Relationships Between Zooplankton and Habitat Conditions in the Upper Mississippi River System

ABSTRACT Zooplankton link microbial primary producers to larger consumers (primarily fish) in aquatic food webs. Despite their importance, assessment of zooplankton populations in large rivers is rare, especially across large spatial scales and during the winter. We collected zooplankton alongside summer, fall, and winter water quality sampling events in the Upper Mississippi River Restoration Program's Long Term Resource Monitoring element key reaches during 2019 and 2020. We used generalized additive models to assess the relationships between measures of zooplankton abundance and local and site‐level habitat variables. Zooplankton abundance was strongly tied to measures of productivity (chlorophyll) and water clarity (suspended solids), but the strength and shape of these relationships were not the same between seasons. Abundance of crustacean zooplankton was greatest during the winter at some locations; however, we did not find strong relationships between abundance and chlorophyll during that season. Crustacean density and biomass were negatively associated with the presence of invasive carps and positively associated with abundant aquatic plants. These findings highlight the variability of zooplankton response to physical and temporal habitat conditions.