Eastern Bering Strait, Barrow Canyon, Beaufort Gyre and Beaufort Shelf. Neocalanus spp., Eucalanus bungii and Metridia pacifica were abundant in the warmer and saltier waters of the Gulf of Alaska and on the Bering Sea Slope, whereas Calanus hyperboreus, Calanus glacialis and Metridia longa were abundant in the cold and fresher Arctic waters on the Beaufort Shelf and in the Beaufort Gyre. Salinity, nitrate and temperature, all of which were strongly correlated with latitude, and water column depth were the main factors influencing variation of zooplankton composition and spatial distribution. The 15 N values were less enriched in the Gulf of Alaska and on the Bering Sea Slope and more enriched in the eastern portion of Bering Strait and in Barrow Canyon. The 13 C values of zooplankton were more depleted in the Gulf of Alaska and in the Beaufort Sea regions and were more enriched in Western Bering Strait. The level of carnivory in the zooplankton community appeared highest in the Beaufort Gyre decreasing on the Beaufort Shelf and lowest in Western Bering Strait and in the Gulf of Alaska. Our results highlight the existence of large intra- and inter-specific differences in zooplankton isotopic signatures in relation to community composition and environmental conditions.

Description: Ecological indicators are used extensively as tools to manage environmental resources. In the oceans, indicators of plankton can be measured using a variety of observing systems including: mooring stations, ships, autonomous floats and ocean colour remote sensing. Given the broad range of temporal and spatial sampling resolutions of these different observing systems, as well as discrepancies in measurements obtained from different sensors, the estimation and interpretation of plankton indicators can present significant challenges. To provide support to the assessment of the state of the marine ecosystem, we propose a suite of plankton indicators and subsequently classify them in an ecological framework that characterizes key attributes of the ecosystem. We present two case studies dealing with plankton indicators of biomass, size structure and phenology, estimated using the most spatially extensive and longest in situ and remote-sensing observations. Discussion of these studies illustrates how some of the challenges in estimating and interpreting plankton indicators may be addressed by using for example relative measurement thresholds, interpolation procedures and delineation of biogeochemical provinces. We demonstrate that one of the benefits attained, when analyzing a suite of plankton indicators classified in an ecological framework, is the elucidation of non-trivial changes in composition, structure and functioning of the marine ecosystem.

Description: To investigate the combined effect of temperature and light availability on organic matter production and degradation during a winter/spring phytoplankton bloom in Kiel Bight, we conducted a mesocosm study applying two temperature regimes, ambient (T + 0) and plus 6°C (T + 6) and three irradiance levels. Rising temperature accelerated the onset of the phytoplankton bloom, while light intensity played only a minor role for the timing and bloom development. Maximum build-up of chlorophyll a and particulate organic carbon were ~20% lower at T + 6 compared with T + 0, probably caused by a combination of elevated heterotrophic processes and enhanced sedimentation during the bloom. The latter is supported by increased TEP concentrations at T + 6 (TEP/POC 0.18 mol C/mol C) compared with T + 0 (0.11 mol C/mol C) during bloom conditions, which may have promoted cell aggregation and sinking. Dissolved organic carbon concentrations increased more rapidly at elevated temperature. For a warmer future ocean, we can hence expect two counteracting mechanisms controlling organic matter flow during phytoplankton blooms: (1) enhanced processing of organic matter via the microbial loop resulting in a faster recycling and (2) depending on the dominating phytoplankton species, enhanced TEP formation resulting in increased particle aggregation and thus export of carbon and nutrients.

Description: Zooplankton carcasses are ubiquitous in marine and freshwater systems, implicating the importance of non-predatory mortality, but both are often overlooked in ecological studies compared with predatory mortality. The development of several microscopic methods allows the distinction between live and dead zooplankton in field samples, and the reported percentages of dead zooplankton average 11.6 (minimum) to 59.8 (maximum) in marine environments, and 7.4 (minimum) to 47.6 (maximum) in fresh and inland waters. Common causes of non-predatory mortality among zooplankton include senescence, temperature change, physical and chemical stresses, parasitism and food-related factors. Carcasses resulting from non-predatory mortality may undergo decomposition leading to an increase in microbial production and a shift in microbial composition in the water column. Alternatively, sinking carcasses may contribute significantly to vertical carbon flux especially outside the phytoplankton growth seasons, and become a food source for the benthos. Global climate change is already altering freshwater ecosystems on multiple levels, and likely will have significant positive or negative effects on zooplankton non-predatory mortality. Better spatial and temporal studies of zooplankton carcasses and non-predatory mortality rates will improve our understanding of this important but under-appreciated topic.

Description: The pelagic phytoflagellate Phacotus lenticularis creates a shell of highly symmetrical calcite crystals at its outer extracellular layer and thereby constitutes a significant source of calcite in temperate alkaline lakes worldwide. Responses of P. lenticularis to a changing lake environment therefore have potential implications for the calcium carbonate flux in lakes. How meteorological variability in interaction with the CaCO 3 saturation state may influence the abundance and phenology of P. lenticularis has not been investigated so far. We measured, biweekly to daily, algal abundance, CaCO 3 saturation, pH, water temperature and nutrient status in three hard-water lakes that varied in lake morphology and wind exposure. Our data provide evidence that increasing supersaturation of CaCO 3 mediates the onset of P. lenticularis exponential growth and stimulates seasonal peak development. However, total cell concentration in the lake water appears to be independent of the CaCO 3 saturation state and was the highest in the most wind exposed lake. Phacotus lenticularis abundance and epilimnetic CaCO 3 supersaturation were significantly positively correlated at a time lag of about 1 day. Wind caused an immediate decline in epilimnetic pH followed by a decline in CaCO 3 saturation about 2 days later. Solar irradiance positively affected both pH and water temperature, but water temperature changes were insufficient to produce a significant CaCO 3 saturation response. We conclude that the CaCO 3 saturation state is a determinant factor driving the timing of P. lenticularis peaks in alkaline lakes.

Description: The value of mechanistic ecosystem modelling has long been appreciated, and in connection with trait-based approaches it has recently stimulated a more process-based understanding of adaptive capacities and trade-offs. Notwithstanding recent advances, even sophisticated state-of-the-art models of plankton ecosystems, some of which include hundreds of idealized species, do not accurately represent the great biodiversity of plankton, or the associated flexible adaptive response of plankton communities. We build on previous reviews to suggest that it may be necessary to discard some common assumptions and try new approaches in order to construct models that can make new and testable predictions about the "adaptive capacity" of plankton ecosystems. Major challenges remain unresolved for modelling interacting communities of producers and consumers. Rather than the common approach of mixing and matching existing model components, each laden with its own legacy assumptions, we suggest that a judicious combination of innovative, mechanistic approaches that combine traits and trade-offs will likely better address such challenges.

Description: Dynamics and sources of fluorescent dissolved organic matter (FDOM) are important for understanding biogeochemical processes in aquatic ecosystems. This study aimed to analyse direct production of FDOM by marine phytoplankton cultures and reveal fluorescent characteristics of exuded FDOM. Axenic cultures of eight species of bloom-forming marine phytoplankton, including two diatoms; a raphidophyte; two dinoflagellates; a chlorophyte; a cryptophyte and a haptophyte, were incubated in an artificial medium. Excitation emission matrices (EEMs) of FDOM in the culture medium were spectrofluorometrically measured. FDOM production was observed in all species, and fluorescent characteristics of the exudates varied considerably among species. Measured EEMs had peaks at 350/450 nm (excitation/emission) for the diatom Ditylum brightwellii and 370/450–470 nm for the raphidophyte Heterosigma akashiwo and the chlorophyte Oltmansiellopsis viridis , which have previously been regarded as the peaks of terrestrially derived humic-like substances. Direct production of FDOM by marine phytoplankton should be considered in future studies of FDOM dynamics in marine systems. Species-specific features of FDOM might be used for early detection of harmful blooms because this method is simple, rapid and suitable for monitoring.

Description: Phytoplankton community analysis is important with respect to natural or human-induced changes in the marine environment. Because of the efforts involved and the limitations of traditional methods, molecular sensing approaches are becoming more popular. Among others, microarray techniques targeting ribosomal 18S sequences have been successfully applied for phytoplankton investigation. In this contribution, we compared the results of two microarray methods targeting 18S rDNA and 18S rRNA with the results obtained from microscopy, HPLC and flow cytometry. On a qualitative basis, the microarrays showed similar or potentially better performance than the non-molecular methods. Quantitatively, our data suggest that microarray signals obtained from 18S rDNA provide relatively rough estimates of phytoplankton abundance. In contrast, when targeting 18S rRNA instead, a robust linear relationship ( r ² = 0.68) between molecular sensing signal and microscopic cell counts could be demonstrated using a probe specific to the genus Pseudo-nitzschia as an example. Thus, for both qualitative and quantitative purposes, microarray techniques can be valuable additions to traditional methods for phytoplankton analysis. Routine monitoring approaches in particular could benefit from advantages like reduced effort, higher taxonomic resolution and a potential for automation.

Description: We determined reproductive and growth rates of three common copepods in the low-salinity zone of the San Francisco Estuary during spring–summer of 2006 and 2007. Rates were low, particularly during summer. The egg production rate of Eurytemora affinis in spring averaged ~3 eggs female –1 day –1 or 0.04 day –1 , while that of Pseudodiaptomus forbesi in summer averaged ~1 egg female –1 day –1 or 0.02 day –1 . Specific growth rates of copepodites were moderate for E. affinis in spring (0.23 and 0.15 day –1 for early and late stages, respectively) and low for P. forbesi in summer (0.15 and 0.03 day –1 , respectively). Growth and egg production rates of both species were generally lower than rates predicted from temperature for well-fed copepods, suggesting chronic food limitation. Previously published estimates for the small cyclopoid Limnoithona tetraspina were also low. None of the measures of growth of any species was related to phytoplankton biomass, primary production or abundance of the species, nor did they differ between the 2 years despite large differences in hydrology. To understand patterns of abundance will require investigation of differential mortality rates.

Description: We studied female fertilization status in North Sea summer populations and laboratory cultures of the marine copepod Temora longicornis and found fractions of fertilized females in both field and laboratory populations that were much smaller (〈50%) than predicted by a theoretical model that assumes random mating. Such low fertilization rates are normally related to environmental factors such as poor food or low densities, which we could not confirm in our experiment. Male density was negatively related to fertilization rate, and a large fraction of males did not mate in laboratory incubations. We therefore suggest that sexual selection, through mate choice or male–male competition could account for low fertilization rates of females in populations of pelagic copepods during some periods of the year.

Description: Few studies have been made on planktonic food webs of temperate ecosystems, especially those from the Southern Atlantic Ocean, using molecular biomarkers. The fatty acid compositions of suspended particulate matter (SPM), microplankton and mesozooplankton were studied during summer and winter at a sewage-impacted and a control site in the Bahía Blanca Estuary (Argentina). The aim was to identify trophic relationships on a spatial and seasonal scale and to detect allochthonous inputs to the food web. Fatty acid trends were consistent with the seasonal succession of the plankton community structure supporting our underlying hypothesis that regional seasonality is mostly responsible for changes in fatty acid composition. Sewage had no clear impact on the fatty acids and may not be a significant source of SPM in the estuary. However, at the sewage site the composition of the SPM was more related to terrestrially derived compounds, diatoms and bacteria, and mesozooplankton fatty acids suggested grazing on terrestrial components and on diatoms over flagellates. Saltmarshes likely have a crucial role as the main contributors to the organic fraction of SPM followed by plankton. The seasonal fatty acid pattern of the mesozooplankton indicated different feeding strategies suggesting an active feeding mode during summer and a more terrestrially associated diet in winter. The fatty acid trophic marker approach provided relevant information to clarify planktonic trophic interactions and to trace the origin of organic matter in this highly dynamic temperate coastal system.

Description: The effects of the colonial peritrich ciliate epibiont, Zoothamnium sp., on fitness components of the copepod Acartia tonsa were examined during a three-week infestation event in Long Island Sound. Infestation rate and intensity (ciliate load copepod –1 ), copepod body size and mating success were measured on six separate occasions in the field. Infestation rates exceeded 60% in both male and female copepods. Infested females had a significantly greater ciliate load than males on three of the six sampling dates. Infested males were significantly smaller than uninfested males midway through the event, whereas infested females were significantly smaller during the peak in infestation rate and intensity. There was no difference in the frequency of unmated females between infested and uninfested individuals on any sampling date. Laboratory experiments were carried out under three food regimes (starvation, food-limited and food-unlimited). Survivorship was significantly lower for infested versus uninfested males under starvation, but not under the food-unlimited condition. Infested females, however, showed significantly lower survivorship than uninfested ones under all three food regimes. In addition, infested females produced significantly fewer eggs than uninfested females under starvation and food-unlimited conditions. Overall, both male and female A. tonsa bearing the ciliate epibiont Zoothamnium experience a significant decrease in components of fitness.

Description: A mesocosm experiment was conducted to investigate the impact of rising f CO 2 on the build-up and decline of organic matter during coastal phytoplankton blooms. Five mesocosms (~38 m³ each) were deployed in the Baltic Sea during spring (2009) and enriched with CO 2 to yield a gradient of 355–862 µatm. Mesocosms were nutrient fertilized initially to induce phytoplankton bloom development. Changes in particulate and dissolved organic matter concentrations, including dissolved high-molecular weight (〉1 kDa) combined carbohydrates, dissolved free and combined amino acids as well as transparent exopolymer particles (TEP), were monitored over 21 days together with bacterial abundance, and hydrolytic extracellular enzyme activities. Overall, organic matter followed well-known bloom dynamics in all CO 2 treatments alike. At high f CO 2, higher POC:PON during bloom rise, and higher TEP concentrations during bloom peak, suggested preferential accumulation of carbon-rich components. TEP concentration at bloom peak was significantly related to subsequent sedimentation of particulate organic matter. Bacterial abundance increased during the bloom and was highest at high f CO 2 . We conclude that increasing f CO 2 supports production and exudation of carbon-rich components, enhancing particle aggregation and settling, but also providing substrate and attachment sites for bacteria. More labile organic carbon and higher bacterial abundance can increase rates of oxygen consumption and may intensify the already high risk of oxygen depletion in coastal seas in the future.

Description: In June 2010 four krill species were collected in Godthåbsfjord, SW Greenland, namely Thysanoessa raschii, T. inermis, T. longicaudata and Meganyctiphanes norvegica. A transect from offshore Fyllas Bank to the inner Godthåbsfjord revealed a zonation of the species in relation to hydrography. Offshore, in the Atlantic-influenced water, the Atlantic T. longicaudata dominated. All individuals of T. longicaudata found offshore were carrying spermatophores. Within the fjord T. raschii and T. inermis were present in high numbers but the former dominated. The boreal M. norvegica were only present in the inner part of the fjord. Presence of only 1-year-old individuals and no fertilized females suggests the population is sustained by advection of M. norvegica from offshore waters. Krill community abundance and biomass were lowest offshore and peaked in the innermost part of Godthåbsfjord. Thysanoessa inermis matured after 2 years, while T. raschii matured after 1 and 2 years, respectively, depending on temperature. The present study suggests that a warmer future will favour Atlantic species and result in a more diverse self-sustainable krill community in the Godthåbsfjord.

Description: First larval stage (Zoea I) of the crab Cyrtograpsus altimanus can obtain photoprotective compounds (PPCs) from their mother (indirect acquisition) and also via feeding on PPC-producers such as phytoplankton (direct acquisition). The bioaccumulation of PPC resulted in higher survival of larvae exposed to ultraviolet radiation (UVR), when comparing larvae with high and low content of PPC. Thus, both ways of acquiring PPC may contribute to maximize survival of Zoea I when they are exposed to natural UVR.

Description: Coccolithophores are one of the most abundant eukaryotic phytoplankton in the oceans and are distinguished by their ability to build calcitic platelets (coccoliths). Of the numerous species, Emiliania huxleyi is considered one of the major calcifiers in the pelagic ocean. There is growing concern that increasing levels of CO 2 in the atmosphere and the subsequent acidification of the ocean may disrupt the production of coccoliths. Furthermore, any change in the global distribution and abundance of E. huxleyi relative to non-calcifying groups of phytoplankton (e.g. diatoms) will have important effects on the biogeochemical cycling of carbon and climatic feedbacks. We review different lines of evidence that suggest E. huxleyi is increasingly expanding its range into the polar oceans. These observations contribute to the debate on the climatic effects on natural coccolithophore populations. We postulate that E. huxleyi may be more sensitive to recent environmental changes such as increasing sea surface temperature and salinity than to changing ocean carbonate chemistry, partly because increased availability of CO 2 (aq) likely alleviates a carbon limitation for the inefficient Rubisco enzyme in these algae. Any potentially important climatic feedbacks of coccolithophores need a better knowledge of the mechanisms and rates of adaptation by natural populations. As more data and modelling work become available, the real significance of this poleward expansion will become clear.

Description: Conventional methods for the estimation of marine phytoplankton diversity include the collection of a small volume of seawater which is analysed under the microscope. We sampled natural communities and also synthetic communities generated under a neutral community model configuration and demonstrate that traditional sampling methods underestimate the species richness of marine phytoplankton communities. In our model, a synthetic community represents an ensemble of individuals enclosed in a parcel of seawater wherein the dynamics of each population is controlled by demographic stochasticity and dispersal. By sampling these synthetic communities, we found that roughly 20–45% of the species is missed by conventional, small volume samples. Consistent with the simulations, field data showed that the number of species increases with sampling effort by up to ~1.5-fold, revealing that these microbial communities might be more diverse than previously estimated. We suggest that conventional sampling methods have limited our ability to delineate the patterns of marine phytoplankton diversity and identify the underlying mechanisms. Improved sampling methods are proposed to obtain more accurate estimates of marine phytoplankton diversity.

Description: Algicidal and growth-inhibiting bacteria (GIB) are promising tools for mitigating the negative impacts of harmful algal blooms in coastal environments; however, there is a paucity of quantitative data describing their ecological effects. Here, we investigate their temporal and spatial distribution in the southwest Yatsushiro Sea during the summer of 2011, by co-culturing bacterial isolates with an axenic culture of a fish-killing raphidophyte Chattonella antiqua . This novel co-culturing method excludes other biological influences, such as promotion of algal growth by some bacteria. The highest density (1.6 x 10 4 cells mL –1 ) of algicidal bacteria (AB) was detected at the surface of the nearshore station on 3 August 2011 when the maximum chlorophyll a concentration was observed, associated with the high abundance of two centric diatoms, low abundance of dinoflagellates and the absence of C. antiqua . On 10 August, total bacterial abundance increased at 10 m, suggesting that the bacteria used decaying organic matter derived from dead microalgae sinking to the bottom to promote their growth. Most of AB (87.5%) were found to be particle associated, suggesting these bacteria attached to the dead phytoplankton particles after killing them. Two types of GIB impaired co-cultured C. antiqua by inducing unusual cell deformations, including round and elongated cell morphologies. Partial 16S rRNA sequencing showed that algicidal and GIB were mainly comprised of "- and a -proteobacteria". This study provides a new perspective on the dynamics of AB, suggesting that they play a significant role in regulating microalgal composition, physiology and abundance in coastal marine environments.

Description: The dinoflagellate Neoceratium is commonly observed in oceanic waters, depleted in major inorganic nutrients such as nitrogen and phosphorus. Using culture isolates, we investigated whether two Neoceratium species ( N. hexacanthum and N. candelabrum ) can grow phototrophically at low nutrient concentrations found in surface waters of oligotrophic subtropical gyres (OSGs). No phototrophic growth (indicated by changes in cell numbers, the presence of dividing cells or cellular protein increase) was observed when N. hexacanthum and N. candelabrum were grown in low nutrient seawater. In separate experiments, to determine survival time under oligotrophic nutrient conditions, 68% of N. hexacanthum cells were able to re-establish growth after spending 1–10 days in North Atlantic gyre seawater; 40% recovered after 11–20 days and only 3% recovered after 21–30 days. The longest period any single cell survived, and then went on to divide, was 26 days. These findings demonstrate that Neoceratium cells could remain viable for 〉3 weeks in surface waters of OSGs, but to sustain their growth nutrients must be obtained periodically from an alternative source: via ephemeral upwelling of nutrient-rich waters, phagotrophy and/or movement to and from the nutricline.

Description: Calanus glacialis initiated feeding and reproduction under extensive sea ice in the northern Bering Sea during late winter despite very low mean water column chlorophyll a concentrations. A peptide nucleic acid (PNA)-PCR and clone library analysis of prey 18S rDNA in the guts of C. glacialis , together with the quantification of the more abundant prey species with quantitative PCR (qPCR), demonstrated feeding on sea ice algae. The ice algal diatoms, Fragilariopsis cylindrus , Fragilaria sp. and Pseudo-nitzschia sp. were the dominant prey species while the most abundant species in the water column, Thalassiosira antarctica , was not important. The total of these species-specific ingestion rates was strongly correlated ( r 2 = 0.93) with, and similar to, those estimated from gut pigments. The increase in feeding and reproduction during the study was significantly related to air temperature lagged by 2 or 3 days. We suggest that warmer temperatures led to a release of ice algae into the water column stimulating feeding and reproduction. The availability of ice algae over an extended period during colder winters when sea ice cover is more extensive and lasts longer will lead to larger population sizes of C. glacialis compared with warmer years with less extensive ice cover.

Description: To examine seasonal changes in the mesozooplankton community, analyses were made on the swimmer samples (〉1 mm) collected by a sediment trap mooring at 184 m depth on the Northwind Abyssal Plain in the western Arctic Ocean during October 2010–September 2011. The zooplankton swimmer flux ranged from 5 to 44 ind. m –2 day –1 and was greater during July to October; copepods were the dominant taxon. Based on the zooplankton swimmer flux, cluster analysis classified samples into three groups (A, B-1 and B-2). The occurrence of each group showed clear seasonality; group A was observed during July to October, group B-1 was seen in November to January and group B-2 during March to June. The seasonal variability in population structures of four dominant copepod swimmers was clearly different between the species. Most Calanus hyperboreus were copepodid stage 6 female (C6F) throughout the year. For Metridia longa and Paraeuchaeta glacialis , C6Fs dominated during January to May, and early copepodid stages increased during June to October. Heterorhabdus norvegicus was dominated by stage C5 during November to February, and C6F/M during March to May. Since Pacific copepods ( Neocalanus cristatus ) occurred in significant number during August–September, possible causes are discussed.

Description: Distribution, growth and mortality of jack mackerel Trachurus japonicus larvae 〈10 mm standard length (SL) were investigated in relation to habitat conditions in the southern East China Sea (ECS) in 2005, 2007 and 2009. In all the years, the center of larval distribution shifted northeastward with growth, indicating northeastward entrainment by the Kuroshio branch current. In 2005, the mean habitat temperature of the larvae was ~2°C lower than the other two years. Mean density of the major prey item Paracalanus spp. in the larval habitat varied interannually, with the highest and lowest values in 2007 and 2005, respectively. The mean absolute growth rate and weight-specific instantaneous growth coefficient ( G w ) were highest in 2007 (0.30 mm d –1 and 0.217 d –1 , respectively) and lowest in 2005 (0.19 mm d –1 and 0.120 d –1 , respectively), corresponding with the habitat temperature and food availability. There was no significant interannual difference in the daily instantaneous mortality coefficient ( Z ) which ranged from 0.107 d –1 in 2009 to 0.136 d –1 in 2007. The ratio G w /Z , an index of stage-specific larval survival, was highest (1.60) in 2007 and lowest (1.02) in 2005. The lowest G w /Z in 2005 corresponded with the lowest recruitment of the juveniles (mainly 50–75 mm SL) in the ECS in 2005. However, G w /Z in 2007 and 2009 did not clearly correspond with recruitment levels, suggesting that a high G w /Z is a necessary, but may not be a sufficient, criterion of a strong year-class.

Description: The relative abundances of small centric diatoms have increased in many Arctic lakes over the past century, with these changes commonly attributed to warming. However, the specific mechanisms by which diatom community structure is changing in response to warming remain unclear. We investigated the responses of two common centric diatoms to nutrient enrichment and incubation depth, the latter used to manipulate light availability which is a key factor that changes with altered mixing depths in lakes. We conducted 2 x 2 factorial experiments, manipulating nutrients (none added or N + P addition) and incubation depth (shallow or deep), and measured changes in growth rates and cell densities of Discostella stelligera and Puncticulata radiosa . A second set of experiments was conducted on D. stelligera in a growth chamber to separate temperature and light effects associated with incubation depth. Puncticulata radiosa was always more abundant in the shallow depth incubations, regardless of nutrient conditions. In contrast, D. stelligera responded most strongly to nutrient additions, and cell densities of this species were affected by interactions between nutrients and incubation depth or light. Our research suggests that processes that alter light availability (such as water clarity and water column stability) and nutrient concentrations are likely to play a major role in controlling the growth of small centric diatoms in Arctic lakes.

Description: Bythotrephes longimanus is an invasive predatory cladoceran that is negatively impacting North American zooplankton in the Laurentian Great Lakes and on the Canadian Shield. Concurrently, algal community composition, which affects zooplankton food quality, is changing in many lakes of the Canadian Shield. The n -3 fatty acid eicosapentaenoic acid (EPA) is highly retained in Bythotrephes , but the effects of EPA limitation on Bythotrephes ' population dynamics are unknown. To test the hypothesis that EPA limitation results in decreased weight and fecundity of Bythotrephes , the green alga Scenedesmus obliquus was cultured in the laboratory, split into EPA-enriched lines and un-enriched controls, then fed to Daphnia ambigua , which were in turn offered to juvenile Bythotrephes. Bythotrephes consuming EPA-enriched daphniids were heavier and had larger clutch sizes than those consuming control daphniids. Both diets supported ontogenesis, but not brood release, of Bythotrephes . To understand why laboratory-reared Bythotrephes did not release broods, we compared their fatty acid profiles with those of field-collected specimens, and found that they were EPA impoverished compared with field-collected conspecifics. Our results suggest that EPA availability influences Bythotrephes population dynamics, establishment success and impacts in nature.

Description: The magnitude and characteristics of the response of Arctic marine ecosystems to the challenges resulting from climate change are not known. Among the drivers of change, temperature plays a fundamental role, influencing biological processes from individual organisms to whole ecosystems, and sets the thresholds for species performance, abundance and distribution, and is responsible for massive shifts in ecosystem structure and function. The metabolic theory of ecology is generally invoked to ascertain the effects of global temperature changes on shifts in ecosystems, from individual size and species composition to global trophic status. However, as generally occurs with most scaling laws, there is a lively debate about its usefulness to predict something more than gross tendencies. In general, to explain variability is much more interesting than to predict average values. The successful performance of species and the trophic status of ecosystems are controlled by the balance between energy gains and losses. The temperature-induced mismatch between the positive and negative terms of the metabolic balance appears to depend on precise characteristics of their respective thermal windows, hardly identifiable by the averaging predictions made by the metabolic theory. As a case study, we discuss the response to temperature changes of the balance between ingestion and respiration rates of the copepod Calanus glacialis , a fundamental component of Arctic pelagic food webs. We suggest using the response of the metabolic balance (at the organismal, community or ecosystem level) to temperature changes to identify thermal thresholds leading to tipping points and nonlinear ecosystem shifts.

Description: Detecting fluctuations in the species composition of bloom-forming jellyfish requires the ability to correctly identify each species in each developmental stage. We verified diagnostic morphological and molecular genetic characters to discriminate Cyanea lamarckii and Cyanea capillata from northern European waters. Intrusions in the subumbrellar muscle folds were present in all C. capillata 〉80 mm r-diameter (between opposite rhopalia tips), but absent in C. lamarckii . Clearly visible wart-like papillae on the central exumbrella were present in all C. lamarckii 〉10–80 mm r-diameter, but absent in C. capillata . Both morphological features were retained in formaldehyde-seawater (4%) preserved medusae which had shrunk by 12.8% (±2.7%) after 1 year of preservation. Our molecular genetic analyses demonstrated that fragments of mitochondrial cytochrome c oxidase subunit I (COI) and nuclear 18S rDNA clearly distinguished C. lamarckii from C. capillata , with intra- and inter-specific pairwise genetic distances of 0.0–1.5% and 15.5–17.0% (COI) and 0.0 and 0.2% (18S rDNA), respectively. The study revealed various bell colours in both species underlining that the identification based on the bell colours can result in misidentification. Our integrated taxonomic approach can help to correctly identify jellyfish species, which is fundamentally important for understanding the causes of jellyfish fluctuations and the development of jellyfish blooms.