ZOOPLANKTON COMMUNITY RESPONSE TO RESERVOIR AGING

Changes in zooplankton diversity and density in response to reservoir aging in Pawnee Reservoir were investigated. Zooplankton samples colle cted from April 1992 through April 1993, were compared to a similar st udy conducted after initial impoundment by Helzer (1971), in 1968-1970 . Since this initial study, increases in turbidity and resulting chang es in biotic interactions significantly altered the zooplankton commun ity. A significant increase in total zooplankton density and a decreas e in species richness were observed between study periods. Density inc reased from 24.6 to 95.4 individuals L(-1), while the number of taxa d eclined from fourteen to ten. During this time period, Cyclops vernali s became the dominant zooplankter in the reservoir. The density of thi s predatory copepod increased significantly, from less than or equal t o 0.1 l(-1) in 1968-1970, to 44.3 l(-1) in 1992-1993, which accounted for most of the increase in total zooplankton density. Though a greate r spring maximum of another dominant, Bosmina spp. was found during th e 1992-1993 study period, the annual density of this cladoceran was no t significantly different between study periods. Similar trends for Da phnia ambigua and D. parvula were also observed, as greater spring max ima levels were attained, however overall annual densities were not si gnificantly different. The dominance of C. vernalis (46% of annual den sity) and Bosmina spp. (33%), indicate that these two zooplankters wer e tolerant of changes in physical conditions resulting from reservoir aging and biotic interactions that followed in the reservoir during th e 22 years between study periods.