Description: Global warming is assumed to alter the trophic interactions and carbon flow patterns of aquatic food webs. The impact of temperature on phyto-bacterioplankton coupling and bacterial community composition (BCC) was the focus of the present study, in which an indoor mesocosm experiment with natural plankton communities from the western Baltic Sea was conducted. A 6°C increase in water temperature resulted, as predicted, in tighter coupling between the diatom-dominated phytoplankton and heterotrophic bacteria, accompanied by a strong increase in carbon flow into bacterioplankton during the phytoplankton bloom phase. Suppressed bacterial development at cold in situ temperatures probably reflected lowered bacterial production and grazing by protists, as the latter were less affected by low temperatures. BCC was strongly influenced by the phytoplankton bloom stage and to a lesser extent by temperature. Under both temperature regimes, Gammaproteobacteria clearly dominated during the phytoplankton peak, with Glaciecola sp. as the single most abundant taxon. However, warming induced the appearance of additional bacterial taxa belonging to Betaproteobacteria and Bacteroidetes. Our results show that warming during an early phytoplankton bloom causes a shift towards a more heterotrophic system, with the appearance of new bacterial taxa suggesting a potential for utilization of a broader substrate spectrum.

Description: We compared the development and fatty acid content of the harpacticoid copepods Tachidius discipes and Tisbe sp. fed with different microalgal species (Dunaliella tertiolecta, Rhodomonas sp., Phaeodactylum tricornutum, Isochrysis galbana and a concentrate of Pavlova sp.), which differed in cell size and fatty acid composition. Tisbe could develop in 11 days with every alga to the same average stage, whereas Tachidius developed poorly when fed with Isochrysis and Dunaliella. Feeding with Phaeodactylum resulted in a fast development of both copepods at low algal concentrations. However, reproduction was higher with Rhodomonas as food than with the other algae. Fatty acid compositions of copepods were influenced by their food source, but both were able to convert docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) from precursors. Tachidius fed with Rhodomonas or Phaeodactylum was closest to the DHA/EPA/arachidonic acid (ARA) ratio of 10 : 5 : 1 considered optimal for some marine fish larvae. Tachidius showed similar development and reproduction capacity as Tisbe, but requested higher absolute fatty acid contents in the diet. Tisbe was superior in the utilization of bacteria as additional food source and the bioconversion of precursor fatty acids. Phaeodactylum and Rhodomonas are recommendable food sources for both copepod species, but Phaeodactylum is more easily cultured.

Description: 1. Different components of the climate system have been shown to affect temporal dynamics in natural plankton communities on scales varying from days to years. The seasonal dynamics in temperate lake plankton communities, with emphasis on both physical and biological forcing factors, were captured in the 1980s in a conceptual framework, the Plankton Ecology Group (PEG) model. 2. Taking the PEG model as our starting point, we discuss anticipated changes in seasonal and long-term plankton dynamics and extend this model to other climate regions, particularly polar and tropical latitudes. Based on our improved post-PEG understanding of plankton dynamics, we also evaluate the role of microbial plankton, parasites and fish in governing plankton dynamics and distribution. 3. In polar lakes, there is usually just a single peak in plankton biomass in summer. Lengthening of the growing season under warmer conditions may lead to higher and more prolonged phytoplankton productivity. Climate-induced increases in nutrient loading in these oligotrophic waters may contribute to higher phytoplankton biomass and subsequent higher zooplankton and fish productivity. 4. In temperate lakes, a seasonal pattern with two plankton biomass peaks – in spring and summer – can shift to one with a single but longer and larger biomass peak as nutrient loading increases, with associated higher populations of zooplanktivorous fish. Climate change will exacerbate these trends by increasing nutrient loading through increased internal nutrient inputs (due to warming) and increased catchment inputs (in the case of more precipitation). 5. In tropical systems, temporal variability in precipitation can be an important driver of the seasonal development of plankton. Increases in precipitation intensity may reset the seasonal dynamics of plankton communities and favour species adapted to highly variable environments. The existing intense predation by fish on larger zooplankters may increase further, resulting in a perennially low zooplankton biomass. 6. Bacteria were not included in the original PEG model. Seasonally, bacteria vary less than the phytoplankton but often follow its patterns, particularly in colder lakes. In warmer lakes, and with future warming, a greater influx of allochthonous carbon may obscure this pattern. 7. Our analyses indicate that the consequences of climate change for plankton dynamics are, to a large extent, system specific, depending on characteristics such as food-web structure and nutrient loading. Indirect effects through nutrient loading may be more important than direct effects of temperature increase, especially for phytoplankton. However, with warming a general picture emerges of increases in bacterivory, greater cyanobacterial dominance and smaller-bodied zooplankton that are more heavily impacted by fish predation.

Description: Three species of marine phytoplankton, Rhodomonas sp., Isochrysis galbana Parke, and Phaeodactylum tricornutum Bohlin, were cultivated in semicontinuous cultures to test biochemical responses (fatty acids; FAs) to five nitrogen (N):phosphorus (P) supply ratios and four growth rates (dilution rates). The characteristic FA profile was observed for each algal species (representing particular algal class), which remained relatively stable across the entire ranges of N:P supply ratios and growth rates. For all species, significant direct effects of N:P supply ratios on FAs were found at lower growth rates. The highest saturated and monounsaturated fatty acid (SFA and MUFA) contents were observed under N deficiency at the lowest growth rate in all three species, while responses of polyunsaturated fatty acids (PUFAs) revealed no consistent pattern. Total FAs (and SFAs and MUFAs) in all species showed significant negative correlations with N cell quota (QN) under N deficiency, but PUFAs had species-specific correlations with QN. The results show that characteristic FA profiles of algal genus or species (representing particular algal classes) underlie fluctuations according to culture conditions. The significant correlation between FAs and QN under N deficiency suggests that elemental and biochemical limitation of phytoplankton should be considered mutually as determinants of food quality for zooplankton in marine ecosystems.

Description: Ocean warming has been implicated in the observed decline of oceanic phytoplankton biomass. Some studies suggest a physical pathway of warming via stratification and nutrient flux, and others a biological effect on plankton metabolic rates; yet the relative strength and possible interaction of these mechanisms remains unknown. Here, we implement projections from a global circulation model in a mesocosm experiment to examine both mechanisms in a multi-trophic plankton community. Warming treatments had positive direct effects on phytoplankton biomass, but these were overcompensated by the negative effects of decreased nutrient flux. Zooplankton switched from phytoplankton to grazing on ciliates. These results contrast with previous experiments under nutrient-replete conditions, where warming indirectly reduced phytoplankton biomass via increased zooplankton grazing. We conclude that the effect of ocean warming on marine plankton depends on the nutrient regime, and provide a mechanistic basis for understanding global change in marine ecosystems.

Description: Conceptual models predict a unimodal effect of consumer abundance on prey diversity with the highest diversity at intermediate consumer abundance (intermediate disturbance hypothesis). Consumer selectivity and prey productivity are assumed to be further important determinants. Preferential grazing on dominant prey species favoured by high nutrient supply is supposed to increase prey diversity, whereas the effect of consumers on prey diversity may be negative under low nutrient conditions (grazer reversal hypothesis). We tested the effect of four common consumers the isopod Idotea baltica, the amphipod Gammarus oceanicus, and the gastropods Littorina littorea and Rissoa membranacea on diversity and composition of epiphytes growing on eelgrass Zostera marina. Consumer density was manipulated (four levels: grazer free control, low, medium, high) based on abundances observed in eelgrass systems. Additionally, we manipulated nutrient supply (three levels) and the presence of Idotea in a factorial experiment. The impact of consumer abundance on epiphyte diversity varied depending on consumer identity and epiphyte evenness was affected rather than species number in this short-term experiment. Idotea reduced epiphyte diversity (Shannon-Wiener index H') and Gammarus increased epiphyte diversity. Littorina had no effect at low and medium abundance, but a negative effect in the high density treatment. Only Rissoa supported the conceptual models as it caused the proposed unimodal pattern in epiphyte diversity. The varying species-specific selectivity of the studied consumers is likely to explain their diverse impact on epiphyte diversity. Nutrients enhanced epiphyte diversity at medium enrichment, whereas higher nutrient supply reduced epiphyte diversity. The effect of Idotea changed from negative at low nutrient concentration to positive at higher nutrient supply, supporting the grazer reversal hypothesis. This study implies that consumer species identity and nutrient concentrations are important in controlling prey diversity and composition. Different consumer selectivity and changes in selectivity with growing consumer abundance and nutrient concentration are the causal factors for this effect.

Description: Three species of phytoplankton, Rhodomonas sp., Phaeodactylum tricornutum Bohlin, and Isochrysis galbana Parke, were cultivated in semicontinuous culture to analyze the response of carbon (C):nitrogen (N):phosphorus (P) stoichiometry to the interactive effect of five N:P supply ratios and four growth rates (dilution rates). The relationship between cellular N and P quotas and growth rates fits well to both the Droop and Ågren’s functions for all species. We observed excess uptake of both N and P in the three species. N:P biomass ratios showed a significant positive relationship with N:P supply ratios across the entire range of growth rates, and N:P biomass ratios converged to an intermediate value independent of N:P supply ratios at higher growth rates. The effect of growth rates on N:P biomass ratios was positive at lower N:P supply ratios, but negative at higher N:P supply ratios for both Rhodomonas sp. and I. galbana, while for P. tricornutum this effect was negative at all N:P supply ratios. A significant interactive effect of N:P supply ratios and growth rates on N:P biomass ratios was found in both Rhodomonas sp. and P. tricornutum, but not in I. galbana. Our results suggest that Ågren’s functions may explain the underlying biochemical principle for the Droop model. The parameters in the Droop and Ågren’s functions can be useful indications of algal succession in the phytoplankton community in changing oceans.

Description: Summary - In our recent contribution to the special issue on plankton dynamics in a fast-changing world, we outlined some general predictions of plankton dynamics in different climate regions now and in future, building on the Plankton Ecology Group (PEG) model (de Senerpont Domis et al., 2013). - We proposed a stylised version of plankton dynamics in Fig. 3 of our article and stated that these patterns need to be further elaborated. Our figure displays annual plankton dynamics now and in future in oligotrophic, mesotrophic and eutrophic lakes in arctic, temperate and tropical climate zones. - We fully agree with Sarmento, Amado & Descy (2013) that more data on tropical regions are needed, and we are looking forward to the emergence of published data from tropical regions to extend our still-limited understanding of plankton dynamics in these regions. - Sarmento et al. (2013) did not agree with our predictions on plankton dynamics for hydrology-driven water systems in the tropics. Unfortunately, however, Sarmento et al. (2013) did not substantiate their statements with the much-needed data on plankton dynamics in the tropics. Moreover, they merely provide an overview of precipitation patterns in the tropics, not an alternative hypothesis for our predictions.

Description: The widely cited Plankton Ecology Group (PEG) model of plankton seasonal succession is often used as a template to explain the seasonal changes in plankton communities outside the cold temperate zone, where it was developed, but this may be inappropriate for lower-latitude lakes. Lower-latitude lakes have high light availability in winter and less pronounced seasonal variations in fish predation on zooplankton. We might therefore expect higher phytoplankton crops in winter and much more predation on zooplankton by fish than in colder lakes. This might lead to less grazing in summer and relatively higher phytoplankton crops. We compared data on phytoplankton biovolume, zooplankton biomass and body size from 18 German and 6 Greek lakes to test these hypotheses.

Description: The main experimental approaches in aquatic community and ecosystem ecology will be presented along a gradient of scale: unenclosed field experiments, mesocosms with a natural mix of species, microcosms with an artificial mixture of species from cultures, and single species experiments in batch and continuous culture. Experimental manipulations usually consist of the addition or removal of (supposed) key organisms, addition of chemicals or alterations of the physical environment. Selecting the appropriate scale of experimentation is not only a question of technical and financial feasibility but also it has to consider the inevitable trade-offs between realism and control. Conclusions will be more widely accepted, if they are supported by experiments at a variety of scales. Key Concepts: The choice of the appropriate experimental scale has to face an inevitable trade-off between realism and control. Unenclosed field manipulations have the highest degree of realism but the least degree of control. Mesocosms operate with natural species assemblages, permit replication and control of experimental manipulations but are limited in temporal scale (weeks). Microcosm experiments operate with artificial communities assembled from cultures, offer a high degree of control, but lack the pre-history a mutual adaptation of the participating species. Experiments with single or few species rely on the 'model organism' concept. Small-scale experiments are biased against the detection of slow and space requiring processes.