RELATIVE IMPORTANCE OF PICOPLANKTON, NANOPLANKTON, AND MICROPLANKTON TO THE PRODUCTIVITY OF MOUNTAIN LAKE, VIRGINIA

Studies of the relative importance of various phytoplankton size class es (pico-, nano-, micro-, and macro-) during thermal stratification at oligomesotrophic Mountain Lake, Giles County, Virginia were conducted from March through October 1994. A detailed time- and depth-series st udy was conducted to compare C-14-fixation rates in these phytoplankto n size fractions. Data indicated that on average the microplankton wer e the most important size class in cell number and cell volume and con tributed approximate to 95% of the total primary productivity. The pic oplankton ranked second in importance constituting approximate to 5% o f total primary productivity, whereas the relatively small numbers of nano- and macroplankton contributed little primary productivity. No ob vious relationships were apparent among cell densities, cell volumes, and chlorophyll a values during late summer thermal stratification, po ssibly due to shifts in community structure at that time. The findings contrast with many other oligo- to mesotrophic ecosystems where the p ico- and nanoplankton have demonstrated significantly greater importan ce, especially in summer.