Advective regulation of zooplankton assemblages in a reservoir series on the Savannah River, South Carolina/Georgia

Three hydropower reservoirs in series provided an ideal environment to investigate the importance of advection in regulating zooplankton assemblages. Data were collected on the distribution of zooplankton and planktivorous fish, zooplankton migration behavior, environmental variables and hydrologic impacts. I addressed the question of how advective current regimes impact zooplankton within these systems on three scales: reservoir-wide seasonal surveys, diel studies at the interface between two reservoirs, and a simulation of transport within one reservoir. Rotifers, 60-300 {dollar}mu{dollar}m in size, were usually the most numerous, with densities of 50-1400 organisms per liter. Copepods, 300-1500 {dollar}mu{dollar}m in size, were dominant in terms of biomass (50-140 {dollar}mu{dollar}g L{dollar}sp{lcub}-1{rcub}).{dollar} Significant increases in abundance and biomass at the Richard B. Russell dam boundary led me to focus my research on the dynamics occurring at this interface between the lower reservoirs.; During conventional hydropower generation, copepodids and cladocerans in the forebay were transported through the dam into the tailwater environment, but they appear to be returned to the forebay during pumpback operations. During generation, cool inflowing water from upstream releases plunged under the surface water and traveled as an underflow downstream. Zooplankton biomass was greatest (up to 250 {dollar}mu{dollar}g L{dollar}sp{lcub}-1{rcub}){dollar} at the leading edge of a surface backflow of epilimnetic water that occurs following generation. Predation by blueback herring (Alosa aestivalis) was greatest during generation near the dam, when they fed mainly on copepodids and cladocerans (11.3 +/{dollar}-{dollar} 0.9 mg stomach{dollar}sp{lcub}-1{rcub}).{dollar} Another planktivore, threadfin shad (Dorosoma petenense) contained more zooplankton prey (0.31 +/{dollar}-{dollar} 0.08 mg stomach{dollar}sp{lcub}-1{rcub}){dollar} when collected during generation in the warm surface water away from the dam.; Zooplankton migration strategies coupled with opposing current regimes influenced their downstream transport. Rotifers generally remain in the epilimnetic water (slow or backflowing currents) and simulations predicted that they were actually transported upstream ({dollar}-{dollar}0.2 km) over the course of one day. Cladocerans vertically migrate down within the region of greatest flow and consequently, they were transported downstream at a rate of 3.1 km d{dollar}sp{lcub}-1{rcub}.{dollar} These data support the importance of advective forces in determining zooplankton assemblage structure in similar Southern reservoir ecosystems.