Recent change in summer chlorophyll a dynamics of southeastern Lake Michigan

Six offshore stations in southeastern Lake Michigan were sampled during a pre quagga mussel Dreissena rostriformis bugensis period (1995-2000) and a post quagga mussel period (2007-2011). Chlorophyll a fluorescence profiles were used to characterize chlorophyll a concentrations during early (June-July) and late (August-September) summer stratification. During the early summer period the average whole water column chlorophyll a, the deep chlorophyll maximum, and the size of deep chlorophyll layer decreased 50%, 55%, and 92%, respectively, between 1995-2000 and 2007-2011. By contrast, in late summer there were no changes in these metrics between periods. Surface mixed layer chlorophyll a in early and late summer did not differ between time periods. On the other hand, chlorophyll a in the near bottom zone (bottom 20 m) declined 63% and 54% between 1995-2000 and 2007-2011 in early and late summer respectively. Changes in total phosphorus between 1995-2000 and 2007-2011 were less dramatic, with declines of 22-27% in early summer and 11-30% in late summer. Changes in the chlorophyll a conditions were attributed to dreissenid mussels which reduced material available from the spring bloom and disrupted the horizontal transport of nutrients to the offshore. Although light availability increased (i.e., increased secchi depths), reduced nutrient availability and spring diatom abundance resulted in a much smaller deep chlorophyll layer in 2007-2011.