Nutrient enrichment exacerbates the effect of climate driven shifts in early spring population size for temperate lake plankton

Background/Question/Methods

One of the most apparent impacts of global climate change on organisms has been the change in seasonal timing of events such as migration or emergence.  Because changes in timing are not consistent across taxa, the relative abundance of organisms can be changed drastically by climate change.  Here, we focus on temperate lake ecosystems, where climate change is altering the ability of zooplankton to overwinter and impacting the timing of emergence, altering the composition of early spring communities.  Simultaneously, many temperate lakes are in various states of recovery from eutrophication brought on by human development.  We test how the water nutrient concentration affects the sensitivity of a lake plankton community to changes in the early spring abundance of the Daphnia, a keystone grazer.  In 265 L mesocosms we establish an array of nutrient levels and Daphnia densities. We stocked these mesocosms with a natural phytoplankton assemblage and followed their development through the spring bloom.  To test whether the changes in the algal community brought on by these manipulations could be sufficient to induce a regime shift between a clear water state and a turbid state, we also monitored light penetration and the growth of algae in the benthos. 

Results/Conclusions

A decrease in the initial population size of Daphnia resulted in an increase in the magnitude of the algal peak across all nutrient levels.  With the exception of the lowest nutrient level, the increase in food resources resulted in an increase in the peak Dapnhia biomass in treatments with the lowest initial Daphnia densities.  Overall, there was a significant positive effect of nutrient enrichment on both chlorophyll a and Daphnia biomass.  Additionally, there was a significant interaction of initial Daphnia density and nutrient level, indicating that the effect of changing the abundance of grazers in the early spring community is most pronounced at high nutrient levels.  Following the patterns in chlorophyll a, light penetration in the mesocosms was reduced significantly by a decrease in the initial density of Daphnia, by an increase in nutrients and there was a significant interaction of the two.  The growth of algae in the benthos similarly followed the patterns in chlorophyll a, possibly due to settling from the water column.  Despite the strong responses in both the phytoplankton and zooplankton, we did not find evidence that changes in spring Daphnia densities are likely to push temperate lakes into alternate states.