Effects of turbidity and nutrients on zooplankton community structure: a mesocosm study Freshwater ecosystems make significant contributions to biodiversity and provide ecosystem services of value to humans. However, 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, with much research limited to controlled single-species laboratory studies or community-level studies in oligotrophic environments. Sediments can adsorb nutrients and simultaneously deliver nutrients to freshwater systems, resulting in turbidity and nutrients simultaneously acting as stressors in lakes. Zooplankton play an important role in aquatic ecosystems, transferring energy from lower to higher trophic levels, and are excellent indicators of freshwater ecosystems. With the objective of exploring the causal relationships between turbidity and nutrients on zooplankton communities, I conducted a mesocosm experiment at the Queen’s University Biological Station. Using 60 mesocosms, I established two 30-increment turbidity gradients, one at ambient (mesotrophic-eutrophic) and one at high (eutrophic-hypereutrophic) nutrient levels. I stocked the mesocosms with a diverse zooplankton community and measured community-level responses (abundance, biomass, and diversity) after 6 weeks. I found no change in total zooplankton abundance or biomass, and no interaction between turbidity and nutrients. Rotifer abundance and biomass declined with turbidity, which correlated with a decline in the concentration of cryptophytes, a preferred food source for rotifers. Cladocera abundance increased with turbidity and there was no change in biomass, indicating compensatory responses within the cladocera community. Changes in the abundance of specific cladocera species further support the presence of compensatory dynamics in response to turbidity. In contrast with my findings, past laboratory studies and oligotrophic community studies found that turbidity had no impact on rotifer abundance and caused a decline in Cladocera abundance. In drawing different conclusions from previous studies, my research provides insights to how a highly diverse zooplankton community in nutrient-rich conditions respond to turbidity, and the potential for nutrients to alter the effects of turbidity on zooplankton communities.
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