Relationships among zooplankton groups and environmental factors at a nearshore site in Lake Michigan during 2007–2022

Sampling was conducted in southeast Lake Michigan at a nearshore 17-m deep site off Muskegon, MI during 2007–2022 to evaluate how the zooplankton assemblage responds to differing environmental conditions, including temperature, tributary flow, and chlorophyll concentration. Three small-bodied groups composed over 65 % of overall zooplankton assemblage: diaptomid copepodites (24 %), Dreissena veligers (21 %), and Bosmina spp. (21 %). Dreissena veligers have become one of the predominant groups in the nearshore, whereas cyclopoid copepodites have become less prevalent compared to historical data from the 1970s and 1990s. Zooplankton composition exhibited strong seasonal patterns that reflected the life cycles of the main groups, with Bosmina, other cladocerans and veligers most important in summer when overall abundance was highest and adult or immature diaptomid copepods dominating the assemblage the rest of the year. Ordination analysis revealed differences among years in zooplankton assemblages, in particular, the three major groups, diaptomid copepodites, Bosmina, Dreissena veligers, were well separated. Both diaptomids and Diacyclops were associated with years with higher potential resource availability (i.e., tributary flow, TP, chlorophyll) and cooler temperatures. On the other hand, Bosmina were associated with years with warm spring air temperatures and low resource availability and veligers were associated with high Secchi depths. Years with strong contributions of both diaptomid and Diacyclops copepodites and adults were separated from years with strong contributions of Cercopagis pengoi, the predominant predatory cladoceran, whereas there was overlap between Bosmina and Cercopagis. Multiple invasive species and environmental factors have complex interactions that affect the current zooplankton assemblage in the nearshore.