Spatial and temporal trends in zooplankton assemblages along a nearshore to offshore transect in southeastern Lake Michigan from 2007 to 2012

Zooplankton were collected at a nearshore (15 m depth), a mid-depth (45 m) and an offshore site (110 m) near Muskegon, Michigan during March-December in 2007-2012. On a volumetric basis, biomass was lower at the nearshore site relative to the mid-depth site, but overall biomass at the nearshore and offshore sites did not differ. Differences in zooplankton assemblages among sites were due largely to Diaptomidae, Limnocalanus macrurus, Daphnia galeata mendotae, Cyclops, and either Bosmina longirostris or Bythotrephes longimanus. Diaptomidae were the most abundant group, accounting for 56-66% of zooplankton biomass across sites. Herbivorous cladocerans accounted for 14-22% of zooplankton biomass across sites, with B. longirostris dominant at the nearshore site and D. g. mendotae dominant at the mid-depth and offshore sites. Bythotrephes was the most abundant predatory cladoceran at all sites although, at the nearshore site, it was only abundant in the fall. There was a higher percentage of large-bodied zooplankton groups in the offshore and mid-depth zones relative to the nearshore zone. Declines in zooplankton biomass relative to the 1970s have occurred across all sites. In addition to seasonal variation within a site, we noted annual variation, especially at the offshore site, with the zooplankton assemblage during 2007-2008 differing from that found in 2010-2012 due to increases in D. g. mendotae and Cyclops and decreases in B. longimanus and L. macrurus in 2010-2012. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.