Short-term variability in coastal community and ecosystem dynamics in northern Lake Michigan

Despite their ecological and economic importance, coastal biological communities and ecosystem processes in large lakes have been understudied compared to those offshore. We investigated temporal variability in the abundance and assemblage composition of coastal organisms in northern Lake Michigan. We sampled nutrients, phytoplankton, zooplankton, periphyton, benthic invertebrates, and fish along the east coast of Beaver Island, Michigan, USA, with high temporal frequency (every d to every 2 wk) over 2 summers. We also examined connectivity between coastal and offshore habitats with both hourly water current data from the Great Lakes Coastal Forecasting System and sampling offshore 2 to 3x per summer. The abundance of littoral zone organisms varied markedly on daily to weekly timescales for both benthic invertebrates and water-column organisms (phytoplankton, zooplankton, fish). For example, zooplankton abundance changed by a mean of 297% between sampling dates at the same location. Within-season peaks in plankton abundance also differed among years. Specifically, peak chlorophyll a (Chl a) concentrations were observed 1 mo earlier in 2016 than 2015, and early summer (Jun) zooplankton abundance was 333% higher in 2016 than 2015. Furthermore, our data suggest that water currents may influence the abundance and assemblage composition of plankton, possibly by altering the flux of nutrients and organisms from offshore to coastal habitats. Specifically, zooplankton abundance increased by a mean of 194% when surface currents moved eastward before we sampled, and eastward surface velocity significantly altered zooplankton assemblage composition. Chl a concentration increased by a mean of 106% when surface currents moved northward prior to sampling. In contrast, the abundance and composition of benthic invertebrates and fish differed among sites and was not significantly related to water currents. The abundance of these organisms also declined throughout the summer. Therefore, benthic and water-column organisms are controlled by different processes, so patterns in their local abundance are out of sync. Understanding variability in coastal communities may enhance monitoring and design of effective protocols for investigating large lake ecology. Further, understanding the importance of regional movement of nutrients and organisms may be key to understanding community and ecosystem structure in coastal areas.