The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the "macroscopic" realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute approximately 50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

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Phytoplankton carbon and particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) (POC : PON : POP) were analyzed in 109 temperate lakes covering a wide span in productivity and other key parameters. Seasonal means of total POC (four samples) ranged from 206 to 7160 g C L Ϫ1 , with a grand mean of 960 g C L Ϫ1 , whereas estimated phytoplankton C ranged 12 to 1,770 g C L Ϫ1 , with a mean of 217 g C L Ϫ1 . Sestonic C : P ratios ranged from 59 to 553 (atom : atom), with a mean of 207. The elemental contributions from phytoplankton and other sestonic compartments (mainly detritus) were analyzed with a simple regression model, in which autochthonous and allochthonous components were separated. Model-derived estimates for N : P ratios of phytoplankton and allochthonous seston compartments were nearly equal (15.4 Ϯ 2.5 and 16.0 Ϯ 2.0) and were not significantly different from the Redfield N : P ratio (16). The estimated C : P ratio of allochthonous detritus was 2.7 times higher than that for phytoplankton (123 Ϯ 15), which again was not significantly different from the Redfield C : P ratio (106). Altogether, this indicates that sestonic components of autochthonous origin should be closer to Redfield proportions in eutrophic than in oligotrophic lakes. It also indicates that major contributions of allochthonous detrital C in oligotrophic lake seston may explain deviations from the Redfield ratio and calls for caution when interpreting elemental ratios in algae versus total seston. The regression model indicates that live phytoplankton cells rarely exceed 40% of total POC, yet it suggests that a major fraction of detritus is derived from autotrophs. This close link between live and dead cells could explain why total seston apparently carries the stoichiometric and biochemical footprints from the phytoplankton. Judged from algal biomass alone, Daphnia would face severe food limitation in a majority of lakes, while if we were to include total seston, Daphnia would be above threshold food levels in all lakes. Likewise, the effect of food quality limitation related to C : P ratios will turn out differently if total seston or only the phytoplankton fraction is considered.

Based on the currently largest available dataset of phytoplankton in lakes in northern Europe, we quantified the responses of three major phytoplankton classes to eutrophication.Responses were quantified by modeling the proportional biovolumes of a given group along the eutrophication gradient, using generalized additive models. Chlorophyll-a was chosen as a proxy for eutrophication because all classes showed more consistent responses to Chlorophyll-a than to total phosphorus.Chrysophytes often dominate in (ultra-) oligotrophic lakes, and showed a clear decrease along the eutrophication gradient. Pennate diatoms were found to be most abundant at moderate eutrophication level (spring-samples). Cyanobacteria often dominate under eutrophic conditions, especially in clear-water lakes at chlorophyll-a levels > 10 µg L -1 (late summer samples).We compare the relationships among types of lakes, based on the lake typology of the northern geographic intercalibration group, and among countries sharing common lake types. Significant differences were found especially between humic and clear-water lakes, and between low-and moderately alkaline lakes, but we could not identify significant differences between shallow and deep lakes.Country-specific differences in response curves were especially pronounced between lakes in Norway and Finland, while Swedish lakes showed an intermediate pattern, indicating that countryspecific differences reflect large-scale geographic and climatic differences in the study area.REBECCA MS no.04

Recent research has highlighted that positive biodiversity -ecosystem functioning relationships hold for all groups of organisms, including microbes. Yet, we still lack understanding regarding the drivers of microbial diversity, in particular, whether diversity of microbial communities is a matter of local factors, or whether metacommunities are of similar importance to what is known from higher organisms. Here, we explore the driving forces behind spatial variability in lake phytoplankton diversity in Fennoscandia. While phytoplankton biovolume is best predicted by local phosphorus concentrations, phytoplankton diversity (measured as genus richness, G) only showed weak correlations with local concentrations of total phosphorus. By estimating spatial averages of total phosphorus concentrations on various scales from an independent, spatially representative lake survey, we found that close to 70 per cent of the variability in local phytoplankton diversity can be explained by regionally averaged phosphorus concentrations on a scale between 100 and 400 km. Thus, the data strongly indicate the existence of metacommunities on this scale. Furthermore, we show a strong dependency between lake productivity and spatial community turnover. Thus, regional productivity affects beta-diversity by controlling spatial community turnover, resulting in scale-dependent productivity-diversity relationships. As an illustration of the interaction between local and regional processes in shaping microbial diversity, our results offer both empirical support and a plausible mechanism for the existence of common scaling rules in both the macrobial and the microbial worlds. We argue that awareness of regional species pools in phytoplankton and other unicellular organisms may critically improve our understanding of ecosystems and their susceptibility to anthropogenic stressors.