Nearshore-offshore trends in Lake Superior phytoplankton

Changes in phytoplankton community composition and structure can have broad-scale ecosystem effects; however, drivers of species diversity in planktonic systems are not well understood. In lakes, a common but not thoroughly tested assumption is that shallow, nearshore waters are much more diverse and productive, and contribute considerably more material and energy to pelagic food webs than deeper waters farther offshore. Lake Superior is a large, cold, oligotrophic freshwater system which can provide insight into community organization under oligotrophic conditions. We used epilimnion and deep chlorophyll layer phytoplankton data from a lake-wide sampling program conducted in 2011 and 2016 to test whether assemblage composition, total algal biovolume, cell concentrations, diversity, and richness vary with depth. Although lake depth was an important factor in structuring assemblage composition, there were no clear nearshore-offshore gradients in cell density or biovolume despite the exposure of nearshore areas to higher concentrations of watershed-derived nutrients. Shannon diversity increased slightly with increasing depth, whereas richness was uncorrelated. Understanding of the nearshore-offshore patterns in phytoplankton community characteristics in the Great Lakes has implications for designing monitoring strategies and for considering how further changes in climate and nutrient deposition would affect the base of the food web.