A COMPARISON OF NATURAL AND HUMAN DETERMINANTS OF PHYTOPLANKTON COMMUNITIES IN THE KENTUCKY RIVER BASIN, USA

Physical, chemical, and biological characteristics of the Kentucky Riv er and its tributaries were assessed for one year to compare effects o f seasonal, spatial, and human environmental factors on phytoplankton. Phytoplankton cell densities were highest in the fall and summer and lowest in the winter. Cell densities averaged 1162 (+/- 289 SE) cells ml(-1). Cell densities were positively correlated to water temperature and negatively correlated to dissolved oxygen concentration and to fa ctors associated with high-flow conditions (such as, suspended sedimen t concentrations). Chrysophytes, diatoms, and blue-green algae dominat ed winter, spring, and summer assemblages, respectively. Ordination an alyses (DCCA) indicated that variation in taxonomic composition of ass emblages was associated with stream size as well as season. Spatial va riation in phytoplankton assemblages and effects of humans was investi gated by sampling 55 sites in low flow conditions during August. Phyto plankton density increased with stream size. Assemblages shifted in co mposition from those dominated by benthic diatoms upstream to downstre am communities dominated by blue-green algae and small flagellates. Hu man impacts were assumed to cause higher algal densities in stream bas ins with high proportions of agricultural or urban land use than in ba sins with forested/mined land use. While density and composition of ph ytoplankton were positively correlated to agricultural land use, they were poorly correlated to nutrient concentrations. Phytoplankton diver sity changed with water quality: decreasing with nutrient enrichment a nd increasing with conditions that probably changed species compositio n or inhibited algal growth. Human impacts on phytoplankton in running water ecosystems were as great or greater than effects by natural sea sonal and spatial factors. Our results indicated that phytoplankton co uld be useful indicators of water quality and ecosystem integrity in l arge river systems.