Plankton ecology was examined during the 1986 winter in Grand Traverse Bay, a 190 m deep, fjordlike bay on Lake Michigan. Before ice cover, algal concentration was low and uniformly distributed with depth, as it is in open Lake Michigan. During ice cover (February and March), a bloom of a typical winter-spring phytoplankton community developed in the upper 40 m, resulting in a 4 to 7-fold increase in feeding rate of adult Diaptomus spp. High algal concentration and zooplankton feeding persisted after ice melt (April). During and after ice cover, lipid concentrations of Diaptomus dropped rapidly from 34% of dry weight to 17 % because of egg production. High incident photosynthetically active radiation (PAR), high (45-50%) PAR transmittance of the ice due to little snow on the ice, and water column stability were probably responsible for the bloom. High ice transparency may be a common feature of large lakes and bays, where strong winds blow snow cover off the ice, or at low latitudes where snowmelt due to occasional rains and warm temperature is common. Winter reproducing calanoid copepods use these blooms to increase their reproductive output.
A simple technique, based on several published studies, is presented to estimate photosynthetically available radiation (PAR: 400-700 nm) at the airlwater and icelwater interfaces on freshwater lakes. Grand Traverse Bay of Lake Michigan of the Laurentian Great Lakes before, during, and after ice cover is used as a case study. The technique depends on assigning PAR transmittances to airlwater or air/ ice surfaces from empirically determined relationships. During ice cover, PAR reaching the water column under the ice exceeded 45 % of incoming PAR, on the average, due to the amount of clear ice present on the bay. Fig. I. Location of Traverse Bay with respect to the other Great Lakes. See Fig. 2 for coordinates.
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