Environmental and Water Quality Operational Studies: Size Distribution of Planktonic Autotrophy and Microheterotrophy in DeGray and West Point Reservoirs: A Comparative Study.

Planktonic autotrophy in both reservoirs was dominated by microalgae, with usually 60% of the total photosynthetic carbon uptake associated with organisms in the 8.0-micrometer size fraction. Microheterotrophic activity in the 0.2- to 1.0-micrometer size fraction fraction (indicative of small, free-living bacterioplankton rather than of large bacteria or bacteria attached to suspended particles) usually accounted for 80% of the planktonic microheterotrophy. Relative to marked uplake-to-downlake gradients in physical and chemical conditions, size distributions of autotrophy and microheterotrophy were remarkably uniform in both reservoirs. Uplake-to-downlake shifts in size distributions appear to correspond to within-reservoir transitions from riverine to lacustrine conditions; however, the specific environmental factors controlling the size distributions of planktonic autotrophy and microheterotrophy ramain uncertain. Our results suggest that additional ecological factors (e.g., size-selective losses of cells by grazing and/or sinking, autotroph-microheterotroph interactions) must be considered, in addition to the availability of nutrients and suspended particles, as potential environment controls on the size distributions of planktonic autotrophy and microheterotrophy.