Natural Plankton Communities Research Articles

Phytoplankton nutritional quality is altered by shifting Si:N ratios and selective grazing

Abstract Diatoms often dominate phytoplankton in temperate, polar and upwelling regions. Decreases in silicate availability or silicon to nitrogen (Si:N) ratios may induce silicon limitation in diatoms and lower their proportion within phytoplankton communities. The effects of such changes on the nutritional quality of phytoplankton are not well understood. To examine how changing Si:N ratios affect plankton nutritional value, we applied a range of Si:N ratios on a natural plankton community and manipulated grazing pressure to assess top-down effects of copepod selective grazing. Diatom proportion in phytoplankton increased with increasing Si:N ratios and so did phytoplankton nutritional quality in terms of major fatty acid concentrations, such as polyunsaturated fatty acids, docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids. However, stoichiometric quality (carbon to nitrogen and carbon to phosphorus ratios), DHA:EPA and omega 3:6 (ω3:ω6) ratios declined with increasing Si:N ratios, suggesting that proportions between essential compounds in copepod diet may be more favorable in lowered Si:N ratios. Copepods had a negative effect on DHA contents, DHA:EPA and ω3:ω6 ratios, indicating possible selective grazing on more nutritious plankton. Our findings show that declining silicate concentrations can affect stoichiometric and biochemical quality of phytoplankton, which copepods can also moderate by selective grazing.

Effects of experimental warming on small phytoplankton, bacteria and viruses in autumn in the Mediterranean coastal Thau Lagoon

To investigate the responses of a natural microbial plankton community of coastal Mediterranean waters to warming, which are still poorly known, an in situ mesocosm experiment was carried out in Thau Lagoon during autumn 2018. Several microorganisms, including virio-, bacterio-, and phytoplankton < 10 µm in size, were monitored daily and analysed using flow cytometry for 19 consecutive days in six mesocosms. Three mesocosms (control) had the same natural water temperature as the lagoon, and the other three were warmed by + 3 °C in relation to the control temperature. The cytometric analyses revealed an unexpected community dominated by picophytoplanktonic cells, including Prochlorococcus-like and Picochlorum-like cells, which had not previously been found in Thau Lagoon. The experimental warming treatment increased the abundances of nanophytoplankton, cyanobacteria, bacteria and viruses during the experiment and triggered earlier blooms of cyanobacteria and picoeukaryotes. Only the abundance of Picochlorum-like cells was significantly reduced under warmer conditions. The growth and grazing rates of phytoplankton and bacterioplankton estimated on days 2 and 8 showed that warming enhanced the growth rates of most phytoplankton groups, while it reduced those of bacteria. Surprisingly, warming decreased grazing on phytoplankton and bacteria at the beginning of the experiment, while during the middle of the experiment it decreased the grazing on prokaryote only but increased it for eukaryotes. These results reveal that warming affected the Thau Lagoon plankton community from viruses to nanophytoplankton in fall, inducing changes in both dynamics and metabolic rates.

Extreme Levels of Ocean Acidification Restructure the Plankton Community and Biogeochemistry of a Temperate Coastal Ecosystem: A Mesocosm Study

The oceans’ uptake of anthropogenic carbon dioxide (CO2) decreases seawater pH and alters the inorganic carbon speciation – summarized in the term ocean acidification (OA). Already today, coastal regions experience episodic pH events during which surface layer pH drops below values projected for the surface ocean at the end of the century. Future OA is expected to further enhance the intensity of these coastal extreme pH events. To evaluate the influence of such episodic OA events in coastal regions, we deployed eight pelagic mesocosms for 53 days in Raunefjord, Norway, and enclosed 56–61 m3 of local seawater containing a natural plankton community under nutrient limited post-bloom conditions. Four mesocosms were enriched with CO2 to simulate extreme pCO2 levels of 1978 – 2069 μatm while the other four served as untreated controls. Here, we present results from multivariate analyses on OA-induced changes in the phyto-, micro-, and mesozooplankton community structure. Pronounced differences in the plankton community emerged early in the experiment, and were amplified by enhanced top-down control throughout the study period. The plankton groups responding most profoundly to high CO2 conditions were cyanobacteria (negative), chlorophyceae (negative), auto- and heterotrophic microzooplankton (negative), and a variety of mesozooplanktonic taxa, including copepoda (mixed), appendicularia (positive), hydrozoa (positive), fish larvae (positive), and gastropoda (negative). The restructuring of the community coincided with significant changes in the concentration and elemental stoichiometry of particulate organic matter. Results imply that extreme CO2 events can lead to a substantial reorganization of the planktonic food web, affecting multiple trophic levels from phytoplankton to primary and secondary consumers.

Metabolomics-derived marker metabolites to characterize Phaeocystis pouchetii physiology in natural plankton communities

Phaeocystis pouchetii (Hariot) Lagerheim, 1893 regularly dominates phytoplankton blooms in higher latitudes spanning from the English Channel to the Arctic. Through zooplankton grazing and microbial activity, it is considered to be a key resource for the entire marine food web, but the actual relevance of biomass transfer to higher trophic levels is still under discussion. Cell physiology and algal nutritional state are suggested to be major factors controlling the observed variability in zooplankton grazing. However, no data have so far yielded insights into the metabolic state of Phaeocystis populations that would allow testing this hypothesis. Therefore, endometabolic markers of different growth phases were determined in laboratory batch cultures using comparative metabolomics and quantified in different phytoplankton blooms in the field. Metabolites, produced during exponential, early and late stationary growth of P. pouchetii, were profiled using gas chromatography-mass spectrometry. Then, metabolites were characterized that correlate with the growth phases using multivariate statistical analysis. Free amino acids characterized the exponential growth, whereas the early stationary phase was correlated with sugar alcohols, mono- and disaccharides. In the late stationary phase, free fatty acids, sterols and terpenes increased. These marker metabolites were then traced in Phaeocystis blooms during a cruise in the Barents Sea and North Norwegian fjords. About 50 endometabolites of P. pouchetii were detected in natural phytoplankton communities. Mannitol, scyllo-inositol, 24-methylcholesta-5,22-dien-3β-ol, and several free fatty acids were characteristic for Phaeocystis-dominated blooms but showed variability between them. Distinct metabolic profiles were detected in the nutrient-depleted community in the inner Porsangerfjord (< 0.5 µM NO3−, < 0.1 µM PO43−), with high relative amounts of free mono- and disaccharides indicative for a limited culture. This study thereby shows how the variable physiology of phytoplankton can alter the metabolic landscape of entire plankton communities.

Coastal Ocean Darkening Effects via Terrigenous DOM Addition on Plankton: An Indoor Mesocosm Experiment

Human activities, increasing precipitation, and changes in land run-off deliver a large input of allochthonous nutrients into coastal waters including terrigenous dissolved organic matter (tDOM). Increased subsidies of tDOM into the coastal water are expected to reduce light availability and thus might be one of the factors causing coastal ocean darkening. To investigate the effect of increased tDOM input and thus limitation in light availability on primary production as well as the transfer to higher trophic levels (zooplankton), we conducted a large-scale indoor mesocosm “Planktotrons” experiment with natural (pelagic and benthic) plankton communities from the North Sea. We simulated a coastal ocean system with daily light and tidal cycles for 35 days. The experimental treatments included a light gradient consisting of three levels of tDOM addition (i.e., low tDOM, medium tDOM and high tDOM) and a control without tDOM addition. Results showed that tDOM addition reduced the light availability by 27% (low tDOM addition), 62% (medium tDOM addition) and 86% (high tDOM addition). Light reduction through tDOM addition negatively influenced phytoplankton biomass during the first half of the experiment (< 18 days) mainly in the ‘medium tDOM’ and ‘high tDOM’ treatments. The tDOM addition changed the phytoplankton community composition, potentially due to adaptations to different light conditions. Neither phytobenthos biomass nor composition was significantly affected by tDOM addition, probably because the tidal cycle assured sufficient light availability during low tide. Overall, our results indicate that the tDOM addition negatively influenced phytoplankton biomass and composition via light availability and tDOM effects were also observed on the zooplankton level (biomass and C:N ratio). Our experiment demonstrates possible implications of coastal darkening under climate-driven environmental changes on primary producers and their interactions in the aquatic food web.

Grazing Pressure Is Independent of Prey Size in a Generalist Herbivorous Protist: Insights from Experimental Temperature Gradients.

Grazing by herbivorous protists contributes to structuring plankton communities through its effect on the growth, biomass, and competitiveness of prey organisms and also impacts the transfer of primary production towards higher trophic levels. Previous evidence shows that heterotrophic processes (grazing rates, g) are more sensitive to temperature than autotrophic ones (phytoplankton growth rates, μ) and also that small cells tend to be more heavily predated than larger ones; however, it remains unresolved how the interplay between changes in temperature and cell size modulates grazing pressure (i.e., g:μ ratio). We addressed this problem by conducting an experiment with four phytoplankton populations, from pico- to microphytoplankton, over a 12°C gradient and in the presence/absence of a generalist herbivorous protist, Oxyrrhis marina. We found that highest g rates coincided with highest μ rates, which corresponded to intermediate cell sizes. There were no significant differences in either μ or g between the smallest and largest cell sizes considered. The g:μ ratio was largely independent of cell size and C:N ratios, and its thermal dependence was low although species-specific differences were large. We suggest that the similar g:μ found could be the consequence that the energetic demand imposed by rising temperatures would be a more important issue than the mechanical constriction to ingestion derived from prey cell size. Despite the difficulty of quantifying μ and g in natural planktonic communities, we suggest that the g:μ ratio is a key response variable to evaluate thermal sensitivity of food webs because it gives a more integrative view of trophic functioning than both rates separately.

Ocean acidification impacts on biomass and fatty acid composition of a post-bloom marine plankton community

A mesocosm approach was used to investigate the effects of ocean acidification (OA) on a natural plankton community in coastal waters off Norway by manipulating CO2 partial pressure (pCO2). Eight enclosures were deployed in the Raunefjord near Bergen. Treatment levels were ambient (~320 µatm) and elevated pCO2 (~2000 µatm), each in 4 replicate enclosures. The experiment lasted for 53 d in May-June 2015. To assess impacts of OA on the plankton community, phytoplankton and protozooplankton biomass and total seston fatty acid content were analyzed. In both treatments, the plankton community was dominated by the dinoflagellate Ceratium longipes. In the elevated pCO2 treatment, however, biomass of this species as well as that of other dinoflagellates was strongly negatively affected. At the end of the experiment, total dinoflagellate biomass was 4-fold higher in the control group than under elevated pCO2 conditions. In a size comparison of C. longipes, cell size in the high pCO2 treatment was significantly larger. The ratio of polyunsaturated fatty acids to saturated fatty acids of seston decreased at high pCO2. In particular, the concentration of docosahexaenoic acid (C 22:6n3c), essential for development and reproduction of metazoans, was less than half at high pCO2 compared to ambient pCO2. Thus, elevated pCO2 led to a deterioration in the quality and quantity of food in a natural plankton community, with potential consequences for the transfer of matter and energy to higher trophic levels

Grazing resistance and poor food quality of a widespread mixotroph impair zooplankton secondary production

Growing evidence suggests that global climate change promotes the dominance of mixotrophic algae especially in oligotrophic aquatic ecosystems. While theory predicts that mixotrophy increases trophic transfer efficiency in aquatic food webs, deleterious effects of some mixotrophs on consumers have also been reported. Here, using a widespread mixotrophic algal genus Dinobryon, we aimed to quantify how colonial taxa contribute to secondary production in lakes. We, therefore, studied the dietary effects of Dinobryon divergens on Cladocera (Daphnia longispina) and Copepoda (Eudiaptomus gracilis), representing two main taxonomic and functional groups of zooplankton. In feeding experiments, we showed that Dinobryon was largely grazing resistant and even inhibited the uptake of the high-quality reference food in Daphnia. Eudiaptomus could to some extent compensate with selective feeding, but a negative long-term food quality effect was also evident. Besides, Eudiaptomus was more sensitive to the pure diet of Dinobryon than Daphnia. Low lipid content and high C:P elemental ratio further supported the low nutritional value of the mixotroph. In a stable isotope approach analysing a natural plankton community, we found further evidence that carbon of Dinobryon was not conveyed efficiently to zooplankton. Our results show that the increasing dominance of colonial mixotrophs can result in reduced dietary energy transfer to consumers at higher trophic levels. In a wider perspective, global climate change favours the dominance of some detrimental mixotrophic algae which may constrain pelagic trophic transfer efficiency in oligotrophic systems, similarly to cyanobacteria in eutrophic lakes.

The impacts of ocean acidification on marine trace gases and the implications foratmospheric chemistry andclimate.

Surface ocean biogeochemistry and photochemistry regulate ocean–atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO2) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N2O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Life under an oil slick: response of a freshwater food web to simulated spills of diluted bitumen in field mesocosms

Heavy crude oil transportation over land is increasing, yet the ecological impacts of spills, particularly of diluted bitumen, in freshwater environments remain poorly understood. We simulated spills of diluted bitumen in 1400 L land-based mesocosms containing water and sediments from a boreal, oligotrophic lake and monitored the response of natural planktonic communities over 11 days. Most species of phytoplankton (chrysophytes and dinoflagellates) and zooplankton (copepods and cladocerans) were sensitive to oil, exhibiting &gt;70% overall abundance reductions in response to the spills. Declines in nano- and microphytoplankton were short-lived and began to recover after the oil sank, whereas picophytoplankton and zooplankton populations remained depressed at the end of the experiment. In contrast, oil spills stimulated bacteria known to degrade hydrocarbons, especially Alphaproteobacteria, whereas Gammaprotobacteria (a common marine oil spill bacterial class) increased less. This is the first experiment to examine the effects of diluted bitumen in a multitrophic freshwater community.

INT Toxicity over Natural Bacterial Assemblages from Surface Oligotrophic Waters: Implications for the Assessment of Respiratory Activity.

Plankton community respiration (R) is a major component of the carbon flux in aquatic ecosystems. However, current methods to measure actual respiration from oxygen consumption at relevant spatial scales are not sensitive enough in oligotrophic environments where respiration rates are very low. To overcome this drawback, more sensitive indirect enzymatic approaches are commonly used as R proxies. The in vivo electron transport system (ETSvivo) assay, which measures the reduction of (2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl tetrazolium chloride salt, INT) to INT-formazan in the presence of natural substrate levels, was recently proposed as an indirect reliable estimation of R for natural plankton communities. However, under in vivo conditions, formazan salts could be toxic to the cells. Here, we test the toxicity of 0.2mM of final INT concentration, widely used for ETSvivo assays, on natural bacterial assemblages collected in coastal and oceanic waters off Gran Canaria (Canary Islands, subtropical North Atlantic), in eight independent experiments. After 0.5h of incubation, a significant but variable decline in cell viability (14-49%) was observed in all samples inoculated with INT. Moreover, INT also inhibited leucine uptake in less than 90min of incubation. In the light of these results, we argue that enzymatic respiratory rates obtained with the ETSvivo method need to be interpreted with caution to derive R in oceanic regions where bacteria largely contribute to community respiration. Moreover, the variable toxicity on bacterial assemblages observed in our experiments questions the use of a single R/ETSvivo relationship as a universal proxy for regional studies.

Changes in food characteristics reveal indirect effects of lake browning on zooplankton performance

AbstractBrowning caused by colored dissolved organic matter is predicted to have large effects on aquatic ecosystems. However, there is limited experimental evidence about direct and indirect effects of browning on zooplankton in complex field settings. We used a combination of an ecosystem‐scale enclosure experiment and laboratory incubations to test how prolonged browning affects physiological and life‐history traits of the water flea Daphnia longispina, a key species in lake food webs, and whether any such effects are reversible. Daphnids and water were collected from enclosures in a deep clear‐water lake, where the natural plankton community had been exposed for 10 weeks to browning or to control conditions in clear water. Daphnid abundance was much lower in the brown than in the clear enclosure. Surprisingly, however, daphnids continuously kept in brown enclosure water in the laboratory showed increased metabolic performance and survival, and also produced more offspring than daphnids kept in clear enclosure water. This outcome was related to more and higher‐quality seston in brown compared to clear water. Moreover, daphnids transferred from clear to brown water or vice versa adjusted their nucleic acid and protein contents, as indicators of physiological state, to similar levels as individuals previously exposed to the respective recipient environment, indicating immediate and reversible browning effects on metabolic performance. These results demonstrate the importance of conducting experiments in settings that capture both indirect effects (i.e., emerging from species interactions in communities) and direct effects on individuals for assessing impacts of browning and other environmental changes on lakes.

Consumer-resource dynamics is an eco-evolutionary process in a natural plankton community

When traits affecting species interactions evolve rapidly, ecological dynamics can be altered while they occur. These eco-evolutionary dynamics have been documented repeatedly in laboratory and mesocosm experiments. We show here that they are also important for understanding community functioning in a natural ecosystem. Daphnia is a major planktonic consumer influencing seasonal plankton dynamics in many lakes. It is also sensitive to succession in its phytoplankton food, from edible algae in spring to relatively inedible cyanobacteria in summer. We show for Daphnia mendotae in Oneida Lake, New York, United States, that within-year ecological change in phytoplankton (from spring diatoms, cryptophytes and greens to summer cyanobacteria) resulted in consumers evolving increasing tolerance to cyanobacteria over time. This evolution fed back on ecological seasonal changes in population abundance of this major phytoplankton consumer. Oneida Lake is typical of mesotrophic lakes broadly, suggesting that eco-evolutionary consumer-resource dynamics is probably common.

Heterotrophic Bacteria Enhance the Aggregation of the Marine Picocyanobacteria Prochlorococcus and Synechococcus.

Marine picocyanobacteria are ubiquitous primary producers across the world’s oceans, and play a key role in the global carbon cycle. Recent evidence stemming from in situ investigations have shown that picocyanobacteria are able to sink out of the euphotic zone to depth, which has traditionally been associated with larger, mineral ballasted cells. The mechanisms behind the sinking of picocyanobacteria remain a point of contention, given that they are too small to sink on their own. To gain a mechanistic understanding of the potential role of picocyanobacteria in carbon export, we tested their ability to form “suspended” (5–60 μm) and “visible” (ca. > 0.1 mm) aggregates, as well as their production of transparent exopolymer particles (TEP)—which are a key component in the formation of marine aggregates. Additionally, we investigated if interactions with heterotrophic bacteria play a role in TEP production and aggregation in Prochlorococcus and Synechococcus by comparing xenic and axenic cultures. We observed TEP production and aggregation in batch cultures of axenic Synechococcus, but not in axenic Prochlorococcus. Heterotrophic bacteria enhanced TEP production as well as suspended and visible aggregate formation in Prochlorococcus, while in Synechococcus, aggregation was enhanced with no changes in TEP. Aggregation experiments using a natural plankton community dominated by picocyanobacteria resulted in aggregation only in the presence of the ballasting mineral kaolinite, and only when Synechococcus were in their highest seasonal abundance. Our results point to a different export potential between the two picocyanobacteria, which may be mediated by interactions with heterotrophic bacteria and presence of ballasting minerals. Further studies are needed to clarify the mechanistic role of bacteria in TEP production and aggregation of these picocyanobacteria.

Impacts of reduced inorganic N:P ratio on three distinct plankton communities in the Humboldt upwelling system

The ratio of inorganic nitrogen to phosphorus (NP) is projected to decrease in the Eastern Boundary Upwelling Systems (EBUS) due to warming of the surface waters. In an enclosure experiment, we employed two levels of inorganic NP ratios (10 and 5) for three distinct plankton communities collected along the coast of central Chile (33°S). The primary effect of the NP treatment was related to different concentrations of NO3, which directly influenced the biomass of phytoplankton. In addition, low inorganic NP ratio reduced the seston NP and Chl a-C ratios, and there were some effects on the plankton community composition, e.g., benefitting Synechococcus spp. in some communities. One of the communities was clearly top-down controlled and trophic transfer to grazers was up to 5.8% during the 12 day experiment. Overall, the initial, natural plankton community composition was more important for seston stoichiometry and trophic transfer than the manipulation of the inorganic NP ratio, highlighting the importance of plankton community structure for marine ecosystem functioning.

Sulfonate-based networks between eukaryotic phytoplankton and heterotrophic bacteria in the surface ocean.

In the surface ocean, phytoplankton transform inorganic substrates into organic matter that fuels the activity of heterotrophic microorganisms, creating intricate metabolic networks that determine the extent of carbon recycling and storage in the ocean. Yet, the diversity of organic molecules and interacting organisms has hindered detection of specific relationships that mediate this large flux of energy and matter. Here, we show that a tightly coupled microbial network based on organic sulfur compounds (sulfonates) exists among key lineages of eukaryotic phytoplankton producers and heterotrophic bacterial consumers in the North Pacific Subtropical Gyre. We find that cultured eukaryotic phytoplankton taxa produce sulfonates, often at millimolar internal concentrations. These same phytoplankton-derived sulfonates support growth requirements of an open-ocean isolate of the SAR11 clade, the most abundant group of marine heterotrophic bacteria. Expression of putative sulfonate biosynthesis genes and sulfonate abundances in natural plankton communities over the diel cycle link sulfonate production to light availability. Contemporaneous expression of sulfonate catabolism genes in heterotrophic bacteria highlights active cycling of sulfonates in situ. Our study provides evidence that sulfonates serve as an ecologically important currency for nutrient and energy exchange between microbial autotrophs and heterotrophs, highlighting the importance of organic sulfur compounds in regulating ecosystem function.

Composition of natural phytoplankton community has minor effects on autochthonous dissolved organic matter characteristics

ABSTRACTDissolved organic matter (DOM) is an important component of nutrient cycling, but the role of different organisms controlling the processing of autochthonous DOM remains poorly understood. Aiming to characterize phytoplankton-derived DOM and the effects of complex pelagic communities on its dynamics, we incubated natural plankton communities from a temperate mesohaline estuary under controlled conditions for 18 days. The incubations were carried out in contrasting seasons (spring and autumn) and changes in the planktonic community (phytoplankton, bacteria and microzooplankton), nutrients and DOM were assessed. Our results highlight the complexity of DOM production and fate in natural planktonic communities. Small changes in DOM composition were observed in the experiments relative to the orders-of-magnitude variations experienced in the phytoplankton assembly. We argue that the tight coupling between microbial processing and DOM production by phytoplankton and grazers stabilizes variations in quantity and characteristics of autochthonous DOM, resulting in apparently homogeneous semi-labile DOM pool throughout the experiments. However, seasonal differences in the production and processing of DOM were observed, reflecting differences in the nutrient regimes and initial DOM characteristics in each experiment, but also likely influenced by changes in the successional status of the pelagic community. Acknowledging that characteristics of the DOM derived from phytoplankton growth can vary broadly, heterotrophic processing and successional status of the community are synergistically important factors for shaping those characteristics, and thus affecting the seasonal signature of the semi-labile autochthonous DOM pool.

The stoichiometric ratios (C:N:P) in a pelagic food web under experimental conditions

Interactions between phytoplankton and zooplankton affect the overall functioning of lakes. Herbivores are habitually confronted with food of inferior quality, usually a result of low nutrient concentrations in plant material. Large-bodied cladocerans are better competitors for food than small-bodied species but they are more vulnerable to low food quality. Understanding the effects of food quality on zooplankton structure and competition between small - large bodied herbivorous is of considerable interest. We want to find out how differences in C:N:P ratios between phytoplankton and zooplankton communities affect their abundances in a freshwater food web. We want also to assess the role of phytoplankton and zooplankton as sinks of the phosphorus and nitrogen. Therefore, we conducted a 31-day mesocosms experiment with water from a mesotrophic and a eutrophic lake (with natural plankton communities). To simulate changes in the plankton communities large-bodied Daphnia magna and Daphnia pulicaria were added. Samples for zooplankton, phytoplankton and water chemistry were taken every 10 days. Samples for elemental analysis (C:N:P) of seston and zooplankton were collected on the first, and on the final day of the experiment.Our mesocosms experiment showed mismatch in C:P between seston (high) and zooplankton (low), which suggests that most of the phosphorus is incorporated in zooplankton biomass. This evidenced that zooplankton is an effective sink of phosphorus, while nitrogen is accumulated mainly by primary producers. Our results also indicated more stability in stoichiometry with increasing trophic levels of organisms. However, there were significant changes in the zooplankton structure. The increasing dominance of large Daphnia resulted in reduction of C:P ratio in zooplankton. Low food quality (C:P) did not limit the growth of large Daphnia in the experimental conditions, which competed effectively with small planktonic cladocerans and with Rotifera. Over time, inedible algae began to dominate resulting in increase of relative biomass of periphyton grazers, which suggests that plankton community is transformed into littoral system in mesocosms for about 30 days.

Validation of the in vivo Iodo-Nitro-Tetrazolium (INT) Salt Reduction Method as a Proxy for Plankton Respiration

Knowledge of the magnitude and variability of plankton respiration is a crucial gap in our understanding of marine carbon cycling. In order to validate the INT reduction method as a proxy for plankton respiration, we have compiled and analysed a dataset (n = 376) of concurrent measurements of dissolved oxygen consumption (CRO2) and in vivo reduction of 2-para (iodophenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride tetrazolium salt (INT) spanning a wide range of oceanic regions and physicochemical conditions. Data were randomly divided into two independent subgroups: two thirds of the data were used to derive a regression conversion between dissolved oxygen consumption and INT reduction (“training” dataset) and one third of the data was used to validate the regression (“test” dataset). There was a significant relationship between the log-transformed dissolved oxygen consumption rates and the log-transformed INT reduction rates (INTT) with the “training” dataset (logCRO2 = 0.72logINTT + 0.44, R2 = 0.69, n = 249, p 8 ˚C and chlorophyll-a concentrations >0.2 µg L-1 and reduced at lower temperatures and chlorophyll-a concentrations. The results of this study endorse the adequate performance and reliability of the INT method for natural plankton communities.

Putting the silicon cycle in a bag: Field and mesocosm observations of silicon isotope fractionation in subtropical waters east of New Zealand

A mesocosm experiment was used to investigate the fractionation of silicon (Si) isotopes in subtropical surface waters east of New Zealand. Surface waters in this region were characterised by relatively low concentrations of silicic acid (Si(OH)4) (~2 μmol L−1) and higher nitrate (~5 μmol L−1) and dissolved iron (Fe) concentrations (~0.4 nmol L−1) prior to development of the annual springtime phytoplankton bloom. To simulate initiation of the bloom, a large (~700 L) mesocosm experiment was undertaken whereby surface seawater containing the natural plankton community was incubated for a 168-h period. During the mesocosm experiment the concentrations of Si(OH)4, nitrate, phosphate and dissolved iron all decreased while the concentration of biogenic silica (BSi) increased 12-fold. Coupled with the increase in BSi was a change in the Si-isotope composition of BSi (δ30SiBSi) which increased from 1.49‰ to 2.64‰ after 168 h. Complementary observations to those made for the mesocosm experiment were made for corresponding surface waters. For these waters, we observed a small decline in the concentrations of nitrate, phosphate and dissolved Fe, but little change in the concentrations of Si(OH)4 and BSi. In contrast to the mesocosm experiment, surface water δ30SiBSi values became lighter during bloom initiation, suggestive of Si(OH)4 being replenished into surface waters. These differences in the drawdown and utilisation of nutrients and dissolved Fe between the mesocosm and surface waters during bloom initiation likely result from favourable Fe and light supply conditions within the mesocosm. In contrast, water column stability (i.e. vertical mixing), and the supply of dissolved Fe are likely to influence bloom initiation and its longevity. The fractionation of Si-isotopes in the mesocosm experiment followed closed-system Rayleigh fractionation kinetics, and an enrichment factor (ε) of −1.13‰ was calculated for the exponential phase of growth for the diatom community, which was marked by the presence of the diatoms Asterionelopsis glacilis and Mellosira moniliformis. The isotope enrichment factor agreed well with previous observations of Si isotope fraction in diatoms from field communities, and appeared to be independent of variations in the ambient Si(OH)4 concentration, and phytoplankton species composition.