SPATIAL HETEROGENEITY OF RESERVOIR ZOOPLANKTON - A MATTER OF TIMING

Description of patterns of zooplankton abundance in reservoirs has usually taken one of two approaches: (1) determining the relative importance of spatial and temporal components of heterogeneity, or (2) relating spatial variation along a reservoir's long-axis to the effects of inflows from major tributaries. This study brings together these two approaches to better understand zooplankton heterogeneity in reservoirs. Spatial and temporal patterns of zooplankton abundance were monitored during the spring of three consecutive years in a large, southeastern United States reservoir. The amount of spatial variation in zooplankton abundance exceeded temporal variation for most taxa. The differences among sites were not fixed - but rather ephemeral, in that they changed over the course of each season. Significant spatial variation was detected on most sample dates for rotifers and crustacean zooplankton. The greatest variation was along the long-axis of the reservoir (uplake vs. downlake), and differences between main-stream and embayment sites were usually secondary contributors to heterogeneity. Spring peaks in zooplankton abundance occurred later at the upstream sites than at downstream sites. This phenomenon could not be explained by either differential rates of warming in the two regions or by temporal variation in inflow rates. This difference in timing of population peaks was the basis of heterogeneity in this reservoir and the determinant of the ephemeral nature of spatial variation observed in this study.