Influences of coastal stressors on phytoplankton in the Great Lakes

We aimed to determine the scale of stressor impacts on phytoplankton in the Laurentian Great Lakes. Algal abundance of 72 pelagic locations was summarized for the seven main phytoplankton classes. The nearest shorepoint to each pelagic location was used to generate reaches of various lengths. Watersheds intersecting each reach were quantified for the six stressors: agriculture, urban area, population, point sources, roads, and a composite stressor. Phytoplankton data were modeled with the stressors using regressions at the various shoreline reach sizes. Model fits were compared among phytoplankton classes, indicators, stressors, shoreline sizes, and seasons. Cryptophytes and diatoms had the strongest relationships to stress; cell density had a stronger model fit than biovolume; agriculture and population were the most important determinants of phytoplankton abundance; phytoplankton were better related to stress quantified from larger reaches; and spring assemblages were more strongly associated with stress than summer assemblages.