ALBERT

Description: Microzooplankton have received increased attention as an important trophic link between the microbial loop and calanoid copepods. On the basis of food size spectra overlap in some microzooplankton groups and calanoid copepods, however, such microzooplankton could function as competitors rather than as food for calanoid copepods (intraguild prey). Mixotrophic flagellates presumably represent a link between the microbial loop and the micro and mesozooplankton. We investigated the effects of microzooplankton and mixotrophy by altering the presence of a heterotrophic dinoflagellate and of a mixotrophic nanoflagellate in artificial food webs with calanoid copepods as terminal consumers. Overall system productivity was manipulated by two levels of nutrient enrichment. The heterotrophic dinoflagellate drastically reduced the nanophytoplankton and enhanced the reproduction of the copepods, suggesting that its role as a competitor is negligible compared to its function as a trophic link. In spite of the presence of heterotrophic nanoflagellates, the mixotroph had a strong negative effect on the picophytoplankton and (presumably) on bacterial biomass. At the same time, the mixotroph enhanced the atomic C:N ratio of the seston biomass, indicating a higher efficiency in overall primary production. Copepod reproduction was enhanced in the presence of the mixotrophic nanoflagellate. Results did not support predictions of the intraguild predation theory: The ratios of the intraguild predators and their preys were not affected by overall system productivity

Description: We assess population dynamics of picophytoplankton groups (≤2 μm diameter; Prochlorococcus, Synechococcus, and picoeukaryote) at a Pacific Ocean coastal site in the Southern California Bight. Weekly sampling (August 2000 to January 2002), dilution experiments, and flow cytometric analysis were combined with an instrument-specific calibration for cell size determination, allowing biovolume and carbon biomass estimation. Synechococcus was almost always numerically dominant, accounting for 60 ± 12 of the total picoplankton cells over time. It had moderately high growth rates (0.52-0.86 d-1) and was subject to low grazing mortality (-0.14 to -0.39 d-1). Prochlorococcus growth and mortality rates were roughly balanced (0.33 ± 0.14 d-1 and -0.36 ± 0.06 d-1, respectively). Picoeukaryotes had the highest growth rates (0.71-1.29 d-1) and were responsible for, on average, 76 of net carbon production (NCP), amounting in up to 32.05 ± 1.31 μg C L-1 d-1 produced and 28.31 ± 2.61 μg C L-1 d-1 consumed. In order to better define the eukaryotic component of these populations, an isolate was characterized via small subunit rRNA gene sequencing, transmission electron microscopy, and growth experiments and was identified as the prasinophyte Ostreococcus, not previously known to the Pacific Ocean. Our results show that although picoeukaryotes do not stand out as particularly important players in this system on the basis of cell abundance, they dominate in terms of picophytoplankton biomass and trophic transfer potential of carbon in this size class.

Description: Denitrification was investigated in the Baltic proper at two stations with different conditions in the deep water. The Gotland Deep was examined as an example of a basin with anoxic, H2S‐containing deep water and station T was taken as an example of low‐oxygen (〈0.2 ml liter−1), sulfide‐free deep water. Denitrification was measured by the acetylene blockage method; in addition, N2O reduction was followed in samples without acetylene. To shed light on the factors limiting denitrification, we compared in situ rates to denitrification after adding nitrate or electron donors. Denitrification was restricted to the layer of the oxic‐anoxic interface in the Gotland Deep and to the water layer near the sediment of station T. For both stations it could be shown that denitrification was not limited by nitrate availability. A lack of available organic C seemed to limit denitrification rates and growth of denitrifiers. As a result of C limitation in the water column, denitrification was restricted to energy‐rich interfaces. In the low‐oxygen water away from energy‐rich interfaces, the less C‐demanding nitrification‐denitrification coupling (NH4+ → N2O → N2) seemed to be favored. Denitrification in the water of the central Baltic seems to be subjected to strong variability due to changing C supply during the course of the year. However, limitation by C availability can be assumed for most of the year and should be taken into account in calculating the N budget of the Baltic.

Description: Environmental evidence suggests that Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton that blooms in coastal seawaters, can outcompete other organisms because of its ability to use abundant dissolved organic nitrogen (DON). To test this hypothesis, we isolated A. anophagefferens in axenic culture and monitored its growth on high-molecular weight (HMW) DON collected from sediment pore waters, a putative source for DON in bays where blooms occur. HMW DON originating from pore water had a substantially higher protein content than surface seawater DON. We found that A. anophagefferens could deplete 25-36% of the available nitrogen in cultures with HMW DON as the sole source of nitrogen and that this corresponded well with the protein fraction in pore-water HMW DON. High rates of cell surface peptide hydrolysis and no detectable N-acetyl polysaccharide hydrolysis, together with the high percentage of hydrolyzable amino acids compared to hydrolyzable aminosugars present in the HMW DON, pointed to the protein fraction as the more likely source of nitrogen used for growth. Whether or not nitrogen scavenging from protein is a common mechanism in phytoplankton is at present unknown but needs to be investigated

Description: According to a recent dynamical model, the depth of a well-mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon-specific sinking velocity and decrease with increasing background turbidity. In two field-enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100-µm screened phytoplankton community of a clear, unproductive, but silica-rich lake to a gradient of mixing depths (1.5- 15 m) during 4-6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon-specific sinking velocity and negatively related to background turbidity.

Description: The uptake and efflux of 64Cu was studied in the marine cyanobacterium Synechoccous strain WH7803 (DC2). Uptake followed classical Michaelis-Menten type kinetics in metal-buffered seawater. The maximum uptake rate, Vmax, was 0.236 ± 0.016 × 10-18 mol Cu cell-1 h-1, with the half-saturation constant, KS, of 10-10.81±0.11 mol L-1. An efflux mechanism was also observed in WH7803, whose growth was inhibited by high internal Cu concentrations. Efflux of Cu enabled WH7803 to maintain homeostasis for Cu at typical seawater ambient free copper concentrations ([Cu2+]f). The sensitivity of WH7803 growth to Cu was related to a simple inability to regulate internal Cu concentrations when external concentrations were 〉10-11 mol L-1.

Description: Sediments from the River Elbe estuary and incubated sediments were extracted with 1 N HCI for 24 h at room temperature. The extracted ferric and ferrous iron was determined with DC and AC polarography. Acid-volatile sulfide was determined from H2S trapped in aqueous zinc acetate solution. Analysis of sediment samples and extraction residues with Mossbauer spectroscopy demonstrated that the Fe oxidation state was conserved during extraction and polarographic determination, siderite and vivianite were completely dissolved, and Fe(II) in chlorite was partially extracted with HCl. Incubation experiments showed that extractable Fe was almost completely oxidized to Fe(III) at the oxic sediment surface and reduced to Fe(II) in deeper anoxic layers within a few weeks. Reactive Fe(III), i.e. that fraction of Fe which was reducible on the time scale of the incubation experiment, was completely extracted with HCl.

Description: Carbon acquisition in relation to CO2 supply was investigated in three marine bloom-forming microalgae, the diatom Skeletonema costatum, the flagellate Phaeocystis globosa, and the coccolithophorid Emiliania huxleyi. In vivo activities of extracellular (eCA) and intracellular (iCA) carbonic anhydrase activity, photosynthetic O2 evolution, CO2 and HCO uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO2 levels of 36, 180, 360, and 1,800 ppmv. Large differences were obtained between species both with regard to the efficiency and regulation of carbon acquisition. While eCA activity increased with decreasing CO2 concentration in S. costatum and P. globosa, consistently low values were obtained for E. huxleyi. No clear trends with pCO2 were observed in iCA activity for any of the species tested. Half saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest for E. huxleyi and lowest for S. costatum, generally decreased with decreasing CO2 concentration. In contrast, K1/2 values for P. globosa remained unaffected by pCO2 of the incubation. CO2 and HCO3- were taken up simultaneously by all species. The relative contribution of HCO3- to total carbon uptake generally increased with decreasing CO2, yet strongly differed between species. Whereas K1/2 for CO2 and HCO3- uptake was lowest at the lowest pCO2 for S. costatum and E. huxleyi, it did not change as a function of pCO2 in P. globosa. The observed taxon-specific differences in CO2 sensitivity, if representative for the natural environment, suggest that changes in CO2 availability may influence phytoplankton species succession and distribution. By modifying the relative contribution of different functional groups, e.g., diatomaceous versus calcareous phytoplankton, to the overall primary production this could potentially affect marine biogeochemical cycling and air-sea gas exchange.

Description: The chemical speciation of dissolved Cu was investigated by voltammetric methods in Gullmar Fjord, Sweden, over the course of a year from September 1996 until August 1997. Sampling was carried out on a roughly monthly basis, with an intensive survey carried out in May 1997. Surface water temperatures ranged from 21 to 22°C, whereas bottom waters in the fjord were approximately 6°C throughout. Macronutrient concentrations in the fjord during the period of the survey were investigated independently by the Göteborgs och Bohus läns Vattenvårdsförbund (Water Quality Association of Göteborg and Bohus). Surface phosphate concentrations were highest in early spring with low levels (〈0.1 mmol kg-1) over the late spring and summer. Nitrate and silicate showed a similar pattern to phosphate with the exception of high concentrations encountered in surface waters when low salinity plumes caused by runoff were encountered. A period of calm, sunny weather in January 1997 saw the initiation of the spring bloom some 2 months earlier than usual. Dissolved Cu speciation was dominated by organic complexation (over 99.8%) throughout this study. Strong Cu binding ligands (log K 〉 12.5) were not detected during the winter or early spring and could be related to the temperature-related seasonal appearance of the cyanobacterium Synechoccocus in these waters. The appearance of the strong Cu ligands led to a decrease in the concentration of free copper, resulting in a seasonal cycle for free copper in the fjord. This is the first study to examine Cu speciation over an annual cycle in a coastal environment

Description: EisenEx�the second in situ iron enrichment experiment in the Southern Ocean�was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate (14C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m-2), low photosynthetic efficiency (Fv/Fm ~ 0.3), and low daily primary productivity (130-220 mg C m-2 d-1). Between 70 and 90% of Chl a was found in pico- and nanophytoplankton. During the induced bloom, Fv/Fm increased up to ;0.55, primary productivity and Chl a reached the maximum values of 790 mg C m-2 d-1 and 231 mg Chl a m-2, respectively. As a consequence, the euphotic depth decreased to ~41 m. Picophytoplankton biomass hardly changed. Nano- and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly 〈40 m, primary productivity was highly correlated with Chl a. In the third week, productivity was much lower than predicted from Chl a, probably because of a reduction in photosynthetic capacity as a consequence of increased physical variability in the upper water column. These results provide unequivocal evidence that iron supply is the central factor controlling phytoplankton primary productivity in the Southern Ocean, even if the mixing depth is 〉80 m.

Description: Flows of the major biogeochemical elements (C, N, P, Si) and of transparent exopolymer particles (TEP) were traced during a bloom of a natural assemblage of marine diatoms in a mesocosm (l m(3)) to determine whether the exudation and subsequent gelation of carbon-rich phytoplankton exopolymers can account for the formation and potential export of carbon in excess of that predicted by Redfield ratios. Exponential growth of the phytoplankton community in the mesocosm extended for 10 d until nitrate concentration fell below detection and concentrations of dissolved inorganic and particulate organic nitrogen and phosphorus remained stable. Tight covariation of particulate organic elements occurred as long as nutrients were replete. But, after nitrate depletion, decoupling of carbon dynamics from that of nitrogen and phosphorus was observed, with a large flow of carbon into TEP An uptake of 72% more dissolved inorganic carbon (DIC) than inferred from nitrate supply and Redfield stoichiometry (referred to as carbon overconsumption) occurred during the study, largely during the postbloom phase, and was almost entirely traced to the particulate organic matter (POM) pool. Marine snow (aggregates 〉0.5 mm) appeared at the onset of nitrate depletion and coincided with rapid increase in TEP concentrations. Elemental composition of marine snow differed from the Redfield ratio by an enrichment in carbon and a depletion in phosphorus relative to nitrogen. It is suggested that sinking of TEP-rich marine snow could be a possible mechanism for export of carbon above calculations that are based on the Redfield stoichiometry.

Description: The carbon isotopic composition of marine phytoplankton varies significantly with growth conditions. Aqueous CO2 concentration [CO2] and algal growth rate (µ) have been suggested to be important factors determining isotope fractionation (ep). Here we examine ep of the coccolithophorid Emiliania huxleyi in relation to CO2 concentration and light conditions in dilute batch cultures. Cells were incubated at different irradiance cycles, photon flux densities (PFDs), and [CO2]. Isotope fractionation varied between 6.7 and 12.3‰ under 16 : 8 h light : dark cycle (L :D) and between 14.7 and 17.8‰ at continuous light. ep was largely independent of ambient [CO2], varying generally by less than 2‰ over a range of [CO2] from 5 to 34 mmol L-1. Instantaneous carbon-specific growth rates (µC) and PFDs, ranging from 15 to 150 mmol m-2 s-1, positively correlated with ep. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between ep and µ. In the present study the effect of PFDs on ep was stronger than that of mand thus resulted in a positive relationship between µ and ep. In addition, the L:D cycle of 16 : 8 h resulted in significantly lower ep values compared to continuous light. Since the observed offset of about 8‰ could not be related to daylength dependent changes in µC, this implies a direct influence of the irradiance cycle on ep. These findings are best explained by invoking active carbon uptake in E. huxleyi. If representative for the natural environment, these results complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications.

Description: Although N2-fixing cyanobacteria contribute significantly to oceanic sequestration of atmospheric CO2, little is known about how N2 fixation and carbon fixation (primary production) interact in natural populations of marine cyanobacteria. In a developing cyanobacterial bloom in the Baltic Sea, rates of N2 fixation (acetylene reduction) showed both diurnal and longer-term fluctuations. The latter reflected fluctuations in the nitrogen status of the cyanobacterial population and could be correlated with variations in the ratio of acetylene reduced to 15N2 assimilated. The value of this ratio may provide useful information about the release of newly fixed nitrogen by a cyanobacterial population. However, although the diurnal fluctuations in N2 fixation broadly paralleled diurnal fluctuations in carbon fixation, the longer-term fluctuations in these two processes were out of phase.

Description: Rates of cellular uptake of CO2 and HCO3- during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO2 partial pressures of 36, 180, 360, and 1,800 ppmv. In addition, in vivo activity of extracellular (eCA) and intracellular (iCA) carbonic anhydrase was determined in relation to CO2 availability. Both species responded to diminishing CO2 supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO2 concentrations. Simultaneous uptake of CO2 and HCO3- was observed in both diatoms. At air-equilibrated CO2 levels (360 ppmv), T. weissflogii took up CO2 and HCO3- at approximately the same rate, whereas CO2 uptake exceeded HCO3- uptake by a factor of two in P. tricornutum. In both diatoms, CO2 :HCO3- uptake ratios progressively decreased with decreasing CO2 concentration, whereas substrate affinities of CO2 and HCO3- uptake increased. Half-saturation concentrations were always 〈=5 mM CO2 for CO2 uptake and 〈700 mM HCO3- for HCO3- uptake. Our results indicate the presence of highly efficient uptake systems for CO2 and HCO3- in both diatoms at concentrations typically encountered in ocean surface waters and the ability to adjust uptake rates to a wide range of inorganic carbon supply.

Description: Redfield ratios of remineralization are calculated based on chemical data analysis on isopycnal surfaces. The concentrations of dissolved inorganic carbon used in this study were corrected for the anthropogenic CO2 content as estimated with a back-calculation technique. The corrections increased the apparent carbon remineralization by 25-30%, thus proving important for the reliable estimation of Redfield carbon ratios in the presence of anthropogenic CO2. Best estimates from this study largely confirm the more recently published Redfield ratios of remineralization. The following results were obtained for the latitude range 3-41°N along 20-29°W in the Northeast Atlantic Ocean: Corg: P ratio = 123 ± 10; Corg : N ratio = 7.2 ± 0.8; -O2 :Corg ratio = 1.34 ± 0.06; -O2 : P ratio = 165 ± 15; N: P ratio = 17.5 ± 2.0. These ratios are in close agreement with the average composition of phytoplankton and represent respiration of organic matter consisting on average of 52% protein, 36% polysaccharide, and 12% lipid.

Description: In contrast to most pelagic primary producers, benthic macrophytes pass through morphologically distinct life stages, which can be subject to different ecological controls. Using factorial field experiments, we investigated how grazing pressure (three levels) and nutrient supply (four levels) interact in controlling the passage of marine macroalgae through an apparent recruitment bottleneck at the germling stage. In comparative experiments, we asked whether relative bottom-up and top-down effects on early life stages (〈4 week germlings) vary (1) between the eutrophic Baltic Sea and the oligotrophic NW Atlantic, (2) across seasons in the NW Atlantic, and (3) among annual and perennial macroalgae. In both systems nutrient enrichment favored and grazers suppressed recruitment of green and brown annual algae; however, enrichment effects were much more pronounced in the Baltic, whereas grazer effects dominated in the NW Atlantic. Grazers induced a shift from grazer-susceptible green to more resistant brown algae in the Baltic without reducing total germling density. In the NW Atlantic, grazers strongly reduced overall recruitment rate throughout all seasons. Effects on perennials were similar in both systems with moderate losses to grazing and no effects of nutrient enrichment. Recruit densities and species composition shifted with season in the NW Atlantic. We conclude that the relative effects of grazers and nutrient enrichment depended on the nutrient status of the system, algal life history strategy, and season. Strong bottom-up and top-down controls shape benthic community composition before macroalgae reach visible size

Description: Iron supply is thought to regulate primary production in high nitrate, low chlorophyll (HNLC) regions of the sea in both the past and the present. A critical aspect of this relationship is acquisition of iron (Fe) by phytoplankton, which occurs through a complex series of extracellular reactions that are influenced by Fe chemistry and speciation. During the first in situ mesoscale Fe-enrichment experiment in the Southern Ocean (Southern Ocean iron release experiment [SOIREE]), we monitored the uptake of Fe by three size classes of plankton and their ensuing physiological response to the Fe enrichment. Rates of Fe uptake from both inorganic Fe (Fe') and organic Fe complexes (FeL) were initially fast, indicative of Fe-limitation. After Fe enrichment phytoplankton down-regulated Fe uptake and optimized physiological performance, but by day 12 they had greatly increased their capacity to acquire Fe from FeL. The increase in Fe uptake from FeL coincided with a sixfold decrease in Fe' that followed the production of Fe-binding organic ligands. Phytoplankton were able to use organically bound Fe at rates sufficient to maintain net growth for more than 42 d. Adaptation to such shifts in Fe chemistry may contribute to bloom longevity in these polar HNLC waters.

Description: Blooms of the marine diatom Skeletonema costatum were initiated in closed-system batch cultures with P-deficient medium under two different initial concentrations of dissolved molecular CO2([CO2,aq]: 20.6 and 4.5 µmol L-1). Algal C: N: P ratios strongly increased with decreasing P concentration. In the exponential growth phase, C: N ratios were 1.3 mol mol-1 higher in the low relative to the high [CO2,aq] treatment. There was no [CO2,aq] effect on C: N: P ratios during P-limited growth. Carbon isotope fractionation («p ) was 2-3‰ higher in the high [CO2,aq] treatment. With growth rate decreasing due to P limitation, ep increased in both [CO2,aq] treatments by 2-3‰ despite decreasing [CO2,aq]. Under these conditions the effect of decreasing growth rate on isotope fractionation strongly dominated over that of declining CO2 availability. When extrapolated to the natural environment, these results imply that systematic changes in algal growth, as occurring during the course of phytoplankton blooms, may affect algal isotope fractionation. These results severely complicate the interpretation of carbon isotope measurements in suspended and sedimentary organic matter

Description: By factorial field experiments we analyzed the relative effects of increased nutrient (N+P) loading and natural grazing pressure on species composition, carbon storage, and nitrogen retention in the Baltic Sea littoral food web, composed of macroalgae, grazers (snails, isopods, amphipods), and predators (shrimps, crabs, fish). Nitrogen was depleted relative to phosphorus throughout most of the year. Increasing nitrogen (6–200% over ambient concentrations) enhanced algal productivity and cover of fast-growing annual algae, grazer, and predator densities, suggesting a three-level bottom-up effect. With increasing nitrogen loading, annual algae increasingly blocked perennial algal recruitment (65–98% decrease) and growth. Grazers counteracted the effects of nutrient enrichment on algal species composition through selective consumption of annual algae. Grazer exclusion had equivalent negative effects on perennial recruitment as a 85% increase in nitrogen loading. Nutrient enrichment increased algal nitrogen content and decreased tissue C: N ratios in spring and summer but not in fall. Carbon storage and nitrogen retention, measured as C and N retained in plant biomass at the end of the growth season, were increased by grazers (C: 39%, N: 24%) but decreased with increasing nitrogen loading (C: -71%, N: -74%). Our results emphasize the important role of grazers in buffering moderate eutrophication effects and illustrate how food web interactions and shifts in species composition are tightly linked to coastal ecosystem function

Description: I tested the extent to which differences in light supply could influence the outcome of nutrient (Si and N) competition between marine phytoplankton. Competition experiments were performed with 11 species of marine phytoplankton at Si: N ratios from 16 to 124 : 1, light intensities from 28 to 225 µmol quanta m−2 s−1, and three different daylengths. Thus, light supply was the composite result of two components: photoperiod and intensity. Diatoms were dominant competitors at higher Si: N ratios, nonsiliceous flagellates at lower ones. Light had no impact on the transition from flagellate to diatom dominance along the Si: N gradient. However, species within those groups were separated along the light gradient. Contrary to theoretical expectations, changes in light intensity and changes in daylength led to similar shifts in species dominance. Therefore, it was possible to describe the light climate by the integral parameter “daily light dose.”

Description: Phytoplankton species composition and the availability of potentially limiting resources were investigatcd in five northern German lakes (Suhrer See, Kellersee, Behler See, PluBsee, and Krummsee) during spring 1988. Species composition was related to resource ratios to test Tilman’s resource-ratio hypothesis. The clearest results were obtained for Aulacoseira spp. Both the time-courses within the lakes and the comparison among the lakes suggest a strong tendency of this genus to become dominant at high Si : light ratios. Fragilariaceae occupied the next position on the Si : light gradient. Such conditions permitted them to become important even when Si : P ratios were quite low. With the onset of stratification diatoms were replaced by flagellates.

Description: During a 10-month study in Plusssee Si, N, and light were found as potentially limiting resources for phytoplankton growth rates. Therefore, three ratios of essential resources (Si : N, Si : light, N : light) and one ratio of substitutable resources (nitrate: ammonium) were compared to changes in species composition to test the hypothesis that the seasonal change of phytoplankton species composition was a response to changing resource ratios. The relationship was analyzed by a rank correlation analysis between the relative contribution of individual species to total biomass and resource ratios. Allowance was made for time lags between changes in resource ratios and changes in relative biomass. Of 16 species, 14 showed a significant response to at least one resource ratio. Time lags ranged from 0 to 6 weeks. Most species seemed to be favored either by minimal or maximal ratios; optimal ratios in the middle of the range were rare.

Description: The formation of large marine snow macroflocs and stringers was monitored at a fixed station in the southern North Sea. During an 18-month investigation, large marine snow floes occurred in the water column on nine occasions for periods lasting between 5 and 16 d. Their formation coincided with both high phytoplankton biomass and low windspeeds. Retention of marine snow in the surface layer was related to slow sinking, neutral buoyancy, and rising of the large floes. Applying a specially developed instrument showed buoyancy to be caused by gas bubbles incorporated in the floes. Marine snow flotation coincided with periods of oxygen supersaturation in the water column, suggesting that oxygen production could be responsible for gas bubble formation within the floes. Disappearance of the large marine snow floes from the surface layer coincided with drastic increases in windspeed, indicating that enhanced wind-induced turbulent mixing in the surface layer could have resulted in the breakup of these floes. Formation and disappearance of marine snow had a marked influence on the light regime in the water.

Description: P-limited chemostat cultures of the green alga Scenedesmus acutus were inoculated with the herbivorous zooplankter Daphnia galeata after steady state had been reached. At low dilution rates (up to 0.5 d-l) and consequently low P content of the food algae (P: C 〈 0.0011) Daphnia could not grow. At higher dilution rates (from 0.55 d-l upward) and higher P content ofthe food, Daphnia developed dense populations which were able to reduce algal biomass more than IO-fold. This reduction was accompanied by an increase in the P content of the algae which gave rise to increased birth rates of Daphnia. The birth rates of P-limited Daphnia depended on algal cell quotas in a saturating fashion which was mathematically similar to Droop’s growth equation. During P limitation the birth rates of Daphnia were positively correlated to population density, which suggests intraspecific facilitation instead of competition.

Description: Nitrate concentration was measured in seawater samples from the euphotic zone at the beginning and end of 12-h, daytime, in situ incubations. The changes in concentration are considered to be measurements of new production. During periods of 2-3 weeks in March-April 1989, important time scales for NO3- input to the euphotic zone (i.e. residence times) and new production were approximately 26 d at 18-degrees-N, 31-degrees-W and approximately 10 d near 33-degrees-N, 21-degrees-W. The average rate of NO3- use in the two areas was 2.63 and 0.62 mmol N m-2 (12 h)-1, or, in carbon equivalents 209 and 49 mg C m-2 d-1, respectively. These values bracket the large-scale estimate by Jenkins of new production in the nearby beta triangle of 150 mg C m-2 d-1.

Description: Maximal growth rates of 15 Antarctic phytoplankton species at 0 degree C ranged from 0.32 to 0.72/d, showing only a weak dependence on cell size. Comparisons were made with two models for size dependence of temperature-corrected rates of maximal growth. Schlesinger's general phytoplankton model predicts a strong size dependence of growth rates and grossly underestimates the maximal growth rates of the larger species, but gives reasonable estimates for the smallest ones. Banse's marine diatom model assumes a weak size dependence of growth rates and gives generally better predictions.

Description: The nutritional status of phytoplankton in a shallow, hypertrophic lake was analyzed by stoichiometry of seston and by enrichment bioassays during a 6-month period. Both methods suggested moderate and temporally interrupted nutrient limitation of reproductive rates. Nitrogen was the most frequently limiting nutrient, phosphorus was next, and silicate limitation of three diatom species occurred only once. The nutritional status of the most abundant individual species could be described by the Monod equation. The nutritional status of the entire phytoplankton assemblage could be described by a modified version of the Droop equation. In accordance with competition theory, phytoplankton species were arrayed along resource ratio gradients. These results are consistent with ecophysiological models derived from culture experiments.

Description: The influence of grazing pressure on the occurrence and outcome of nutrient competition among planktonic algae was studied in two-chamber microcosms where there was a flow in both directions between a light reactor without out zooplankton and a dark reactor with zooplankton (Daphnia longispina and Daphnia magna). The phytoplankton inoculum was a mixed, natural assemblage. Zooplankton could influence the dynamics of phytoplankton both by selective grazing and by differential excretion of limiting nutrients. Grazing pressure did not prevent the occurrence of nutrient limitation in algae and, hence, of nutrient competition between them. Zooplankton did, however, influence the outcome of competition by lowering Si:P ratios. A comparison with my previous experiments shows that diatoms need higher Si:P supply ratios for dominance over green algae in the presence of grazers than in grazing-free competition with steady or weekly pulsed nutrient supply

Description: Growth rates of Rhodomonas minuta v. nannoplanctica and Cryptomonas ovata were estimated by determining the fraction of cells undergoing cell division during 24 h. Acetocarmine was very useful for identifying stages of division (especially double nuclei). Division stages of R. minuta were found in Lake Constance during the night over a period of 16 or 17 h, of C. ovata over almost the entire 24-h cycle. A comparison of potential growth rates with observed changes in population density indicates that the higher temporal variability of loss rates is more influential for changes in population size

Description: Chemostat competition experiments with natural phytoplankton communities are compared to experiments in which either one (phosphorus) or two (phosphorus and silicon) key nutrients were added discontinuously at l-week intervals. In all types of experiments wide ranges of Si:P ratios were tested. Deviation from steady state was found not only to increase the number of coexisting species, but also to shift the regions of dominance of species and of higher taxa along the gradient of Si:P ratios. Pulsed nutrient addition was mainly to the advantage of green algae and to the disadvantage of diatoms.