SEASONAL-VARIATIONS IN SIZE STRUCTURE AND PROKARYOTIC DOMINANCE IN SULFUROUS LAKE CISO

To be useful in understanding natural communities, the analysis of size spectra should reveal seasonal and interlake variability. We calculated size spectra for the planktonic community of Lake Ciso. Data from 3 yr were used to analyze seasonal patterns of size distribution. The average spectrum showed peaks of biomass at 0.5-0.7, 4, 15, 32, 64, 85, and 200-mu-m of equivalent spherical diameter. These peaks corresponded to the dominant organisms in the lake: heterotrophic bacteria, purple phototrophic bacteria, Cryptomonas phaseolus, small ciliates including Coleps hirtus, large ciliates, rotifers including Anuraeopsis fissa, and copepods. The peak at 4-mu-m was the largest (3 or more times the biomass of the others). We used these data to test the utility of the size spectral approach to describe seasonal changes in the lake. Both linear and polynomial equations were fitted to the normalized spectra. Although the slope of the linear fit did not show any seasonal trend, the second-order coefficient of the polynomial fit presented a clear pattern that made it a useful index of the degree of procaryotic dominance. The lake alternated between anaerobic conditions during mixing, when procaryotes dominated and polynomial fits were best, and stratification, when polynomial and linear curves fit equally well.