Positive effects of biodiversity on ecosystem function are described from an increasing number of systems, but the underlying mechanisms frequently remain elusive. A truly predictive understanding of biodiversity-ecosystem function relationships requires the a priori identification of traits conferring specific (and possibly complementary) functions to individual species. Although planktonic organisms are responsible for approximately half of the world's primary production, few studies have reported on the relationship between phytoplankton biodiversity and planktonic primary production. We argue that taxon-specific differential equipment with photosynthetically active pigments provides a biochemical mechanism of resource use complementarity among phototrophic microorganisms, enabling more diverse communities to more completely harvest the light spectrum. In line with this, more diverse phytoplankton communities showed higher pigment diversity, higher biomass-specific light absorbance, and higher rates of primary production and biomass accrual.

We conducted laboratory experiments with 85 assembled phytoplankton communities composed of species from four predefined functional groups (chlorophytes, diatoms, cyanobacteria, chrysophytes) to distinguish the relative importance of species diversity from functional group diversity on carbon uptake. We separated the observed diversity effects on carbon uptake into those caused by species with particularly important traits (selection effect) and those caused by positive interactions among species (e.g., complementary resource use or facilitation [complementarity effect]). Additionally, we measured the composition of photosynthetically active pigments and light absorbance in communities and monocultures, and related them to species and functional diversity effects on carbon accrual. Biodiversity effects were weak or even absent in pure cyanobacterial and diatom communities compared to strong effects in chlorophytes. Complementarity effects and light absorbance increased as functional (i.e., phylogenetic) diversity increased. There was a positive correlation between complementarity effects on carbon accrual and light absorbance. These findings support hypotheses regarding biodiversity-productivity relationships in phytoplankton communities based on niche separation along spectral light gradients.

Abstract. There is widespread concern that loss of biodiversity can influence important ecosystem services. A positive relationship between diversity and productivity has been observed in investigations of terrestrial and aquatic plant communities. However, an increase in primary production (carbon assimilation) does not necessarily result in higher nutrient uptake by primary producers. There is a loose coupling between carbon assimilation and nutrient uptake in autotrophs, and their biomass carbon-to-nutrient ratios (stoichiometry) are flexible. We performed controlled laboratory experiments to investigate the effect of phytoplankton biodiversity on phytoplankton stoichiometry. Our results indicate that biodiversity influences carbon assimilation and nutrient uptake of phytoplankton communities in different ways, resulting in variations of biomass stoichiometry. Data from 46 lake communities also support this link. Shifts in the biomass stoichiometry of phytoplankton communities are generally attributed to environmental fluctuations in resources. However, our results show that biodiversity is also important in determining their stoichiometry.

00000 ăWOS:000386021300002International audience1. Comparison between algal pigment analyses using HPLC and subsequent CHEMTAX analysis with microscopic counts from different lakes ranging from ultra-oligotrophic to eutrophic allowed testing of pigment: chlorophyll a ratios for lakes of different trophic conditions. Microscopic counts and pigment analyses were positively correlated for all groups except for the chrysophytes. 2. When examining the oligotrophic lakes only, microscopic analyses and pigment analyses of chrysophytes were positively correlated, which showed that the oligotrophic pigment ratios were well suited for determining the abundance of the chrysophytes in oligotrophic lakes. In the eutrophic lakes, where chrysophytes constituted less than 10% of the biomass, the weak correlation was most likely related to counting inaccuracies. 3. Chrysophytes, generally indicative of oligotrophic conditions, were three times more abundant in the oligotrophic lakes. They constituted 28% of the total phytoplankton biomass as chlorophyll a measured by the pigment method and 26% of the total phytoplankton biomass measured as biovolume by microscopy. In mesotrophic/eutrophic lakes, chrysophytes constituted only 9% (pigment method) and 8% (microscopy). 4. Similarity analyses comparing the results of these two methods on functional group level documented the high accuracy of the pigment method in detecting all the functional groups present. 5. The results of the pigment analyses showed a more even composition of phytoplankton groups than found by microscopy, where often one or two of the less abundant groups were absent or only sporadically present. The Shannon's diversity index, H', for functional groups determined from results of the pigment method were higher than the H' values based on microscopic counts, confirming a more continuous composition of the phytoplankton groups when detected by the pigment method. The pigment method was found to be a useful alternative, which supplemented microscopic counts