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Accelerated eutrophication stimulated by pollution inputs is causing silica depletion in the surface waters of Lake Michigan during summer stagnation. Limitation of the reproduction of the presently dominant phytoplankton organisms, which require silica, may lead to drastic and, on the whole, undesirable changes in the ecosystem.

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Electron microscopic morphometric techniques were used to measure accurately the cytoplasm and frustule volumes of 12 naturally occurring planktonic diatoms in the Laurentian Great Lakes. In addition, cytoplasm volumes were measured in 6 other algal species. Estimates of carbon content were then made by converting measured cytoplasm volumes to carbon biomass. From these measurements and conversions, the average percent carbon by dry weight was found to be 19% (range lo-30%) for diatoms and 35% (range 29-43%) for the nondiatom species. These values are lower than those previously reported. This is probably due to two facts: our estimates do not include extracellular carbon that may be associated with the cells, and vacuole relative volume (% of total cell volume) ranged from 22 to 70% in diatoms and 2 to 29% for nondiatoms. The carbon content of cells with lower vacuole volumes more closely approximate those previously reported. Our estimations of percent silica by dry weight of diatoms averaged 60% (range 40-78%). Our values for silica content are closer to those reported for naturally occurring populations than to those for cultured populations.

The importance of the zooplankton-protozoan trophic coupling was determined experimentally by measured changes in protozoan growth rates with increasing zooplankton biomass. In five of six experiments conducted in Lake Michigan, a significant inverse relationship between protozoan growth and zooplankton biomass was observed (avg r2 = 70%), Zooplankton clearance rates on protozoan assemblages (range, 1.0-6.2 ml (pg dry wt)-I d '1 were comparable to those previously measured for phytoplankton which suggested that protozoa are important prey for zooplankton. Clearance rates on individual protozoan taxa [O-15.6 ml (Fg dry wt)-I d-l] were size-dependent. Rates were greatest for taxa ~20 grn in size (mainly nanoflagellates and small ciliates). In contrast to findings for phytoplankton, no evidence emerged for grazer resistance nor growth enhancement by planktonic protozoa in response to grazing. The high flux rates for macrozooplankton on heterotrophic nanoflagellates observed in all experiments (0.2-6.0 pg C liter-' d-l) provided evidence that a large fraction of picoplankton C may be directly transferred to higher trophic levels via a picoplankton-flagellate-zooplankton coupling.

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