Recent changes in primary production and phytoplankton in the offshore region of southeastern Lake Michigan

Index words: Biomass Cyanobacteria Chlorophytes Cryptophyceae Chrysophyceae Diatoms Phytoplankton abundance, composition, and productivity were monitored on a bi-weekly basis from March/ April throughNovember/December at twooffshore stations in southeastern LakeMichigan in 1983–1987, 1995– 1998 and 2007–2008 (exception 1983–1984 which were sampled from May to August). During the spring isothermal mixing period, surface-mixed layer (SML) chlorophyll a and phytoplankton biomass (carbon) and water columnprimary productivity decreased substantially in 2007–2008 as compared to 1995–1998 (66%, 87%, and 70% decrease, respectively). Smaller or no decreases were noted between 1983–1987 and 1995–1998 (chlorophyll a 23% decrease, phytoplankton biomass 5% increase, and production 22% decrease). Phytoplankton composition also changed during the spring isothermal mixing period in 2007–2008 as compared to 1983–1987 and 1995–1998; all phytoplankton groups with the exception of cyanobacteria and chlorophytes exhibited dramatic reductions in 2007–2008. The pronounced changes in phytoplankton properties during spring mixing in 2007–2008 were attributed to the filtering activities of the quagga mussel (Dreissena rostriformis bugensis). Duringmidand late thermal stratification periods, SML phytoplankton chlorophyll a and phytoplankton carbon and water column primary production exhibited only one significant change across all decades (midstratification production in 2007–2008 as compared to 1995–1998 and 1983–1987). Phytoplankton compositional changes in the SML also were limited during thermal stratification. The size of the deep chlorophyll layer (DCL) in2007–2008was similar to or smaller than those in 1983–1987 and 1995–1998. However, phytoplankton composition in theDCL changed as net diatoms constituted b5%of total phytoplankton in the 2007–2008DCL but over 50% in 1983–1987 and 1995–1998.