POPULATION-DYNAMICS OF AUTOTROPHIC PICOPLANKTON IN A SOUTHEASTERN US RESERVOIR

Reservoirs typically exhibit a gradient along their longitudinal axis in turbidity, nutrient flux, and algal biomass. We utilized these char acteristics to examine factors influencing temporal and spatial patter ns in abundance, biomass, composition, and production of epilimnetic a utotrophic picoplankton (APP) in Sardis Reservoir, Mississippi, USA. O ver a 18-month period, APP abundance varied between about 15,000 and 7 00,000 cells ml(-1). Both APP abundance and APP biomass were closely l inked to APP production and temperature. On an annual basis, the contr ibution of APP to total algal biomass and light-standardized productio n ranged between 15-47%, and 5-40%, respectively. Prokaryotes comprise d more than 95% of all APP in summer, but eukaryotes dominated the APP community in winter. During the nutrient-depleted summer period, APP decreased in number but tended to increase in the percentage of total algal biomass and production, from the uplake riverine zone to the dow nlake lacustrine zone. Only in the second year of the study, when rese rvoir water residence time was more than four times greater than in th e first year, were there significant differences in biomass and produc tivity of APP between the uplake and downlake regions. We suggest that , particularly in years or at times of the year when water-flow throug h the reservoir is slow, the importance of APP in Sardis Reservoir ref lects a spatial and temporal gradient in nutrient availability.