ALBERT

Description: Although copepods have been considered tolerant against the direct influence of the ocean acidification (OA) projected for the end of the century, some recent studies have challenged this view. Here, we have examined the direct impact of short-term exposure to a pCO 2 / pH level relevant for the year 2100 ( pH NBS , control: 8.18, low pH : 7.78), on the physiological performance of two representative marine copepods: the calanoid Acartia grani and the cyclopoid Oithona davisae . Adults of both species, from laboratory cultures, were preconditioned for four consecutive days in algal suspensions ( Akashiwo sanguinea ) prepared with filtered sea water pre-adjusted to the targeted pH values via CO 2 bubbling. We measured the feeding and respiratory activity and reproductive output of those pre-conditioned females. The largely unaffected fatty acid composition of the prey offered between OA treatments and controls supports the absence in the study of indirect OA effects (i.e. changes of food nutritional quality). Our results show no direct effect of acidification on the vital rates examined in either copepod species. Our findings are compared with results from previous short- and long-term manipulative experiments on other copepod species.

Description: We analyzed the genetic structure of the radiolarian morphospecies Larcopyle buetschlii from the surface to deep waters (up to 2000 m) in the Japan Sea using the internal transcribed spacer (ITS) regions of ribosomal RNA genes. Each individual had several ITS variants, but these polymorphisms show no vertical phylogeographic structure, suggesting a single biological species. Its rapid clonal reproduction suggested by high ITS variation likely plays a pivotal role in maintaining its wide vertical distribution.

Description: Phronima sedentaria is a hyperiid amphipod that diel migrates into a pronounced oxygen minimum zone (OMZ) in the Eastern Tropical North Pacific. In this study, oxygen consumption and lactate production were measured in P. sedentaria to estimate the aerobic and anaerobic contributions to total metabolism under conditions that mimic its day- (1% oxygen, 10°C) and night-time (20% oxygen, 20°C) habitat. When exposed to hypoxia and low temperature, the total metabolism of P. sedentaria was depressed by 78% compared with normoxic conditions. The metabolic enzymes citrate synthase (CS) and lactate dehydrogenase (LDH) were also measured as indicators of aerobic and anaerobic metabolism, and compared with specimens collected from the California Current and the North Atlantic to assess potential adaptations to low oxygen. LDH activity was not significantly different between regions. Significant differences in CS activity may be due to variation in food availability. Climate change is predicted to increase surface temperatures and cause the expansion of OMZs. This will result in vertical compression of the night-time range for P. sedentaria and is likely to have the same impact on other diel migrators. Habitat compression will reduce zooplankton contribution to carbon cycling and alter oceanic ecology, including predator–prey interactions.

Description: Many phytoplankton exploit phosphorus (P) from organic sources when dissolved inorganic P (DIP) is depleted. This process is, however, rarely considered in ecological and biogeochemical models. We present a mechanistic model describing explicitly the ability of phytoplankton to use dissolved organic P (DOP) when DIP is limiting, by synthesizing alkaline phosphatase (AP) that releases DIP from DOP. This model, applicable to any phytoplankton species expressing AP, is here specifically developed for the colony-forming Phaeocystis globosa. It describes the main processes related to P metabolism, including DIP transport, intracellular accumulation and assimilation. Model behaviour is explored in DIP-limiting batch-type conditions for different DOP ranging between 0 and 1.5 mmol P m –3 . Simulations show that the DOP-derived DIP increases the maximum biomass reached and extends the period of net growth. The magnitude of the enhanced biomass production is controlled by the DOP initially present as well as the released DOP, the latter being recycled by lysis of P. globosa cells. We also present a simplified model version derived from the mechanistic model, which involves fewer state variables and parameters. The latter is directly usable in both variable (quota-type) and fixed stoichiometry descriptions of phytoplankton growth.

Description: Zooplankton diel vertical migration (DVM) is often explained as a balance between predator avoidance and resource acquisition. However, recent studies suggest that ultraviolet radiation (UV) may also be important in driving zooplankton DVM in some systems. Williamson et al. ( Williamson et al ., 2011 ) proposed the "transparency-regulator hypothesis," which integrates UV into our current understanding of the drivers of DVM and predicts that the relative roles of UV and visual predation pressure will vary systematically across a gradient of lake transparency. To assess this hypothesis, we conducted in situ mesocosm experiments in five different lakes: two lakes without fish and three lakes with fish that spanned a range of UV and visible light transparency. We used an open-bottomed mesocosm design that allowed for the direct manipulation of UV that did not constrain visual predators or the amplitude or timing of natural migrations. Consistent with the transparency-regulator hypothesis, we found that UV is an important driver of Daphnia DVM in highly UV transparent lakes with and without fish but not in low transparency systems. Our results also suggest that UV and visual predation pressure may interact in systems of intermediate transparency.

Description: Competition for resources can lead to species exclusion. However, this exclusion may be avoided if species show differential adaptation to physical environment. Empirical studies on competition are difficult when species are phylogenetically close and have complex life cycles. This is the case of B. plicatilis and B. manjavacas , two cryptic rotifer species differing in their salinity niches and in life-history traits related to sex and diapause. These differences have been suggested to promote the stable co-occurrence observed in natural populations of these species. However, in a previous empirical study, the outcome of competition between both species was always exclusion. Here, we theoretically explored the effect of complex life-history traits and salinity fluctuations on the long-term competitive outcome of B. plicatilis and B. manjavacas . We developed a model and simulated ecological scenarios combining different growing period lengths, levels of crossed induction of sex between species and salinity regimes. Results show that a fluctuating salinity regime, an intermediate length of growing season and a low level of crossed induction of sex are essential conditions to take into account to explain coexistence.

Description: Colonization of new habitats through dispersal of phytoplankton cysts might be limited, if resident populations outcompete invaders during germination. We reciprocally transferred Gonyostomum semen (Raphidophyceae) cysts from three lakes into native and foreign waters originating from the respective habitats. Germination rate and germling growth were impacted by water origin, but there was no preference for native water. Gonyostomum semen 's ability to germinate in different conditions might explain its expansion in northern Europe.

Description: Measuring zooplankton biomass and physiological rates is of paramount importance in biological oceanography in order to assess the role of this community in, e.g. carbon fluxes. Classical methods (incubations) are very time-consuming and cannot match the frequency of physical and chemical measurements. Attempting to solve this, a variety of methods (e.g. egg production, RNA/DNA ratio or enzyme activities) have been developed over the last decades. These methods also show uncertainties and hitherto only incubation methods have been widely accepted. Predictive equations relating physiological processes and body weight (bw) and temperature are a rough alternative, normally used to ascertain the role of these organisms in the oceanic ecosystem. However, using imaging systems and empirical relationships to determine bw allows the application of physiological models to each individual, obtaining reliable estimates for taxonomic groups and size classes. In this study, we developed predictive equations suitable for growth and respiration estimations in subtropical regions. In addition, biomass and physiological rates assessed from empirical equations in combination with an image-based system (ZooImage) were compared with standard and enzymatic methods, respectively. We observed a consistent agreement between methodologies, the former resulting in an inexpensive and faster procedure for the appraisal of biomass and community carbon fluxes at large spatial and temporal scales.

Description: Carbon-specific prey clearance and ingestion rates of 1.5-mm tentaculate larvae of the ctenophore Mnemiopsis leidyi increased linearly between 6 and 25°C but declined between 25 and 30°C. Both absolute (length) and carbon-specific growth rate increased linearly with increasing temperature. The latter was 0.87 d –1 at 25°C. Extremely low or negative growth rates observed at 6 and 30°C help define the thermal limits to population growth of this successful biological invader.

Description: We have supplemented available, concurrent measurements of fresh weight ( W , g) and body carbon (C, g) (46 individuals, 14 species) and nitrogen (N, g) (11 individuals, 9 species) of marine gelatinous animals with data obtained during the global ocean MALASPINA 2010 Expedition (totalling 267 individuals and 33 species for the W versus C data; totalling 232 individuals and 31 species for the N versus C data). We then used those data to test the allometric properties of the W versus C and N versus C relationships. Overall, gelatinous organisms contain 1.13 ± 1.57% of C (by weight, mean ± SD) in their bodies and show a C:N of 4.56 ± 2.46, respectively, although estimations can be improved by using separate conversion coefficients for the carnivores and the filter feeders. Reduced major axis regression indicates that W increases isometrically with C in the carnivores (cnidarians and ctenophores), implying that their water content can be described by a single conversion coefficient of 173.78 gW(g C) –1 , or a C content of 1.17 ± 1.90% by weight, although there is much variability due to the existence of carbon-dense species. In contrast, W increases more rapidly than C in the filter feeders (salps and doliolids), according to a power relationship W = 446.68C 1.54 . This exponent is not significantly different from 1.2, which is consistent with the idea that the watery bodies of gelatinous animals represent an evolutionary response towards increasing food capture surfaces, i.e. a bottom-up rather than a top-down mechanism. Thus, the available evidence negates a bottom-up mechanism in the carnivores, but supports it in the filter feeders. Last, N increases isometrically with C in both carnivores and filter feeders with C:N ratios of 3.89 ± 1.34 and 4.38 ± 1.21, respectively. These values are similar to those of compact, non-gelatinous organisms and reflect a predominantly herbivorous diet in the filter feeders, which is confirmed by a difference of one trophic level between filter feeders and carnivores, according to stable N isotope enrichment data.

Description: I explored mortality estimation for stage-structured populations, building on previous work that applied vertical life-table methods to populations of copepods. A new Bayesian approach for estimating mortality rates accounts for uncertainties in stage duration and number counted by stage, which have not been fully incorporated into previous analyses. This method assumes that mortality is similar among similar life stages. Results using simulated data show that realistic values of the standard deviation of stage duration and number of individuals counted result in reliable mortality estimates, though with wide confidence intervals. This uncertainty obscures variation in estimated mortality between successive stages and can also obscure bias due to violation of underlying assumptions such as that of a stable stage distribution. More importantly, the uncertainty calls into question many previous mortality estimates across pairs of life stages that do not account for these sources of uncertainty. The method was applied to an introduced population of the brackish-water cyclopoid copepod Limnoithona tetraspina in the San Francisco Estuary. Despite the uncertainties, results were interpretable: mortality was highest in nauplii and lowest in adults, probably because of high vulnerability of nauplii to invertebrate predators and low vulnerability of adults to fish predation.

Description: The strong La Niña of 2010–2011 provided an opportunity to investigate the ecological impacts of El Niño-Southern Oscillation on coastal plankton communities using the nine national reference stations around Australia. Based on remote sensing and across the entire Australian region 2011 (La Niña) was only modestly different from 2010 (El Niño) with the average temperature declining 0.2%, surface chlorophyll a up 3% and modelled primary production down 14%. Other changes included a poleward shift in Prochlorococcus and Synechococcus . Along the east coast, there was a reduction in salinity, increase in nutrients, Chlorophytes and Prasinophytes (taxa with chlorophyll b , neoxanthin and prasinoxanthin). The southwest region had a rise in the proportion of 19-hexoyloxyfucoxanthin; possibly coccolithophorids in eddies of the Leeuwin Current and along the sub-tropical front. Pennate diatoms increased, Ceratium spp. decreased and Scrippsiella spp. increased in 2011. Zooplankton biomass declined significantly in 2011. There was a reduction in the abundance of Calocalanus pavo and Temora turbinata and increases in Clausocalanus farrani , Oncaea scottodicarloi and Macrosetella gracilis in 2011. The changes in the plankton community during the strong La Niña of 2011 suggest that this climatic oscillation exacerbates the tropicalization of Australia.

Description: Jellyfish are effective predators on mesozooplankton and release large amounts of dissolved organic matter. Nevertheless, jellyfish initiated trophic cascades and bottom-up influences impacting lower trophic levels have received limited attention. We conducted a mesocosm experiment to quantify simultaneous top-down and bottom-up effects of a common jellyfish, Cyanea capillata , in a natural plankton community during autumn. Treatments were 0, 2 or 5 jellyfish per 2.5 m 3 mesocosm, four replicates each, with initial additions of inorganic nutrients. Primary and bacterial production, species abundance and composition of several trophic levels and nutrient and carbon dynamics were followed during the 8-day experiment. Multivariate statistics and generalized additive mixed modelling were applied to test whether jellyfish carbon concentration (0–1.26 mg jellyC L –1 ) in the mesocosms affected the variables monitored. Unexpected negligible predatory impact of jellyfish on mesozooplankton was observed, potentially related to jellyfish senescence. Community compositions of bacteria, phytoplankton and mesozooplankton changed with time, but did not differ between treatments. However, nutrient regeneration by jellyfish was evident, and jellyfish had a positive impact on total and specific bacterial production, total primary production and the 〉10 µm chlorophyll a fraction. Bottom-up influences from abundant jellyfish could thus stimulate productivity in nutrient depleted autumnal surface waters.

Description: The feeding ecology of Blackfordia virginica was evaluated concurrently with their ecophysiological condition in a temperate estuary. The diet of B. virginica is composed not only of metazooplankton, as commonly observed for other jellyfish species, but also of phytoplankton, ciliates and detritus. This feeding behavior might explain their good nutritional condition and sustainable growth during bloom peaks, when zooplankton abundance has already decreased significantly.

Description: The pelagic dynamics of the cosmopolitan scyphozoan Aurelia sp . was investigated in three French Mediterranean lagoons, Thau, Berre and Bages-Sigean, which harbour resident populations. The annual cycles showed a common univoltine pattern in all lagoons where the presence of pelagic stages in the water column lasted ~8 months. Field observations showed a release of ephyrae in winter time followed by pronounced growth between April and July, when individuals reached the largest sizes, before disappearing from the water column. Maximum abundance of ephyrae and medusae were registered in Thau. Medusae abundance attained a maximum of 331 ind 100 m –3 in Thau, 18 ind 100 m –3 in Berre and 7 ind 100 m –3 in Bages-Sigean lagoons. Temperature and zooplankton abundance appeared as leading factors of growth, where Bages-Sigean showed the population with higher growth rates (2.66 mm day –1 ) and maximum size (32 cm), followed by Thau (0.57–2.56 mm day –1 ; 22.4 cm) and Berre (1.57–2.22 mm day –1 ; 17 cm). The quantification of environmental windows used by the species showed wider ranges than previously reported in the Mediterranean Sea, which suggests a wide ecological plasticity of Aurelia spp. populations in north-western Mediterranean lagoons.

Description: Two lobate ctenophores, Bolinopsis infundibulum and Mnemiopsis leidyi , occur in the North Sea. Stomach contents of field-collected B. infundibulum were recorded and clearance rates for cladocerans and copepods calculated. In starvation experiments, daily body carbon losses of 2.2 and 1.2% and total carbon content losses of 76 and 63% were observed for B. infundibulum (after 68 days) and M. leidyi (after 67 days), respectively.

Description: The ctenophore Mnemiopsis leidyi is characterized by high growth rates and a large reproductive capacity. However, reproductive dynamics are not yet well understood. Here, we present laboratory data on food-dependent egg production in M. leidyi and egg hatching time and success. Further, we report on the reproduction of laboratory-reared and field-caught animals during starvation. Our results show that the half-saturation zooplankton prey concentration for egg production is reached at food levels of 12–23 µgC L –1 , which is below the average summer food concentration encountered in invaded areas of northern Europe. Furthermore, starved animals continue to produce eggs for up to 12 days after cessation of feeding with high overall hatching success of 65–90%. These life history traits allow M. leidyi to thrive and reproduce in environments with varying food conditions and give it a competitive advantage under unfavourable conditions. This may explain why recurrent population blooms are observed and sustained in localized areas in invaded northern Europe, where water exchange is limited and zooplankton food resources are quickly depleted by M. leidyi . We suggest that these reproductive life history traits are key to its invasion success.

Description: Surface-dwelling colonies of Velella velella occur throughout tropical to cold-temperate oceans of the world and sometimes are stranded in masses along hundreds of kilometers of beaches. Large-scale blooms in the Western Mediterranean Sea in 2013 and 2014 allowed the study of diet, prey digestion times and predation rates. Gastrozooid content analyses showed that 59% of the 769 identified prey were euphausiid larvae (calytopsis and furcilia) captured at night. Copepods (41%), fish eggs (2.2%) and larvae (0.5%) were captured both at day and night. Digestion times at ambient temperature (~17°C) of calytopsis, furcilia and copepods were estimated to be 〉6.5, 4.4 and 3.9 h, respectively. Estimated prey consumption was substantially lower in 2014 than in 2013 (41 vs. 75 prey day –1 colony –1 ). Velella velella and other gelatinous species bloomed in the Mediterranean Sea and the northeastern Atlantic and Pacific oceans in 2013 and 2014. Because of the wide distribution of V. velella colonies, their mass occurrences, potential importance as predators and competitors of fish, additional production from symbiotic zooxanthellae and stranding on beaches, they could be important in open-ocean carbon cycling and in transport of pelagic production to landmasses.

Description: Several hypotheses exist that describe phytoplankton spring blooms in temperate and subpolar oceans: the critical depth, shoaling mixed layer (ML), critical turbulence, onset of stratification and disturbance-recovery hypotheses. These theories appear to be mutually exclusive and none of them describe the annual cycle of phytoplankton biomass. Here, we present a model of the annual cycle in phytoplankton that recognizes that phytoplankton are not always mixed throughout the so-called ML, and that it is important to distinguish between the surface biomass and depth-integrated phytoplankton. Once these important distinctions are made, the annual cycles and blooms in surface and depth-integrated phytoplankton can be described straightforwardly in terms of the physical drivers and biotic responses.

Description: Impacts of climate change on marine ecosystems have become increasingly apparent during the past decades. In consequence, it is necessary to study how these alterations can affect the habitat and population dynamics of key organisms. Here we used a video plankton recorder (VPR) to investigate the effect of climate-induced habitat changes on the copepod Pseudocalanus acuspes , a key species in the Baltic Sea. The VPR allowed the observation of reproducing copepod females, identified by attached egg sacs, usually lost during traditional net sampling. We compared the small-scale distribution of our target species during non-inflow and inflow periods. Our study showed a large increase in the availability of suitable habitat after the inflow event due to improved oxygen and salinity conditions. Furthermore, increased copepod abundance and a deeper and wider vertical distribution was apparent. Applying a new approach to estimate in situ egg production rates from VPR-derived images revealed no changes. However, we observed increased offspring survival with improved hydrographic conditions pointing toward the importance of salinity and oxygen for the population dynamics of Baltic P. acuspes . Our observations illustrate the strong impact that climate change can have on the habitat of key marine ecosystem species, important for overall ecosystem dynamics.

Description: Research on nutrient controls of planktonic productivity tends to focus on a few standard fractions of inorganic or total nitrogen (N) and phosphorus (P). However, there is a wide range in the degree to which land-derived dissolved organic nutrients can be assimilated by biota. Thus, in systems where such fractions form a majority of the macronutrient resource pool, including many boreal inland waters and estuaries, our understanding of bacterio- and phytoplankton production dynamics remains limited. To adequately predict aquatic productivity in a changing environment, improved standard methods are needed for determining the sizes of active (bioavailable) pools of N, P and organic carbon (C). A synthesis of current knowledge suggests that variation in the C:N:P stoichiometry of bioavailable resources is associated with diverse processes that differentially influence the individual elements across space and time. Due to a generally increasing organic nutrient bioavailability from C to N to P, we hypothesize that the C:N and N:P of bulk resources often vastly overestimates the corresponding ratios of bioavailable resources. It is further proposed that basal planktonic production is regulated by variation in the source, magnitude and timing of terrestrial runoff, through processes that have so far been poorly described.

Description: Understanding the factors that regulate the abundance, size structure and community structure of cladocerans is an important goal of aquatic ecologists. While both top-down and bottom-up factors help to structure cladoceran communities, there may be interactions between these factors. We conducted a mesocosm study to determine how alien large-bodied Daphnia , zebra mussels and fish affected cladoceran community and size structure. We found that large-bodied Daphnia reduced algal resources and the fecundity of smaller bodied cladocerans. Fish removed the large-bodied Daphnia magna from the mesocosms and shifted the cladoceran community to a smaller body size. Fish also appeared to promote increases in cladoceran diversity through the coexistence of several smaller bodied taxa. In contrast, zebra mussels increased cyanobacteria and helped to promote the success of the alien Daphnia , but reduced the biomass of small-bodied cladocerans. Zebra mussels reduced the carbon (C):phosphorus (P) ratio of the phytoplankton in the mesocosms which may have favored the relatively P-limited Daphnia . Combined, our results highlight the complex interactions of multiple factors that help to regulate cladoceran community and size structure.

Description: In freshwater systems, Daphnia has been demonstrated to show adaptive responses following the light–dark cycle. The adjustment of these responses to the change of day and night is probably transmitted via the hormone melatonin. The rate-limiting enzyme in melatonin synthesis is the arylalkylamine N-transferase (AANAT). We identified three genes coding for insect-like AANATs in Daphnia , of which we measured the gene expression in an ecologically relevant light–dark cycle. We demonstrated that Daphnia 's insect-like AANAT gene expression oscillated in a daily manner, and that the highest peak of expression after the onset of darkness was followed by a peak of melatonin production at midnight. Moreover, we could show an oscillation of endogenous melatonin synthesis in Daphnia . In most organisms, melatonin synthesis is due to rhythmic expression of genes of the circadian clock, since transcription of aanat s is directly linked to a circadian transcription factor. We could demonstrate that putative clock genes and insect-like AANAT genes of Daphnia were equally expressed. Therefore, we propose that melatonin synthesis is coupled to the expression of Daphnia clock genes, and that insect-like AANATs of crustaceans have a similar function as AANATs of vertebrates: The initiation of melatonin synthesis. In future studies with Daphnia , it will be necessary to take the time of day into account since melatonin concentrations might influence stress responses.

Description: It has been hypothesized that terrestrial particulate organic matter (t-POM) makes important contributions to Daphnia production in some lakes. We conducted a series of feeding experiments to explore the fatty acid responses in Daphnia to diets comprised of different terrestrial resources (i.e. Alnus rubra , Phragmites australis , Betula nana and Betula pendula ) and mixed diets with terrestrial and phytoplankton ( Scenedesmus or Cryptomonas ) resources. When fed 100% phytoplankton, Daphnia had very similar ( r 2 〉 0.80) fatty acid profiles to their diets, whereas Daphnia that consumed t-POM diets had weak correlations ( r 2 = 0.002–0.56) with the corresponding diet sources. Unusual 16 carbon chain polyunsaturated fatty acids (16:26, 16:33 and 16:43), linoleic acid (18:26) and α-linolenic acid (18:33) were diagnostic fatty acids for Scenedesmus and Daphnia that consumed this alga. Stearidonic acid (18:43) and eicosapentaenoic acid (20:53) were diagnostic for Cryptomonas and Daphnia that consumed this diet. All of the t-POM resources were characterized by a high content of saturated fatty acids (SAFA; 79 ± 12%), especially the diagnostic long-chain SAFA (20:0, 22:0, 24:0, 26:0, 28:0). Daphnia that consumed t-POM assimilated very little of these terrestrial biomarkers, but the shorter chain SAFA 16:0 and 18:0 were very prevalent in juvenile and adult Daphnia that consumed terrestrial plant matter. The -3:-6 ratios were distinctive between terrestrial (0.3–1.6) and phytoplankton resources (3–15), and this ratio in Daphnia was strongly associated with their diets ( r 2 = 0.88). These results suggest that Daphnia , and perhaps zooplankton in general, preferentially retain algae-derived 3 fatty acids, and low -3:-6 ratios in Daphnia indicate a mainly terrestrial diet or poor nutritional condition.

Description: In order to trace community dynamics and reticulate evolution in hybrid species complexes, long-term comparative studies of natural populations are necessary. Such studies require the development of tools for fine-scale genetic analyses. In the present study, we developed species-diagnostic SNP-based markers for hybridizing freshwater crustaceans: the multispecies Daphnia longispina complex. Specifically, we took advantage of transcriptome data from a key species of this hybrid complex, the annotated genome of a related Daphnia species and well-defined reference genotypes from three parental species. Altogether eleven nuclear loci with several species-specific SNP sites were identified in sequence alignments of these reference genotypes from three parental species and their interspecific hybrids. A PCR-RFLP assay was developed for cost-efficient large population screening by SNP-based genotyping. Taxon assignment by RFLP patterns was nearly perfectly concordant with microsatellite genotyping across several screened populations from Europe. Finally, we were able to amplify two short regions of these loci in formaldehyde-preserved samples dating back to the year 1960. The species-specific SNP-based markers developed here provide valuable tools to study hybridization over time, including the long-term impact of various environmental factors on hybridization and biodiversity changes. SNP-based genotyping will finally allow eco-evolutionary dynamics to be revealed at different time scales.

Description: Prey size selectivity in piscivorous fish larvae is important to both aquaculture and fisheries science, but laboratory experiments are few. We analyzed selective foraging in Atlantic bluefin tuna larvae ( Thunnus thynnus ) using two larval fish prey species. The experiments revealed that selective foraging of prey sizes differed among bluefin tuna predator sizes (15–25 mm SL) and prey species, bonito ( Sarda sarda ) and seabream ( Sparus aurata ). The observed pattern suggest a general preference for small bonito prey larvae but large seabream prey. Thus, prey size alone is not the only trait responsible for size selectivity in piscivorous fish larvae.

Description: Using a unique 50-year high-resolution time series of daily kom-fyke catches, long-term patterns of scyphomedusae in the western Dutch Wadden Sea were analysed and related to changes in environmental conditions [eutrophication in the 1980s–1990s and recent climate change (increased water temperature)] in the area. Over the years, species composition and general pattern of appearance has remained the same: the first species that occurred in spring was Aurelia aurita , followed by Cyanea lamarckii / C. capillata. Chrysaora hysoscella and Rhizostoma octopus occurred from June to July onwards. All species appeared earlier in recent decades and first appearance and peak occurrence of A. aurita was in part inversely related to previous winter seawater temperature. Last occurrence of C. hysoscella was related to summer seawater temperature and the species is present longer in recent decades. Phenological relationships might have been decoupled since the seasonality of the phytoplankton bloom did not change. All species showed large inter-annual abundance fluctuations, with prolific years followed by sparse years. Peak catches of the coastal species A. aurita occurred in the late 1970s–early 1990s when eutrophication peaked, however, without a significant relationship with total nitrogen input into the area. Unlike for phenology, the patterns of mean abundance of any species did not show a relationship to climate change in the area. This might imply that population regulating mechanisms do not operate during the planktonic phase but during the sessile demersal polyp stages.

Description: In situ fluorometers are the optimal means of providing high-frequency estimates of phytoplankton communities. However, they may be subjected to measurement biases originating from variations in the physiological states of cells, the use of spectral fluorescence signatures (SFS) defined on the basis of inappropriate phytoplankton groups and the lack of linear independence between selected sets of SFS. We assessed correction procedures for measurement biases in mono and mixed cultures of five freshwater phytoplankton species. We investigated the impacts of total Chl a levels, the lack of linear independence between SFS and varying physiological states on the accuracy of the Chl a estimates that were provided by the FluoroProbe (bbe Moldaenke GmbH, Germany). The use of species-specific SFS allowed for the correction of quantification and classification biases. In some cases, the procedure led to a lack of linear independence between SFS, which significantly reduced estimation accuracies. A convenient method to evaluate linear independence between SFS is provided. Differences in the physiological states of phytoplankton cultures following light pre-acclimation and/or N-starvation appeared to be species specific. Light pre-acclimation led to an underestimation of biomass (up to –28.5%) through fluorescence quenching. The responses of the phytoplankton cultures to N-starvation varied depending on the species (from –40.3 to +336% biases in Chl a quantification). Overall, the application of appropriate corrective measures increased data accuracy. However, optimal data reliability can only be achieved by estimating phytoplankton community composition and associated environmental conditions.

Description: Cyanobacteria are the primary taxa responsible for freshwater harmful algal blooms (HABs), with several genera capable of producing potent intracellular toxins and off-flavor compounds. There is considerable growing interest in methods to rapidly quantify cyanobacteria in water samples. Past studies have demonstrated poor correlations between phycocyanin in vivo fluorescence and cyanobacterial cell densities. We conducted a series of laboratory experiments aimed at refining a protocol that uses benchtop fluorometry to measure the cyanobacterial pigment, phycocyanin, to accurately estimate cyanobacterial biovolume. In our study, we found strong correlations between phycocyanin concentration and cyanobacterial biovolume (but not for cell densities) both within and across ponds, which varied widely in productivity and algal diversity. Thus, benchtop fluorometry of phycocyanin is a viable method for water resource managers to quickly estimate cyanobacterial biovolume.

Description: Several dinoflagellate species in the genus Blastodinium are gut parasites of marine planktonic copepods. However, there is only limited information on the occurrence and infection frequencies of Blastodinium spp. in the field and almost no information on the functional impact on their hosts. We report upon the effects of Blastodinium sp . infection on Calanus finmarchicus from the northeastern Atlantic coast off southern Norway during April 2013 and 2014. Up to 58% of C. finmarchicus were infected near the coast, while 〈5% were infected several kilometers offshore. Ingestion rates of infected females were below detection limits and significantly lower than uninfected females. Blastodinium sp . -infected females showed characteristic symptoms of starvation, including lower respiration rates (implying a lower metabolic rate), production of smaller and fewer fecal pellets and significantly fewer eggs than uninfected females. A few females in this study were able to void the infection, however the extended period of starvation is likely to have longer-term repercussions on egg production rates well after the copepod clears the infection. The degree to which the infection affects C. finmarchicus recruitment depends on the extent of the spatial distribution of the infection. Monitoring of parasitic infection during routine field surveys will be required in order to clarify this.

Description: Knowledge of copepod in situ diet is critical for accurate assessment of trophic linkages and transfer efficiencies of the marine food web but is limited due to technical challenges. Here we report, using a recently developed eukaryote-universal copepod-excluding ectobiotic ciliate-blocking protocol, to investigate the natural diets of the copepods Temora turbinata , Subeucalanus subcrassus and Canthocalanus pauper in coastal waters in Sanya Bay, China. Analysis of the resultant 18S rDNA clone libraries revealed diverse diet composition for all the three copepod species, with 11 prey species for C. pauper , 9 for T. turbinata and 9 for S. subcrassus . The ingested materials included land plants, green algae, Metazoa, Euglenozoa and Rhizaria, although species numbers from each of these lineages differed. Broussonetia sp. (land plant), which might have been ingested in the form of pollen or fresh detritus were common among all three copepods, and accounted for a significant proportion (〉55%) of the clones sequenced. These results suggest that copepods in Sanya coastal waters might use terrigenous detritus as supplementary food sources when phytoplankton production is limited. However, the significance of the plant detritus as a nutrition source of these copepods remains to be determined by analyzing carbon-based proportion and digestion/assimilation rate of the ingested plant materials.

Description: Predation is considered an important source of mortality for plankton, but documenting variation in planktonic predation, particularly across interacting environmental cycles, remains logistically difficult, thus our understanding remains limited. To test for the combined effects of prey life history stage, diel or light level phase (including crepuscular periods) and seasonal upwelling on the risk of predation, we deployed tethered adult and larval brine shrimp Artemia franciscana using dock-based plankton tethering units (PTUs). Risk was higher overall during upwelling, but life history stage also interacted with season. There was no seasonal difference in risk for adults. Larvae were at significantly lower risk of predation during non-upwelling than during upwelling. Larvae were also at lower risk during non-upwelling than were adults during either season. During upwelling, there was no significant difference in risk between the two prey categories. With respect to the diel cycle, dusk was safer than daytime. For larvae, the diel pattern in risk remained consistent across seasons while risk for adults at night was slightly lower during upwelling than during non-upwelling. Variation in planktonic predation risk across diel and seasonal cycles differs for different life history stages and thus, generalizations fail to capture the complexity of interactions between factors.

Description: Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially different implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations ( Prochlorococcus , Synechococcus , picoeukaryotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 ± 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodological constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells.

Description: Dispersal of native species from the regional pool can recover invaded communities to a pre-invaded state by supplementing declining populations or providing resistant species. However, dispersal may also exacerbate the negative effects of an invader. Introduced species can open or create new niche space, which could facilitate the establishment of competitors or predators that previously could not succeed in the uninvaded local community. To investigate the interaction between dispersal and invasion by a non-native consumer, we conducted a field mesocosm experiment that introduced zebra mussels into native zooplankton communities. Regional zooplankton were collected and added to both invaded and uninvaded communities. In uninvaded communities, zooplankton dispersal reduced cladoceran diversity by ~40%, rotifer abundance by ~65% and copepod nauplii abundance by ~80%. In invaded communities, dispersal increased cladoceran diversity by ~60%, but also further exacerbated the negative effects of zebra mussels on rotifer abundance. This experiment illustrates the potential for dispersal to both positively and negatively affect local communities, and how these effects may change with disturbance and the taxa or community metric of study.

Description: Pulse amplitude-modulated (PAM) fluorometry was used to obtain rapid light curves (RLCs) for phytoplankton from small Canadian lakes of varying water clarity, including metalimnetic communities from two clear lakes. RLCs were measured before and after exposure to a number of experimental spectra containing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). Photochemical (qP) and non-photochemical (NPQ) quenching during RLCs were positively and significantly correlated with recent in situ light history, but the maximum quantum efficiency of Photosystem II (α) and maximum relative rate of electron transport (rETR max ) were not. rETR max and α were diminished by experimental exposures to UVR and/or high PAR, but significantly less so in phytoplankton from brighter environments. UVR exposures diminished the inducible (photoprotective) NPQ of most epilimnetic phytoplankton. Both the inducible and total (photoprotective + photoinhibitory) NPQ of metalimnetic phytoplankton were stimulated by spectral exposures. Our results are the first obtained from natural communities of freshwater phytoplankton to show that aspects of PSII photophysiology (as inferred from RLCs) vary according to in situ light history.

Description: Inducible defenses are a common phenotypically plastic response to a heterogeneous predation risk. Once induced, these defenses cannot only lose their benefit, but even become costly, should the predator disappear. Consequently, some organisms have developed the ability to reverse their defensive traits. However, despite extensive research on inducible defenses, reports on reversibility are rare and mostly concentrate on defensive behavior. In our study, we investigated the reversibility of morphological defenses in the freshwater crustacean Daphnia barbata . This species responds to Notonecta glauca and Triops cancriformis with two distinctively defended morphotypes. Within the numerous defensive traits, we found both trait- and predator-specific reversibility. Body torsion and tail-spine-related traits were highly reversible, whereas helmet-related traits remained stable, suggesting different physiological constraints. However, in general, we found the defenses against Triops mostly reversible, while Notonecta -induced defenses were persistent and grew further, even in the absence of a predator.

Description: Marine phytoplankton are a taxonomically and functionally diverse group of organisms that are key players in the most important biogeochemical cycles. Phytoplankton taxa show different resource utilization strategies (e.g. nutrient-uptake rates and cellular allocation) and traits. Therefore, acknowledging this diversity is crucial to understanding how elemental cycles operate, including the origin and dynamics of elemental ratios. In this paper, we focus on trait-based models as tools to study the role of phytoplankton diversity in the stoichiometric phenomenology observed in the laboratory and in the open ocean. We offer a compilation of known empirical results on stoichiometry and summarize how trait-based approaches have attempted to replicate these results. By contrasting the different ecological and evolutionary approaches available in the literature, we explore the strengths and limitations of the existing models. We thus try to identify existing gaps and challenges, and point to potential new directions that can be explored to fill these gaps. We aim to highlight the potential of including diversity explicitly in our modeling approaches, which can help us gain important knowledge about changes in local and global stoichiometric patterns.

Description: Highly intermittent spatial variability of phytoplankton is observed ubiquitously in marine ecosystems, especially when measurements are performed at the micro-scale level. Therefore, theoretical developments and new modelling tools are required to understand the observed small-scale vertical structure and its relationship to ecosystem behaviour. Nearly all current ecosystem models are formulated entirely based on the mean field approximation, ignoring sub-grid scale variability. Even if such approximation may be reasonable for meso-scales (and above), it cannot account for micro-scale dynamics, which may also impact macroscopic properties at the larger scale. To consider intermittency of variables in plankton ecosystem models, we apply a newly developed modelling approach called the closure approach. Detailed simulations were conducted, combining fluid-dynamics of the 1D water column with the nutrient-phytoplankton closure ecosystem model for application to a site in the northern North Sea. Compared with a control model, which does not account for such intermittency, the closure model produced substantially different spatio-temporal patterns of mean phytoplankton biomass and growth rate, which depended on the overall level of variability. In this study, we (i) seek to explore the effects of sub-scale variability coupled with physical transport and (ii) begin to address the yet unresolved question of how to consistently model the advection and diffusion of the variances and co-variances used to represent sub-scale variability in the closure approach. Our results suggest that it may be necessary to account explicitly for the intermittent distribution of plankton and nutrients, even in large-scale biogeochemical models.

Description: Seawater viscosity is influenced by temperature as well as through excretion of exopolymers by some plankton. We examined the role of viscosity on the movement patterns and sensory abilities of the dinoflagellate Heterocapsa triquetra , manipulating the viscosity of seawater to simulate a 10 ± 1.5°C temperature change. In a second treatment, we seeded the water with microbeads to examine swimming behaviours in the presence of a mechanical stimulus. Increased viscosity reduced distances between conspecifics 4.7-fold and increased distances between protists and microbeads by 3.4-fold. Increased viscosity also affected other aspects of motility, with an overall reduction in swimming speed of 2.0- and 7.0-fold for treatments with and without mechanical stimuli. Higher viscosities were associated with upward vertical migration, in both the presence and absence of microbeads. Cells were highly sensitive to disturbances to the velocity field, by as little as 1.5%, and different approach distances of H. triquetra to conspecifics over mechanical stimuli suggest sensory capacity to distinguish types of particles. Mediation of motility and migratory behaviours through viscosity implies ramifications for the distribution of protists and their encounters with resources, predators and conspecifics triggered by events such as temperature changes and phytoplankton bloom events.

Description: Parasites are not typically considered to be important components of polar marine ecosystems. It was therefore surprising when 18S rDNA surveys of protists in the West Antarctic Peninsula in winter revealed high abundances of parasite sequences. Parasite sequences made up, on average, over half (52%) of sequence reads in samples from deep water in winter. Winter surface water and sediment samples contained relatively fewer, but still strikingly high, parasite sequence reads (13 and 9%, respectively), while surface water samples in summer contained fewer parasite sequences (1.8%). A total of 1028 distinct parasite Operational Taxonomic Units were observed in winter, with the largest abundances and diversities within Syndiniales groups I and II, including Amoebophrya . Less abundant parasite sequence groups included Apicomplexa, Blastodinium , Chytriodinium , Cryptocaryon , Paradinium , Perkinsidae, Pirsonia and Ichthyophonae . Parasite sequence distributions suggested interactions with known hosts, such as diatom parasites which were mainly in the sediments, where resting spores of Chaetoceros spp. diatoms were abundant. Syndiniales sequences were correlated with radiolarian sequences, suggesting parasite–host interactions. The abundant proportions of parasite sequences indicate a potentially important role for parasites in the Antarctic marine ecosystem, with implications for plankton population dynamics, the role of the microbial loop, carbon flows and ecosystem responses to ongoing anthropogenic climate change.

Description: Metabarcoding (large-scale taxonomic identification of complex samples via analysis of one or few orthologous DNA regions, called barcodes) is revolutionizing analysis of biodiversity of marine zooplankton assemblages. Metabarcoding relies on high-throughput DNA sequencing (HTS) technologies, which yield millions of DNA sequences in parallel and allow large-scale analysis of environmental samples. Metabarcoding studies of marine zooplankton have used various regions of nuclear small- (18S) and large-subunit (28S) rRNA, which allow accurate classification of novel sequences and reliable amplification with consensus primers, but- due to their relatively conserved nature- may underestimate species diversity in a community. To discriminate species, more variable genes are needed. A limited number of metabarcoding studies have used mitochondrial cytochrome oxidase I (COI), which ensures detection of species-level diversity, but may require group-specific primers and thus result in inconsistent amplification success rates. Reference databases with sequences for accurately-identified species are critically needed to allow taxonomic designation of molecular operational taxonomic units (MOTU) and comparison with previous studies of zooplankton diversity. Potential and promising applications of metabarcoding include rapid detection of impacts of climate change, monitoring and assessment of ecosystem health, calculation of biotic indices, characterization of food webs and detection of introduced, non-indigenous species.

Description: In peninsular Florida, USA, rainfall is coupled with the Pacific Sea Surface Temperature Anomaly (SSTA), and rainfall affects mean depth and residence time of shallow lakes. We examined effects of two cycles of variation in rainfall using a 15-year data set from a shallow eutrophic lake dominated by small zooplankton, cyanobacteria and omnivorous fish. In high rainfall periods, the lake was deeper and cladoceran biomass was significantly higher than in dry periods. One factor was correlated with reduced biomass of cladocerans: a 3-fold higher biovolume of cyanobacteria. This led us to examine how variation in rainfall affects cyanobacteria. When cyanobacteria biovolume was high, the movement of water through the lake was low and invariant. Cyanobacteria grew unchecked. When cyanobacteria was reduced and cladocerans attained high biomass, there were intermittent flushing events that may have disrupted algal growth. Water color was elevated ~6-fold during the same time periods. Greater color may have made conditions less favorable for cyanobacteria by increasing light attenuation, and also more favorable for cladocerans, by reducing risk from fish. This study provides insight into how future variability in rainfall and drought, which may be exacerbated by global warming, could affect plankton in shallow subtropical lakes.

Description: We investigated the effects of temperature on weight-specific respiration rates ( R sp ) of the euryhaline calanoid copepod Pseudodiaptomus annandalei collected from the Matang Mangrove Forest Reserve (Malaysia). We employed a simple experimental approach consisting of a short temperature exposure time from 24 to 36°C. The relationship between temperature and R sp of acclimated copepods under conditions of excess food fitted an exponential function and indicated a complete lack of acclimation at the higher temperature range. Both fine-scale temperature measurements and piecewise linear regression enabled the detection of a breakpoint produced by a significant increase in respiratory rates. It is argued that thermally stressed copepods can be detected by both a higher Q 10 above 32°C and monitoring this breakpoint in the metabolic response ( R sp ). Similarly, the lower Q 10 (≤2) indicates some degree of independence of their metabolic rates between 26 and 32°C, which likely corresponds to their optimal thermal window. Results are discussed in relation to body mass, feeding status and gender.

Description: Restored ponds are uniquely suitable for the study of successional processes on aquatic communities. In a set of dune slacks of different restoration ages, we monitored monthly the environmental and community shifts of one newly restored pond over 4 years and compared it with older ponds. Questions were (i) how does the community change with time in the newly restored pond? (ii) Are these changes associated with the local environment? (iii) Was the pattern comparable to older ponds? Restoration exerted a strong effect in the pond, since the main changes were observed at the early stages of succession. Later on, the pond started to resemble older ponds in the area as it moved toward a more complex community.

Description: P-transport through the microbial food web was investigated in a mesocosm experiment in which orthophosphate was added to oligotrophic Cretan Sea water. As non-exclusive alternatives to traditional phytoplankton–zooplankton succession, two mechanisms for P-transport have previously been proposed: a stoichiometric shift in food quality (Tunnelling); and a transport through a predatory food chain from heterotrophic bacteria, bypassing phytoplankton (Bypass). Following P-addition, particulate C:P-ratio dropped from 436 to 44 (molar) within 1 day, and egg production increased after 2 days. This confirms the hypothesized stoichiometric shift and rapid copepod response of a "Tunnelling" scenario. Bacterial abundance responded positively to P-addition on Day 1, ciliates increased after Day 5 and new egg production peaks occurred on Days 5 and 9; a succession suggesting additional Bypass transport although the response expected in heterotrophic flagellate abundance was not confirmed. A small, but statistically not significant, increase in Chl a in the 0.6- to 2-µm size fraction also suggests possible additional P-transport through a phytoplankton–zooplankton succession. On the basis of the magnitude of the stoichiometric change and the short delay in egg production response, we consider the Tunnelling mechanism to be the most likely signal initiating egg production, but a Bypass, and possibly a traditional succession, may have contributed to maintain the elevated egg production.

Description: The fatty acid (FA) composition of zooplankton has taxon-specific characteristics but may also be influenced by various environmental factors. Abiotic properties of lakes (location, morphometry, water chemistry and temperature) shape the phytoplankton community structure. We studied how this may be manifested in the FA composition of the common freshwater calanoid copepod Eudiaptomus spp. The proportions of saturated and polyunsaturated FA in Eudiaptomus, sampled from 25 lakes in boreal and subarctic regions, showed large variation (ca. 20–60 and 30–70%, respectively), while the proportion of monounsaturated FA was less variable (5–15%). The FA composition of Eudiaptomus differed significantly between subarctic and boreal lakes. Eudiaptomus from subarctic lakes had a higher proportion of 22:63 and lower proportions of 18:26, 18:33 and 20:53 than Eudiaptomus from boreal lakes. In the boreal lakes, the 3:6 ratio in Eudiaptomus increased with increasing nutrients, chl a and dissolved organic carbon, presumably due to parallel changes in phytoplankton community composition, but only in the large and clearwater lakes, and not in the small and humic ones. Copepods are seasonally important prey for planktivorous fish, and the observed among-lake differences in the proportions of essential FA may influence fish growth and reproduction.

Description: The widespread increase in the relative abundances of Cyclotella sensu lato taxa in Northern Hemisphere lakes suggests that these taxa are strong indicators of global change, yet the specific factors affecting the success of these species have been difficult to identify. The primary objective of this study was to investigate the interactive effects of temperature (7–10°C or 11–15°C), light (high or low) and nutrients (control or nitrogen plus phosphorus) on cell densities of various Cyclotella sensu lato taxa. We observed varying interactive effects of temperature, light and nutrients on Lindavia bodanica , Lindavia comensis , Lindavia ocellata and Discostella stelligera . For example, when nutrients were provided, Lindavia radiosa was more abundant under high light conditions, whereas L. ocellata was more abundant under low light conditions. The secondary objective was to determine whether different phytoplankton communities (one dominated by diatoms and the other by a mixed assemblage) altered the response of D. stelligera to abiotic factors. High temperature, light and nutrients only increased cell densities of D. stelligera when this species occurred in the diatom-dominated community. This study provides further ecological insight on Cyclotella species that serve as important indicators of environmental change.

Description: Meroplankton is often neglected in Arctic zooplankton studies, so our knowledge about their seasonal dynamics is limited. To investigate the highly dynamic nature of meroplankton, we sampled the zooplankton community in a high-Arctic fjord in West-Spitsbergen bi-weekly from December 2011 to December 2012. Five significantly different seasonal meroplankton assemblages were identified, representing winter, early spring, spring, summer and autumn. Meroplankton persistently dominated the zooplankton community during the productive seasons in both abundance (41–91%) and biomass (54–98%). Cirripedia and Bivalvia larvae occurred in particularly high numbers and dominated during spring and summer respectively. In the remaining seasons, the meroplankton diversity and abundance were comparatively poor, but interestingly Bryozoa larvae were present mainly during winter. Chlorophyll a and day length were identified as the main environmental variables structuring the meroplankton assemblage, followed by hydrography. Timing of the spring bloom determined the onset of the "meroplankton-boost" and from the observed hydrography, local rather than advective processes shaped the meroplankton community. Our investigation suggests that benthic invertebrate larvae play a significant role in the pelagic ecosystems in Arctic coastal regions, tightly linking the pelagic and benthic realms, thus deserving more attention in future studies.

Description: Fish introduction and nutrient enrichment are two major environmental stressors for many freshwater lakes. Thus far, their ecological impacts have largely been evaluated independently, despite that fish invasion and predation may interact with lake trophic status. We examined paleoecological records from four large lakes of Southwest China to quantify the strength and interaction of icefish invasion and eutrophication in affecting zooplankton over time. There was a significant and positive relationship ( P 〈 0.01) between sediment Chl a concentration and bosminid flux in each lake with bosminid size structure being greater in more productive lakes, indicating a strong bottom-up control. Furthermore, a consistent decrease in bosminid body sizes after fish introduction was found in Fuxian where icefish grew most successfully among our lakes. The variance partitioning results showed a decrease in the relative importance of planktivorous predation in driving zooplankton production with increased trophic status across lakes, with their joint effect showing a unimodal model in strength. In all, there was regional heterogeneity in the population status and ecological impact of invasive icefish that varied with lake trophic status over time. Therefore, nutrient status needs to be considered in assessing long-term ecological status and consequences of fish invasion in stressed freshwaters.

Description: The relationship between the co-occurrence of cryptic zooplankton species and environmental variables was studied in a lake in China. Lake Jinghu was sampled for zooplankton and water chemistry at 3 or 4-day intervals over a year, and samples of the rotifer Brachionus calyciflorus were obtained at 8-day intervals and clonally cultured in the laboratory. We found that B. calyciflorus occurred in winter and spring, and was a species complex composed of cryptic species BcI and BcII based on mtDNA COI sequence divergence and the GMYC model analyses. Both cryptic species cohabited in Lake Jinghu when they were present. The densities of BcI and BcII were not correlated, indicating that there might be no strong competition between them. Based on principal component analysis (PCA) and generalized linear models (GLM), only BcII density was impacted by chlorophyll a (Chl a ) concentration (of the lake water filtered through a 25 μm net) alone with a time lag, and the interaction of temperature and Chl a concentration without a time lag. Our results suggest that a distinct response to Chl a reduces the intensity of competition between cryptic B. calyciflorus species and promotes their coexistence in Lake Jinghu over time.

Description: Calanus sinicus , the key zooplankton species in the Yellow Sea, China, over-summers in the Yellow Sea Cold Water Mass (YSCWM). Here, we compared the metabolic rates, morphological characteristics and relative expressions of seven genes associated with molting, gonad development, lipid catabolism and stress tolerance of C. sinicus captured both inside and outside the YSCWM in summer. With a large oil sac, low metabolic rate and suppressed molting development, the C5-stage copepods inside the YSCWM were probably quiescent. The gene expressions revealed differences in physiology between quiescent and active copepods in the two regions. When quiescent, the gene associated with molting [ecdysteroid receptor (EcR)] was down-regulated, while genes related to lipid catabolism (Hydroxyacyl CoA dehydrogenase) and stress tolerance (ferritin) were up-regulated. C5s at the margin of the YSCWM up-regulated EcR expression and this could be in response to the elevated Chl a concentration, suggesting that elevated food condition may serve as a trigger that terminated the over-summering of C. sinicus in the Yellow Sea. In conclusion, this study revealed the physiological processes of quiescent and active C. sinicus via morphological, physiological and molecular methods simultaneously, providing a foundation for future investigations of the mechanisms involved in over-summering in YSCWM.

Description: Fourteen years of data collected by the Atlantic Zone Monitoring Program served to model the habitat of four dominant copepod species ( Calanus glacialis , Calanus hyperboreus , Calanus finmarchicus , Paracalanus sp.) on the continental shelf and slope waters in the Northwest Atlantic Ocean. Generalized additive mixed models (GAMMs) were applied to abundance and presence–absence data for C. hyperboreus , C. glacialis and Paracalanus sp. and abundance for C. finmarchicus , to describe the optimal environmental productivity envelopes associated with the occurrence and/or the net productivity of these species. The models for Calanus species considered two main phases of their life cycle: (i) an active population growth phase dominated by early stages that occur primarily in surface layers, and (ii) a dormant phase dominated by overwintering stages generally found in deeper layers. GAMMs identified a marked contrast in environmental envelopes occupied by arctic and temperate species. Our analyses underline the importance of using data representative of all the copepodid developmental stages and occupied habitats in order to accurately model the distribution of Calanus species. The value of our models as tools to understand past events in the Northwest Atlantic or to predict future distributions of the species is also discussed.

Description: Here we compare the functional biology of the sympatric krill species, Meganyctiphanes norvegica and Thysanoessa inermis . For M. norvegica, we investigated functional responses on diatoms and copepods, together with prey size spectra on plankton 〈400 µm and copepods in the size range 500–3220 µm. For T. inermis , only prey size spectrum on plankton 〈400 µm were investigated. The prey size ranges of both species include organisms 〈400 µm, and they consequently graze on several trophic levels. However, T. inermis feed on cells 〈10 µm equivalent spherical diameter (ESD), whereas M. norvegica only feed on cells 〉10 µm. Meganyctiphanes norvegica show maximum predation on 800–1600 µm sized copepods, corresponding to a predator:prey size ratio of 17.0 ± 2.2. Functional response experiments with M. norvegica follow a Holling type III functional response, both when feeding on diatoms and copepods, but with an order of magnitude higher ingestion rate on the copepod prey. The two functional groups, M. norvegica and Thysanoessa spp., overlap in prey size spectra. However, there are differences in their ability to exploit different prey classes. Here, we present clearance rates of both krill species on natural plankton illustrating the two species' wide particle range spectra.

Description: We have studied the in vivo respiratory oxygen consumption (RO2) and the potential respiration rates (), measured by the enzymatic activity of the electron transport system (ETS), in epipelagic marine zooplankton from several marine systems. Furthermore, we have determined the intracellular concentration of the ETS substrates, i.e. the pyridine nucleotides (PNs), in these organisms. Both the RO2 and and the relationship between them (RO2/) showed a major dependency on habitat temperature, with significantly higher values in the warm waters of North Atlantic. Conversely, the pattern described by the PN concentration was mainly associated with the productivity regime that ultimately determined the food availability for the zooplankton. We further studied seasonal variability of the RO2 , and PN concentrations in the coastal waters of the Canary Islands. Larger values were measured during the late winter bloom period, whereas they decreased between 30 and 40% with the stratification of the water column. Shedding light on the processes that drive the respiratory control at the physiological level will help in understanding the variability of the respiration rates in marine organisms as well as the impact that these rates have on the remineralization of the organic matter.

Description: This study investigates whether feeding on the domoic acid (DA)-producing diatom Pseudo-nitzschia seriata affects the faecal pellet (FP) production (proxy for grazing) and fecundity of Calanus finmarchicus and Calanus glacialis. Female copepods were fed a saturating concentration of food (400 µg C L –1 ) in two combinations (i) natural phytoplankton spiked with 50% P. seriata and (ii) only the non-toxic phytoplankton community. The copepods ingested DA, as illustrated by DA accumulation in their FPs, and transferred a share of DA to their eggs and body tissue. DA was mainly excreted through FPs in both species. Compared with C. finmarchicus , C. glacialis accumulated substantially more DA in its body tissue. For both species, egg production and hatching success were unaffected by grazing on the toxic diatom. This suggests that feeding on DA-containing P. seriata does not affect copepod fecundity, despite increasing DA concentrations of P. seriata during the experiment.

Description: Mortality is notoriously difficult to estimate for zooplankton populations in the open ocean due to the confounding effect of advection. The vertical life table (VLT) approach is commonly used, but has been shown to be sensitive to both spatial and temporal trends in recruitment. Here, we estimate mortality rates of Calanus finmarchicus copepodites from spatiotemporally resolved data from the highly advective Norwegian Sea–Barents Sea in spring and summer. We apply both the VLT and a statistical regression approach (SRA), specifically taking into account the effects of advection and spatiotemporal trends in recruitment. Testing the two methods on a simulated dataset shows that the SRA performs better than the most commonly used version of the VLT when trends in recruitment are present. Overall, the SRA appears to be a robust method for spatiotemporally resolved survey data influenced by advection and spatiotemporal trends in recruitment. The estimated mortality rates are relatively low (0.03–0.07 d –1 ) and indicate increased mortality for the oldest stage pair (copepodites CIV–CV) compared to earlier stages. The results are discussed in the light of previous mortality rate estimates for C. finmarchicus .

Description: Spatiotemporal patchiness of plankton and appropriate sampling strategy are crucial considerations for long-term studies of plankton dynamics, determining the confidence with which statistically significant ecological changes will be detected and attributed to the drivers. We analyze the spatiotemporal variability of Baltic Sea zooplankton using historical data from various monitoring programs. We determined the scales of highest temporal and spatial variability, focusing on distances and time periods shorter than 100 km and 90 days. Analyses were made for different hydrological regions (small lagoons, larger gulfs, Baltic Proper), and with differently sized zooplankton groups (large and small copepods and cladocerans). Temporal variability in one place usually exceeded the synoptic spatial variability. Smaller, faster reproducing cladocerans varied more in abundance than larger, slower reproducing copepods. The variability increased with time and distance between samples. For copepods, a temporal cycle of 60–70 days emerged, implying the need for sampling after every 20–23 days. For cladocerans, we propose 14 days as a sampling interval, the time during which the average abundance differences between samples doubled. In the future, these guidelines should be used in conjunction or as an addendum to HELCOM (Helsinki Commission) guidelines, to optimize the zooplankton sampling effort in the Baltic Sea.

Description: Diazotrophy-related studies in the North Atlantic have largely focused on its western tropical area, leaving the subtropics and the east undersampled. We studied the longitudinal distribution of Trichodesmium , UCYN-A, UCYN-B, the putative Gammaproteobacterium -24774A11 and Richelia (Het1) along 24.5°N, using quantitative polymerase chain reaction on different size fractions (10, 10–3 and 3–0.2 µm) and additional filament counts for Trichodesmium. Trichodesmium was the most abundant phylotype, followed by UCYN-A, -24774A11 and Het1, with maximum abundances of 8.8 x 10 5 , 2.0 x 10 5 , 3.3 x 10 3 and 3.4 x 10 2 nifH copies L –1 , respectively, whereas UCYN-B was mostly undetected. A clear shift in the diazotroph community was observed at ~30°W, coinciding with the transition between the North Atlantic Subtropical Gyre boundary and inner core. This transition zone divided the transect into an eastern half dominated by UCYN-A and western half dominated by Trichodesmium and -24774A11. -24774A11 was only detected in the 10–3 µm fraction, suggesting their association with larger microbes or aggregates. Our results indicate that typical size fractionation by 10 µm is not optimal for reconciling diazotroph phylotypes to N 2 fixation rates and that non-cyanobacterial diazotrophs may contribute importantly to bulk diazotrophic activity in the western subtropical North Atlantic.

Description: We investigated the effect of diel variation in cell properties of picoplankton prey and the intrinsic grazing rhythm of heterotrophic nanoflagellates (HNF) on the diel periodicity of HNF grazing. To achieve this aim, 2-point dilution experiments with short incubation time were performed over 24 h in the South China Sea and Hong Kong (HK) coastal water to assess the prey cell properties and HNF grazing. Fluorescently labeled bacteria (FLB) were added for inference of the intrinsic diel rhythm of HNF feeding. Cell sizes of in situ prey at both study sites and the carbon to nitrogen molar ratio (C:N) of picoplankton prey in HK coastal water exhibited an increasing trend during the day, indicating cellular growth of prey during the period. Higher grazing rates on FLB during the day compared with night at both study sites suggested higher intrinsic HNF feeding during the day. Diel patterns of HNF grazing with increasing trend during the day and decreasing trend at night were observed for all in situ prey at both study sites. Statistical analysis suggests that both time of day and prey C:N could have significant effects on the diel variation of HNF feeding.

Description: The genus Pseudocalanus (Copepoda, Calanoida) is among the most numerically dominant copepods in eastern North Pacific and Pacific-Arctic waters. We compared population connectivity and phylogeography based on DNA sequence variation for a portion of the mitochondrial cytochrome oxidase I gene for four Pseudocalanus species with differing biogeographical ranges within these ocean regions. Genetic analyses were linked to characterization of biological and physical environmental variables for each sampled region. Haplotype diversity was higher for the temperate species ( Pseudocalanus mimus and Pseudocalanus newmani ) than for the Arctic species ( Pseudocalanus acuspes and Pseudocalanus minutus ). Genetic differentiation among populations at regional scales was observed for all species, except P. minutus . The program Migrate-N tested the likelihood of alternative models of directional gene flow between sampled populations in relation to oceanographic features. Model results estimated predominantly northward gene flow from the Gulf of Alaska to the Beaufort Sea for P. newmani . Model scenarios that allowed bidirectional gene flow between sampled populations gave the best Bayesian predictions for P. acuspes , P. mimus and P. minutus . Under current warming trends, biogeographical boundaries and barriers for Pseudocalanus species may shift, allowing habitat range expansion or contraction and resulting in altered population connectivity between Arctic and sub-Arctic populations.

Description: High Calanus finmarchicus abundances were recorded in wintertime in Vestfjorden, close to the main cod breeding grounds off Lofoten and Vesterålen, northern Norway. The mean abundance for locations with water depth 〉500 m was ~37000 ind. m –2 (range: 26700–49000 ind. m –2 ). To our knowledge, this is the first report of massive overwintering of C. finmarchicus on the Norwegian shelf. Because of the observed size and location of this population, we argue that local overwintering on the northern Norwegian shelf can contribute significantly to sustain a C. finmarchicus population on the shelf during the period of first feeding for cod larvae. This is supported by a particle tracking model.

Description: Trichodesmium , a colonial cyanobacterium typically associated with tropical waters, was observed between January and April 2014 in the western English Channel. Sequencing of the heterocyst differentiation ( hetR ) and 16S rRNA genes placed this community within the Clade IV Trichodesmium , an understudied clade previously found only in low numbers in warmer waters . Nitrogen fixation was not detected although measurable rates of nitrate uptake and carbon fixation were observed. Trichodesmium RuBisCO transcript abundance relative to gene abundance suggests the potential for viable and potentially active Trichodesmium carbon fixation. Observations of Trichodesmium when coupled with a numerical advection model indicate that Trichodesmium communities can remain viable for 〉3.5 months at temperatures lower than previously expected. The results suggest that Clade IV Trichodesmium occupies a different niche to other Trichodesmium species, and is a cold- or low-light-adapted variant.

Description: We compare the long-term and seasonal patterns of abundance and phenology of the cyclopoid copepod Oithona similis at the L4 site (1988–2013) in the North Atlantic and at the LTER-MC site (1984–2013) in the Mediterranean Sea to investigate whether high temperature limits the occurrence of this species with latitudinal cline. The two sites are well suited to testing this hypothesis as they are characterized by similar chlorophyll a concentration (Chl a ) but different temperature [sea surface temperature (SST)]. The abundance of O. similis at L4 was ~10 times higher than at LTER-MC. Moreover, this species had several peaks of abundance during the year at L4 but a single peak in spring at LTER-MC. The main mode of temporal variability in abundance was seasonal at both sites. The abundance of O. similis was negatively correlated with SST only at LTER-MC, whereas it was positively correlated with Chl a at both sites. Oithona similis had a temperature optimum between 15 and 20°C reaching maximum abundance at ~16.5°C at LTER-MC, but showed no Chl a optimum at either site. We conclude that the abundance of O. similis increases with prey availability up to 16.5°C and that temperature 〉20°C represents the main limiting factor for population persistence.

Description: The deep-sea anoxic brines of the Red Sea comprise unique, complex and extreme habitats. These environments are too harsh for metazoans, while the brine–seawater interface harbors dense microbial populations. We investigated the adjacent pelagic fauna at two brine pools using net tows, video records from a remotely operated vehicle and submerged echosounders. Waters just above the brine pool of Atlantis II Deep (2000 m depth) appeared depleted of macrofauna. In contrast, the fauna appeared to be enriched at the Kebrit Deep brine–seawater interface (1466 m).

Description: We investigated how fast the 15 N and 13 C stable isotope signature of copepods changes and how fast the isotope signals in microzooplankton and copepods change in the field. In experiments C1, copepodites of the copepod Acartia tonsa raised on Rhodomonas baltica were switched to a diet of Oxyrrhis marina and the 15 N and 13 C signature followed for 8 days. The 15 N increased by 0.18 ± 0.03 day –1 in animals fed O. marina while it decreased by 0.14 ± 0.02 day –1 with a R. baltica diet and equilibrium was reached after 5 days. Enrichment of nitrogen 15 N ( 15 N) was 2.4 ± 0.26 with R. baltica and 4.1 ± 0.38 with O. marina as food, and corresponding factors for carbon ( 13 C), 1.7 ± 0.38 and 2.3 ± 0.30. 15 N in the field, using seston 0–10 µm as the baseline, ranged from 2.4 to 3.1 for copepods and 1.0 to 1.6 for the microzooplankton (10–90 and 90–200 µm). Nitrate input resulted in low 15 N for copepods (2.4) and high for microzooplankton (1.4–1.6), indicating that copepods were feeding indiscriminately on phyto- and microzooplankton after the event. The second period showed higher 15 N (3.1) and a higher trophic level of copepods, but lower of microzooplankton (1.0–1.1).

Description: Variations in mesozooplankton abundance and community structure in response to water quality improvement and to variations in hydro-climatic conditions were studied in the inner estuary of Bilbao from 1998 to 2011. A process of recolonization was observed with a marked increase in copepod abundance and a smaller increase in the abundances of appendicularians, meroplanktonic bivalves and gastropods, at the expense of the decrease in groups of gelatinous predators. Within the copepods, an initial phase in this recolonization occurred through an increase in neritic copepods. However, in a second phase, the non-indigenous species Acartia tonsa and Oithona davisae increased and became dominant. In the last 2 years, a third non-indigenous species Pseudodiaptomus marinus was observed in lower densities, and Calanipeda aquaedulcis abundance increased becoming co-dominant with A. tonsa . All non-indigenous copepod species had summer/autumn peaks and were characteristic of brackish conditions, which suggests that unsaturated ecological niches in brackish waters were important for the settlement of non-indigenous species. Pollution abatement allowed for zooplankton recolonization, but variations in community structure, both at the broad taxonomic group and at the copepod species level, were greatly influenced by hydro-climatic factors, temperature being the single best explanatory variable of zooplankton variations.

Description: Among the theories conceived to explain the natural world using a few and relatively simple assumptions and parameters, the Metabolic Theory of Ecology (MTE) stands out. It assumes metabolism as the biological pacemaker of processes from molecular to evolutionary scales. The MTE basically uses the old, well-known allometric scaling relations among metabolism, organism mass and temperature to explain the structure and function of cells, individuals, populations, communities and ecosystems. Although some predictions made by the MTE are remarkably accurate, especially for simple processes and when broad ranges of individual biomass and temperature are considered, like any general model the MTE sacrifices precision and thus realism to achieve its generality. MTE has logical inconsistencies and unexplained discrepancies with empirical data that are especially important regarding pelagic marine ecosystems in high latitudes. Interspecific and intraspecific differences in the responses of zooplankton metabolic processes to identical temperature and individual-mass changes, differences not accounted for by the MTE, could result in crucial deviations in ecosystem energy balance and biogeochemical cycles, implying low predictive value. The objective of this paper is to draw attention to the applicability and limits of the MTE as a predictive tool, especially for complex, second-order ecological processes.

Description: The marine ecological research of Dr Miquel Alcaraz has spanned nearly four decades, and his analyses and insights have strongly influenced many of us in ocean sciences, particularly through the seminal 1980 paper that stimulated new research and led to significant advances in our understanding of behavioral plasticity in zooplankton. That behavioral plasticity must exist as a consequence of selective pressures in the environment. Advances in water column observational approaches have resulted in the physical and biological characterization of the upper water column at centimeter vertical scales, and have shown that thin layers (〈3 m in thickness) of phytoplankton are reasonably common and can persist for many hours. This paper describes vertical patterns of distribution of both particulate material (represented by chlorophyll fluorescence) and dissolved material [represented by the fluorescence of chromophoric dissolved organic matter (CDOM)] within Monterey Bay, CA, in August–September, 2005. In the vertical profiles that possessed distinct phytoplankton thin layers, CDOM fluorescence commonly displayed steep vertical gradients at one or more depths within the water column, with ~40% of those CDOM gradients offset from the phytoplankton layer by 1–5 m. These offsets between dissolved and particulate matter illustrate the complex structure of the feeding environment for zooplankton.

Description: Copepods are important grazers on phytoplankton and contributors to carbon export, but their role is poorly understood in the Western Antarctic Peninsula (WAP), a region of high productivity and rapid climate warming. We conducted grazing and egestion experiments with large, dominant copepods each January from 2012 to 2014. We found higher gut evacuation rates ( k ), initial gut pigment and ingestion rates ( I ) for Calanus propinquus and Rhincalanus gigas compared with Calanoides acutus . Since k and I linearly increased with chlorophyll a for most species, ingestion rates were 4–70 times greater in more productive coastal regions than offshore, slope waters. Copepods have a low grazing impact on phytoplankton biomass (〈1%) and productivity (1%, up to 11%) compared with the dominant WAP macro- and microzooplankton. Egestion rates were high (0.8–37.3 µgC ind. –1 day –1 ); however, ~58% of fecal pellets are retained in the upper water column. Daily carbon rations of ~1% indicated feeding on other carbon sources (protozoans and metazoans) to meet metabolic demands. However, during a coastal phytoplankton bloom, daily C rations increased to up to 13%, indicating increased reliance on phytoplankton. Future changes in the WAP plankton community may affect food web carbon flow and export.

Description: The marine planktonic diatom order Thalassiosirales are used as climate proxies with reported characteristic temperate and Polar species. However, a systematic analysis of the genetic variability and species limits is lacking. Here, we combined molecular and morphological approaches to identify Thalassiosirales from 7 Beaufort Sea sea-ice samples and 27 water-column samples collected from across the Canadian Arctic at different times of the year and the Scotia Shelf, North Atlantic in spring. While many species were found in both oceanic regions, there were two distinct Thalassiosirales communities: the first from sea ice and ice-influenced water columns and the second Arctic late summer and North Atlantic spring communities. Thalassiosirales formed genetically coherent Boreal-Arctic meta-communities, inclusive of intra-specific variants. Genetic similarity between the Arctic and North Atlantic meta-communities would be reinforced by counter-clockwise dominant surface current systems. Local patterns of spatial and temporal distributions of planktonic thalassiosiroids were best explained by latitudinal gradients and phosphate and silicate concentrations, suggesting that climate-mediated changes in Arctic and North Atlantic hydrography could have a marked effect on species distribution. Our analysis also revealed a need for taxonomic revision of several species complexes reported from the region before they can be reliably used as bio-indicators.

Description: Mixotrophic protists combine photo-autotrophy and phago-heterotrophy. Even though only in oligotrophic systems mixotrophs overcome strictly autotrophs, they are represented all along the trophic spectrum. We hypothesize that lakes with different chlorophyll a (Chl a ) and dissolved organic carbon (DOC) concentrations harbor different assemblages of mixotrophs. We examined the composition of mixotrophs in 24 lakes in two Patagonian regions (Argentina), with a range of conditions of Chl a and DOC. The categorization of a predominant type of nutrition in each mixotroph was based on published evidence. Additionally in four lakes we analyzed the bacterivory activity of mixotrophs by performing ingestion experiments. We found potentially mixotrophic taxa in all lakes, however the assemblages varied depending on their trophic-DOC features. Primarily heterotrophic mixotrophs (e.g. chrysophytes) attained a higher proportion in oligotrophic lakes, declining towards higher trophic status. In contrast, primarily autotrophic mixotrophs (e.g. cryptophytes) dominated in eutrophic systems. Mixotrophs actively ingested bacteria in all experiments, however cell-specific grazing rates were higher in oligotrophic lakes. Mixotrophs accounted for more than half of total flagellate grazing in oligo-mesotrophic environments. Our results suggest that there is a replacement of mixotrophic taxa along a gradient of increasing trophic state, from primarily heterotrophic to primarily autotrophic mixotrophs, whereas their relative contribution to total flagellate grazing decreases.

Description: The tolerance traits of zooplankton co-occurring with toxic cyanobacteria are poorly understood. To evaluate the tolerance mechanisms of Notodiaptomus iheringi , an abundant tropical copepod coexisting with toxic cyanobacteria blooms across South America, grazing and egg production rates were measured in the laboratory using single- and mixed-food diets containing either a microcystin producing (MC+) or lacking (MC–) strain of Microcystis with Cryptomonas as control food. Pure Microcystis diets resulted in no egg production. In mixed-food diets, the MC+ strain had no effect, while the MC– strain increased egg production relative to the controls. Clearance rates were highest on Cryptomonas , intermediate on the MC– strain and lowest on the MC+ Microcystis . Notodiaptomus iheringi selectively avoided Microcystis in mixed diets. Total grazing was not inhibited by Microcystis abundance as long as Cryptomonas was available. Egg production was unaffected by the ingestion of Microcystis but depended on the abundance and ingestion of Cryptomonas . Consequently, both selective grazing on nutritious food and the detoxification of ingested cyanobacteria may be critical traits for N. iheringi to tolerate blooms of toxic cyanobacteria. The results contrast with reports showing strong negative effects of Microcystis on zooplankton fitness and highlight the diversity in zooplankton tolerance traits.

Description: Sterols are essential dietary nutrients for most aquatic invertebrates and an inadequate dietary sterol supply can adversely influence various life history traits. The sterol composition of microalgae is highly diverse, which may have consequences for zooplankton nutrition because sterols differ in their nutritional quality. We investigated the sterol content and composition of a number of eukaryotic freshwater microalgae belonging to different classes. The aim was to increase our knowledge of sterol profiles and biomass-related sterol concentrations in freshwater algae and to reveal sterol-mediated nutritional constraints that consumers potentially have to cope with. Our data suggest that zooplankton consumers are potentially confronted with low dietary sterol concentrations, depending on the phytoplankton species composition. Total sterol concentrations were found to be low especially in representatives of the genera Chlamydomonas , Choricystis and Gomphonema . Moreover, our findings suggest that the predominance of certain chlorophytes ( Scenedesmus , Ankistrodesmus and Monoraphidium ) in phytoplankton assemblages may cause sterol metabolic constraints in consumers because of high dietary concentrations of nutritionally inadequate 7 and 7,22 sterols. The sterol profiles we present here can serve as reference profiles for particular algae to assess their biochemical food quality for freshwater consumers and provide a first step toward assessing the significance of sterol-mediated nutritional constraints under field conditions.

Description: We addressed how the extreme environmental conditions of the Red Sea impact or alter patterns of vertical distribution and vertical migration of five euphausiid species that are known from other oceans. Euphausia diomedeae was abundant and performed diel vertical migration (DVM) from 〉200 m in daytime to 〈100 m at night, similar to its pattern in other ocean regions. Euphausia sibogae and Euphausia sanzoi also showed consistent patterns of DVM across their ranges in the Red Sea and elsewhere. Two species, Stylocheiron affine and Stylocheiron abbreviatum , did not exhibit DVM. DNA barcode sequences for mitochondrial cytochrome oxidase I (COI) were used to confirm species identifications for four species (no previous barcode data exist for E. sanzoi ). COI sequence differences averaged 2.8% (SD 3.1%) within species and 16.6% (SD 0.7%) between species, similar to previous studies of euphausiids. Red Sea specimens of S. affine matched morphological descriptions of a western equatorial form and differed 14% from Atlantic and Pacific specimens, suggesting possible cryptic species-level variation within this taxon. Widely distributed species of zooplankton may exhibit broad tolerance ranges for key environmental variables, and have considerable potential to adapt to variable and changing conditions across their geographic range.

Description: Morphological species delineation within the cladoceran genus Daphnia is often blurred by phenotypic plasticity, frequently associated with predator presence. We focused on phenotypic variation and genetic relatedness of nine Chinese populations of Daphnia ( Ctenodaphnia ) similoides sinensis . The typical form of this taxon inhabits fishless pools, a habitat characteristic for most Ctenodaphnia species. However, we demonstrate that D. similoides sinensis also occurs in large lakes where it coexists with multiple predators (planktivorous fish, Cyclops and Leptodora ). Individuals from such lakes differ substantially phenotypically (to the extent they have been considered a distinct undescribed species), being significantly smaller and exhibiting prominent recurvate helmets. Genetic variation of D. similoides sinensis , based on mitochondrial cytochrome c oxidase subunit 1 sequences, was low; all Chinese populations studied, regardless of phenotype or environment, apparently recently expanded from a single mitochondrial lineage. This is consistent with other Daphnia species in which body and helmet size are phenotypically plastic traits responding to predator presence. The general phenotype of lake populations of D. similoides sinensis is strikingly similar to North American Daphnia retrocurva , an unrelated species of the subgenus Daphnia . We presume that similar predation pressures in lake habitats triggered convergent evolution of body shapes in both species.

Description: We describe the stage-specific mortality, survival and recruitment patterns for a cyclopoid and a calanoid copepod through the use of vertical life table analysis. Over a 2-year period in Lake Saint-Jean, Québec, Canada, we show that during the growing season, Diacyclops bicuspidatus thomasi  (May–June) had its highest daily mortality rate during metamorphosis (N6–C1), while Leptodiaptomus ashlandi (June–August) had its highest daily mortality rates during the naupliar stages. Patterns in daily recruitment rates differed between the species with the early stages of L. ashlandi experiencing a greater population loss during the growing season. Recruitment of D. bicuspidatus thomasi was controlled by reproduction, while recruitment of L. ashlandi was governed by reproduction and mortality/survival although also influenced by seasonal patterns of phytoplankton biomass. A multiple regression model suggested that cannibalism influenced the mortality of the early stages of D. bicuspidatus thomasi during their growing season. Temperature, competition and cannibalism influenced the mortality of the early stages of L. ashlandi . However, there are interannual differences likely due to interannual difference in seasonal patterns of phytoplankton biomass. Our study emphasizes the importance of interactions between bottom-up and top-down controls governing copepod population dynamics.

Description: We investigated the ontogenetic feeding migration of Euphausia pacifica within the epipelagic zone (0–200 m depth) in the East Sea (Japan Sea) during the autumn using stable isotope analysis (SIA). SIA indicated that the ontogenetic stages of E. pacifica from egg to adulthood were closely related to the vertical profile of three cell-sized fractions of particulate organic matter (POM): pico- (〈2 μm), nano- (2–20 μm) and microPOM (20–200 μm). From nauplius to metanauplius, the 13 C and 15 N values of larvae were slightly lower than those of eggs, indicating the consumption of yolk reserves in the eggs. From calyptopis I, the first feeding stage to furcilia I, larvae mainly distributed in the surface layer and fed mainly on surface layer picoPOM. During developmental progress from CI to adulthood, the residence depth range deepened from the surface to deeper layers with the exception of juveniles, which stayed in the surface layer. The ontogenetic feeding migration pattern of E. pacifica may reduce intraspecific competition for food items, resulting in coexistence of the ontogenetic stages in the East Sea during the autumn.

Description: On 24 June 2013 and on 20 May 2014 near Joga-shima, Japan, we deployed two microstructure profilers, TurboMAP-L and the TurboMAP-Glider, to obtain vertical and quasi-horizontal measurements of turbulence and chlorophyll a fluorescence. The deployments were clustered at two stations (one near the coast and another further offshore). Both instruments carried two fluorescence sensors with different spatial resolutions: the light-emitting diode sensor (LED) with a spatial resolution of 2 cm and the laser sensor with a spatial resolution of 2 mm. We observed that vertical and quasi-horizontal averages as small as centimeters are statistically equal, regardless of the sensor. However, using the laser sensor, quasi-horizontal sampling showed higher fluorescence intensity peaks than vertical sampling at millimeter scale. We hypothesized that layered structures in the upper ocean and the existing shear tend to orient chlorophyll a patches horizontally. Also, we found that large patches tend to have a random distribution in the water column, while small patches are likely to be found near large patches in a non-random distribution. The understanding of the spatial distribution of fluorescence patches is an important step forward to achieve a reliable picture of phytoplankton microscale. Additionally, it provides observational evidence for those modeling phytoplankton microstructure.

Description: We compared and evaluated the performance of a Continuous Automatic Litter and Plankton Sampler (CALPS) against the traditional ring net vertical haul. CALPS is a custom-made semi-automatic sampler, which collects water using a pump system at a single depth along a predetermined transect as the ship sails. CALPS underestimated species abundance compared to the ring net by a factor 1.61, but both datasets illustrated a similar species composition, community size structure and good agreement in the spatial distribution of abundance. Our analysis suggests that avoidance of the CALPS is likely to be the main factor responsible for the observed difference in sampling efficiency, but other factors, such as depth, area sampled and zooplankton patchiness, are also likely to play their part. We conclude that whilst the CALPS is not suitable for investigations that require accurate measures of abundance, it is an ideal tool to identify and quantify changes in plankton communities and diversity. A particular advantage over more traditional vertical sampling methods is that it can be integrated within existing multidisciplinary surveys at little extra cost, thus making the CALPS particularly valuable as part of integrated monitoring programmes to underpin policy areas such as the EU Marine Strategy Framework Directive.

Description: The ratio of elements such as carbon:nitrogen:phosphorus (C:N:P) in phytoplankton is known to vary substantially within single isolates and across environmental gradients. In addition, C:N:P is known to vary throughout the day due to diel patterns in nutrient acquisition and storage. It has been hypothesized that small phytoplankton such as marine cyanobacteria have relatively invariable elemental ratios during a 24 h period, whereas larger phytoplankton have a greater capacity to store elements and thus a wider diel range of C:N:P. To test this hypothesis, we examined diel variability in cellular C:N:P, using a chemostat culturing system, for one of the most abundant marine cyanobacteria, Synechococcus (WH8102) during two 24 h periods. The cellular C quota nearly doubled during the 14 h light period and was subsequently reduced during the dark period. The cellular N quota also varied considerably, whereas the P quota remained relatively stable. These daily changes in elemental quotas led to highly variable C : N cell and C : P cell . Furthermore, the magnitude of variability in cellular elemental stoichiometry of Synechococcus was positively related to the growth rate. We constructed a model to test the extent to which variation in C : N cell and C : P cell is related to reserve carbon accumulation and depletion over each light–dark cycle. Results imply that, in addition to growth-related respiratory losses, Synechococcus also purges excess C during the dark period in order to maintain a nutritive balance within cells. Our data suggest that diel variation in C : N cell and C : P cell of Synechococcus is of the same order of magnitude as stoichiometric variation within plankton communities between major ocean environments.

Description: The view that current species inventories (lists of binomial species) of planktonic protists significantly underestimate true diversity is widespread. However, the existence of synonyms in species lists, due to repeated descriptions of the same form, and polymorphism in which different "forms" or life-history stages are but one species, are generally underappreciated. Recent studies have shown that synonyms are probably common, significantly inflating species lists, and taxa previously thought to be distinct species based on morphology have been revealed to be simple variants of single species. Given here are examples from among tintinnid ciliates, dinoflagellates and foraminifera. The commonality of synonyms and the phenomena of both polymorphism and crypticism underscore a need for taxonomic reviews as well as species-focused studies involving molecular biologists, taxonomists and ecologists.

Description: Functional trait studies are proliferating in plankton ecology, especially studies analysing body size. Yet, empirical studies comparing species versus individual-size structuring are scarce. Here, we conducted a comparison of copepod species-based and individual-size-based community structuring in the East China Sea, and found that: (i) Species, species-based nominal-size and individual-size distributions exhibit very different patterns, and that juveniles (neglected in species counts due to limitation of recognition) tend to dominate in a certain size range. (ii) Species-based structuring is more strongly shaped by physical conditions, while individual-size-based structuring is more strongly shaped by food availability. (iii) Despite these differences, the congruence (i.e. degree of match) between species-based and individual-size-based structuring remains statistically significant. Finally, we tested intrinsic factors potentially explaining the degree of mismatch (i.e. species richness and proxies for: size partitioning of species without accounting for intraspecific variability, intraspecific variability without accounting for ontogeny and ontogeny). (iv) The frequency of juveniles (proxy for ontogeny) is the only intrinsic factor significantly explaining the mismatch between species and size structuring, highlighting the problem of the classic species-based analysis that unavoidably neglects juveniles in the species counts. These results support individual-size as a useful complementary descriptor to species-based studies.

Description: Plankton patch dynamics strongly influences rates of trophic transfer and many ecological processes, yet patchiness is poorly described, especially on fine-scales (cm to 10s of m). We deployed the In Situ Ichthyoplankton Imaging System beneath the Mississippi River plume to measure horizontal distributions of zooplankton across three depth zones (10, 25 and 35 m). We found that larval fishes were significantly aggregated in the shallow (10 m) and deep (35 m) waters, but were randomly distributed at 25 m. In contrast, gelatinous organisms were typically randomly distributed, but shrimps and chaetognaths showed a strong degree of aggregation on fine-scales at all depths. One deep-water transect (35 m) was marked by a ~1.5-km zone of hypoxia (O 2  〈 2.0 mg L –1 ) that contained no larval fishes and few zooplankton that were otherwise abundant throughout the sampling area. The exceptions to this trend were the shrimps and cumaceans, which aggregated within the hypoxic zone, suggesting they were able to tolerate those conditions. Correlation analyses of zooplankton showed differing relationships of zooplankton abundance to physical variables among sampling depths; however, zooplankton were consistently positively correlated with one another, indicating the presence of taxonomically heterogeneous patches.

Description: As an introduction to the following Themed Section on the significance of planktonic organisms to the functioning of marine ecosystems and global biogeochemical cycles we discuss the ramifications size imparts on the biology of plankton. We provide examples of how the characteristics of these microscopic organisms shape plankton population dynamics, distributions, and ecosystem functions. Key features of the marine environment place constraints on the ecology and evolution of plankton. Understanding these constraints is critical in developing a mechanistic understanding and predictive capacity of how planktonic ecosystems function, render their capacities in terms of biogeochemical cycling and trophic transfer, and how planktonic communities might respond to changing climate conditions.

Description: Recent studies have analysed valuable compilations of data for the size-scaling of phytoplankton traits, but these cannot be employed directly in most large-scale modelling studies, which typically do not explicitly resolve the relevant trait values. Although some recent large-scale modelling studies resolve species composition and sorting within communities, most do not account for the observed flexible response of phytoplankton communities, such as the dynamic acclimation often observed in laboratory experiments. In order to derive a simple yet flexible model of phytoplankton growth that can be useful for a wide variety of ocean modelling applications, we combine two trade-offs, one for growth and the other for nutrient uptake, under the optimality assumption, i.e. that intracellular resources are dynamically allocated to maximize growth rate. This yields an explicit equation for growth as a function of nutrient concentration and daily averaged irradiance. We furthermore show how with this model effective Monod parameter values depend on both the underlying trait values and environmental conditions. We apply this new model to two contrasting time-series observation sites, including idealized simulations of size diversity. The flexible model responds differently compared with an inflexible control, suggesting that acclimation by individual species could impact models of plankton diversity.

Description: Processes governing productivity at the base of the pelagic food web of the southern Indian Ocean are influenced primarily by physical–chemical conditions with implications for the structure and function of the entire pelagic food web. Here, we report observations along a great circle transect from Cape Town, South Africa, to Broome in north western Australia. Primary production was tightly linked to water column stability and nutrient availability, with high productivity (1144 mg C m –2 day –1 ) in the sub-tropical convergence zone, and falling off by an order of magnitude in the sub-tropical gyre and tropical waters off northwest Australia. Primary production was largely confined to the GF/F fraction (GF/F 〉75% of total production and usually much higher) and the photosynthetic parameters showed adaption to the prevailing light levels. Bacterial production ranged from 19 to 155 mg C m –2 day –1 . No relationships were found between bacterial production and primary production or phytoplankton biomass and bacterial production seem more related to the state of the phytoplankton community as high bacterial production was observed in a post-bloom situation. The average BP:PP ratio was 31% (range 3.5–71%).

Description: Disko Bay is a highly productive inlet at the border between high- and low-arctic sectors of west Greenland. The physical/chemical conditions in the bay have changed during recent decades, specifically during an inflow event in 1996–1997 that led to increased deep-water temperatures. To further understand the consequences of physical changes to the plankton community in the bay, we examine findings from a field study on mesozooplankton and fish larvae in the areas of Disko Bay and Disko Bank carried out in 1997. We sampled 31 stations over 5 days along four transects and assessed horizontal and vertical distribution patterns, community composition and plankton trophodynamics. Plankton abundance was enhanced near-coast and across the pycnocline, and communities differed between regions. Polar cod ( Boreogadus saida ) and the sandeel ( Ammodytes sp.) were among the abundant fish larvae. Productivity/growth estimates of key species of copepods and fish larvae showed no apparent relationship to food availability; they reached weight specific values of ~6% day –1 for copepods and ~14% day –1 for fish larvae. Overall, we found a rich and dynamic plankton community, strongly influenced by the complex hydrography of the area. Thus, changes in physical characteristics of the bay could threaten the resilience of its ecosystem.

Description: The current melting of glaciers and ice sheets is a consequence of climatic change and their turbid meltwaters are filling and enlarging many new proglacial and ice-contact lakes around the world, as well as affecting coastal areas. Paradoxically, very little is known on the ecology of turbid glacier-fed aquatic ecosystems even though they are at the origin of the most common type of lakes on Earth. Here, I discuss the consequences of those meltwaters for planktonic organisms. A remarkable characteristic of aquatic ecosystems receiving the discharge of meltwaters is their high content of mineral suspensoids, so-called glacial flour that poses a real challenge for filter-feeding planktonic taxa such as Daphnia and phagotrophic groups such as heterotrophic nanoflagellates. The planktonic food-web structure in highly turbid meltwater lakes seems to be truncated and microbially dominated. Low underwater light levels leads to unfavorable conditions for primary producers, but at the same time, cause less stress by UV radiation. Meltwaters are also a source of inorganic and organic nutrients that could stimulate secondary prokaryotic production and in some cases (e.g. in distal proglacial lakes) also phytoplankton primary production. How changes in turbidity and in other related environmental factors influence diversity, community composition and adaptation have only recently begun to be studied. Knowledge of the consequences of glacier retreat for glacier-fed lakes and coasts will be crucial to predict ecosystem trajectories regarding changes in biodiversity, biogeochemical cycles and function.

Description: Within models, zooplankton grazing is typically defined as being dependent on total prey concentration, with feeding selectivity expressed only as a function of prey size. This behavior ignores taxonomic preferences shown by the predators and the capacity of some zooplankton to actively select or reject individual prey items from mixtures. We carried out two model experiments comparing impacts of zooplankton displaying passive and active selection, which resulted in contrasting dynamics for the pelagic system. Passive selection by the grazer resulted in a top down control on the prey with a fast turn-over of nutrients. Active selection, on the other hand led to a bottom-up control, with slower nutrient turnover constraining primary production by changing the system toward export of particulate matter. Our results suggest that selective feeding behavior is an important trait, and should be considered alongside size and taxonomy when studying the role of zooplankton impact in the ecosystem.

Description: Larvae of the western rock lobster ( Panulirus cygnus ) that occur in the south-east Indian Ocean offshore of Western Australia have been found to be in poorer nutritional condition in anticyclonic compared with cyclonic mesoscale eddies. The reason for this is unknown, but culture-based experiments have shown that diet composition and water temperature are key determinants of phyllosoma health and survival. Whether differences in prey composition are the cause of poor phyllosoma condition in situ was tested in the present study by high-throughput sequencing of 18S rDNA amplified from the gut contents of 48 lobster larvae from two cyclonic and two anticyclonic eddies. We determined that phyllosoma prey composition did not vary significantly between anticyclonic and cyclonic eddies and that any variation was at the level of sample site (permutational multivariate analysis of variance F 2,35 = 5.12, P 〈 0.0001). We therefore reject the hypothesis that prey composition alone during the late larval phase determines larval condition. This indicates that the competing hypotheses are more likely: for example, that eddy water temperature is responsible for reducing the condition of phyllosomas directly, or indirectly by impoverishing the nutritional value of their foodweb.

Description: Plankton fractions from a saline lake in Argentina were studied using a combined trophic marker approach. A strong seasonality of biomarkers was characteristic for the different fractions, particularly the variations in the 18:4( n – 3) and 20:4( n – 3) fatty acids and the 13 C values. The primary production in the lake was mainly driven by diatoms, reflected by the close relation of 13 C, chlorophyll a and diatom fatty acid markers. The combined approach of 13 C and 20:4( n – 3) enabled processes in the lipid metabolism of the copepod Boeckella poopoensis to be inferred. The polyunsaturated fatty acid 22:6( n – 3) and the 15 N separated the trophic levels in this food web with copepods at higher trophic level. Nutritional stress and omnivory of B. poopoensis partially explained the 15 N variations in mesozooplankton. The 15 N signature was probably driven by cyanobacteria in the microplankton and by microbial processes in the nanoplankton fraction. Warmer temperatures may favour the saturation of microalgae fatty acids and the abundance of plankton groups richer in saturated fatty acids. The tendency to unsaturation in mesozooplankton at colder temperatures was probably influenced by diet and metabolic requirements. Future temperature increase and eutrophication-like processes may increase the importance of cyanobacterial and bacterial markers under climate change scenarios.

Description: We investigated the composition of the smallest size fraction (〈3 µm) of eukaryotic plankton communities of five pools located in the Rancho Hambre peat bog in Argentinean Tierra del Fuego with an IlluminaHiSeq massive sequencing approach applied to the v9 region of the eukaryotic SSU rRNA gene. Communities were generally dominated by chrysophytes, with a good representation of Perkinsea and Cercozoa clade NC-10. A community composition analysis performed using GUniFraC separated minerotrophic and ombrotrophic sites, reflecting perfectly the classification of the sites based on environmental data. However, this separation disappeared when more weight was given to abundant phylotypes, suggesting that subordinate phylotypes were responsible for site discrimination. The 5% best indicators for, respectively, minerotrophic and ombrotrophic environments were searched using an IndVal analysis. Among these, autotrophic taxa were more common in minerotrophic environments, whereas mixotrophic taxa represented best ombrotrophic water bodies. However, the ecological traits of many taxa have still not been determined, and still needs to be investigated for a better understanding of freshwater systems ecology.

Description: Molecular analyses of small-sized copepods (≤1 mm) generally involve the complete destruction of the specimens. Consequently, incongruences between the molecular and morphological results cannot be investigated since no specimen vouchers remain. The present study provides a modified column-based DNA extraction method to retain the exoskeleton of the specimen and thus, to enable molecular and morphological analysis of the same specimens. The method has been tested on ethanol preserved specimens of nine pelagic copepod genera.

Description: The effect of P-limitation and viral infection on prokaryotic production (PP) and respiration was determined during the early winter in the Northwestern Mediterranean Sea. Addition of P enhanced prokaryotic growth and viral production. The concentrations of total dissolved nitrogen increased linearly with decreasing viral abundance, PP and growth efficiency across treatments. Our data suggest that microbial growth can also be P-limited in winter and that viral lysis products can be readily used by prokaryotes depending on nutrient availability.

Description: Collection of zooplankton is done using a wide array of instrumentation. To ensure the long-term value of zooplankton data, metadata about what the data are and when, where, and how the data were collected, plus the use of a domain-specific, controlled vocabulary is essential. It is especially important to use a controlled "deployment" vocabulary when plankton nets are used to collect data, and here we present a vocabulary of net deployment terms.