Freshwater ecosystems make significant contributions to biodiversity and provide ecosystem services of value to humans. However, due to human-caused ecosystem change, freshwater habitats are among the most threatened globally. Among the many ways that humans are altering freshwater ecosystems, elevated turbidity from suspended clay-sized sediments remains relatively under-explored. Human activities that are accelerating the delivery of sediments to lakes include, but are not limited to, agriculture, mining, and urban development, and researchers around the world have documented the mostly harmful effects of suspended sediments on aquatic ecosystems. Sediments can adsorb nutrients and simultaneously deliver nutrients to freshwater systems, suggesting turbidity and nutrients may interact as stressors in freshwater ecosystems.
Zooplankton play an important role in aquatic ecosystems, transferring energy from lower to higher trophic levels. Zooplankton are known to be especially sensitive to physical and chemical changes. With the objective of exploring the causal relationships between turbidity and nutrients on zooplankton communities, we conducted a mesocosm experiment in 2021 near Kingston, Ontario. Using 60 mesocosms, we established 60-increment turbidity and nutrient gradients. We stocked the mesocosms with a diverse zooplankton community and measured community-level responses (total and relative abundances, diversity metrics, and biomass) after 6 weeks.
Results/Conclusions
Due to an issue with the location of the intake pump in the mesotrophic lake from which we sourced water for the mesocosms, all the mesocosms were eutrophic or hypereutrophic (TP: 63.34 - 313.68 ug/L, TN: 0.504 - 1.900 mg/L), while the zooplankton were sourced from oligotrophic and mesotrophic lakes. Preliminary results indicate that neither turbidity nor nutrients had an effect on total zooplankton abundance, and that there was no interaction between the stressors. While turbidity had no effect on zooplankton diversity, diversity increased with nutrients. In comparison to our findings, past studies found that increased turbidity in oligotrophic lakes decreased total zooplankton abundance and diversity. This suggests that in eutrophic and hypereutrophic systems, which see greater primary and secondary production, zooplankton communities are controlled by nutrients rather than turbidity. Furthermore, our findings suggest that hypereutrophic systems support a more diverse zooplankton community compared with eutrophic systems. These preliminary findings begin to provide insight into the independent and interactive effects of turbidity and nutrients on zooplankton communities.
