Phytoplankton biodiversity promotes zooplankton growth and diversity

Background/Question/Methods

Diversity – productivity and diversity – stability relationships are well described for primary producers with a strong bias towards terrestrial systems. Here, we test whether diversity effects of aquatic primary producers (lake phytoplankton) extend to higher trophic levels (zooplankton). Based on theoretical considerations we expect differences in diversity of the phytoplankton community to have the following consequences for zooplankton consumers: Phytoplankton diversity is expected to have a dual control on zooplankton via affecting both resource quantity as well as stoichiometrically expressed resource quality. The relative importance of those two controls will depend on light and nutrient availability. Additionally, we expect increasing phytoplankton diversity to positively effect zooplankton growth by separate mechanisms acting on both trophic levels. First, individual zooplankton specimens are expected to be positively affected by a higher functional diversity of the phytoplankton community. Second, in a multi-species zooplankton community, complementarity among zooplankton taxa (niche diversification due to different traits of zooplankton species) can act in an increasingly greater potential niche space (phytoplankton diversity) and will further positively affect zooplankton growth. This should result in higher zooplankton growth and diversity and in reduced variation of zooplankton growth among communities (by statistical averaging) at high phytoplankton diversity.  To test this we performed a mesocosm experiment using natural phytoplankton communities from lakes of different trophic states. We conducted complementary nutrient addition bioassays to investigate the effects of phytoplankton diversity, nutrients, and biomass on zooplankton growth. Finally, we performed a laboratory experiment with a diverse zooplankton community under controlled food conditions where only phytoplankton biodiversity was manipulated while food quantity and food quality in terms of biomass stoichiometry were maintained constant.

Results/Conclusions

In the mesocosm experiment the effect of natural phytoplankton diversity on Daphnia growth was positive in low productive systems but negative in highly productive systems. Diversity effects were mediated by phytoplankton productivity which increased resource quantity but also diminished resource quality in terms of carbon to phosphorus ratios at higher productivity. In the additional laboratory experiment phytoplankton diversity increased the mean but suppressed the variance of zooplankton growth and abundance, and sustained higher zooplankton diversity. Likely explanations include complementarity effects among resource (phytoplankton) entities affecting individual Daphnia specimens as well as to complementarity effects among Daphnia species. Thus, by affecting the productivity as well as the reliability of the next trophic level, biodiversity of primary producers may have far-reaching consequences within the whole aquatic food web.