It is expected that climate change will increase the frequency and intensity of precipitation events during the next century. The alterations in precipitation events will likely cause increasing sediment and nutrient (e.g., nitrogen, phosphorus) runoff from the terrestrial landscape. The effects of increased sediment and nutrient runoff will be especially pronounced in areas like the Midwestern US, where extensive row-crop agriculture exposes broad swaths of bare soil susceptible to erosion. Increased sediment and nutrient runoff will have marked effects on aquatic ecosystems, as external resource subsidies strongly influence ecosystem processes and community structure. We can predict how the effects of increased sediment and nutrient addition may work in isolation, but prediction how they may work in concert remains a significant challenge. For instance, increased nutrient enrichment promotes algal growth, thereby increasing resource availability for grazing zooplankton. Conversely, increased sediment loading can negatively affect zooplankton by inhibiting feeding. To address this knowledge gap we employed an 8-week replicated mesocosm experiment in 5000 L tanks. We factorially manipulated sediment and nutrient concentrations. We hypothesized that nutrient enrichment (in isolation) would positively enhance zooplankton populations, and that sediment enrichment (in isolation) would negatively effect zooplankton populations.
Results/Conclusions
We observed an overall increase in zooplankton abundance over the course of the experiment in the Nutrient addition treatment (N+). In contrast, zooplankton abundance decreased in the control (C) and sediment enrichment treatments ( S+;S+N+). The greatest decrease in zooplankton abundance occurred in the control, while the lowest decrease occurred in the S+N+ treatment. Cladoceran abundance trends mimicked those of total zooplankton, while copepod abundance was enhanced by nutrient addition, but not affected by sediment enrichment. We also observed changes in species composition, mainly cause by a negative effect of sediment addition on Daphnia abundance and a positive effect on Chydorus. These results partially support our hypothesis, overall nutrient enrichment enhanced zooplankton abundance. However, our results suggest that the effects of sediment enrichment are taxa-specific.