THE EXCLUSION OF LIMNETIC CLADOCERA FROM TURBID GLACIER-MELTWATER LAKES

Glacial Alaskan lakes, turbid (≥5 nephleometric turbidity units) with suspended particles (1-30 μm), have both lower May-November levels of chlorophyll a and temperatures compared to nonglacial systems. Macrozooplankton densities are also lower and dominated by Cyclops and Diaptomus. Regardless of the presence or absence of planktivorous fish, filter-feeding cladocerans (eg Bosmina, Daphnia and Holopedium) were only absent from the limnetic zooplankton community of the glacial lakes. Turbidity reduced Daphnia survival and recruitment. Nondiscriminating filter feeders ingest glacial silt because the size-range overlaps that of the phytoplankton. Such an inefficient foraging strategy, especially when silt levels are high and algal numbers low, lowers energy extractable from ingested food below maintenance levels. Thus, the limnetic macro-zooplankton community of most Alaskan glacial lakes is restricted to either the selective herbivore Diaptomus and the raptorial feeding Cyclops, or to just Cyclops. -from Authors