Zooplankton are critical to the functioning of aquatic ecosystems and are an important part of lake and reservoir food webs. We used data from our 20-year long monitoring program on Kentucky Lake, a large mainstem reservoir on the Tennessee River system, to describe spatial and temporal patterns in zooplankton abundance with emphasis on the genus Daphnia, particularly the invasive D. lumholtzi. Twelve sites have been sampled every 16 days in spring through autumn months and every 32 days in winter months. Limnological parameters, including temperature, dissolved oxygen and chlorophyll a concentrations, are collected by standard methods. Zooplankton are collected in triplicate at mid-water column or 5 meters depth using a 15-L Schindler trap. Zooplankton are relaxed with carbonated water and preserved in buffered formalin. The entire contents of each trap is identified, enumerated, and archived. Descriptive statistics were extracted and preliminary analyses conducted via repeated-measures ANOVA in R. Years for which no discernable maxima could be determined were excluded.
Native Daphnia, primarily D. retrocurva, densities ranged from <1 to 40.8 L-1, with annual maximum densities occurring between the 111th and 204th days of the year. Daphnia lumholtzi first appeared in 1991 and subsequently occurred in densities ranging from <1 to 26.6 L-1, with maxima between the 150th and 287th days of the year. Daphnia lumholtzi occurred at very low levels (<1 L-1) between 1998-2001 and 2003-2006. Native daphnids did not experience such reductions in density. Significant differences existed between taxa (native daphnids versus D. lumholtzi) and years in both densities and timing of peak densities. Native daphnids peaked in abundance at higher densities and at cooler temperatures than D. lumholtzi . Both daphnids attained peak abundance across all sites within 80 days of one another. Native daphnids typically achieved higher maximum densities than D. lumholtzi. Native Daphnia exhibited more consistent patterns across sites, while D. lumholtzi attained lower densities and exhibited several years of very low abundance. Previous research involving Daphnia in Kentucky Lake had shown that D. lumholtzi occurred in a narrower range of suitable temperatures, which may explain why their peak abundances occur later in the year. Food availability, annual temperature patterns, water residence time, and predation pressure also may contribute to differences among years and sites in reservoir environments.