Zooplankton Density Increases in an Irrigation Reservoir During Drought Conditions

Harlan County Reservoir, located in south-central Nebraska, entered a drought in 2003, providing an opportunity to monitor the effects of drought on the zooplankton community in this irrigation reservoir. We sampled the zooplankton community at 15 standardized locations every other week from April through the third week of October from 2003-2011. Total zooplankton densities were higher (131.8 ±13.1 L-1) in drought reservoir conditions (2003-2006) than under normal conditions (66.6 ± 9.0 L-1) (2007-2011). The zooplankton community was dominated by copepods throughout the study, with adult and immature (nauplii) copepods contributing 86.5% of the total zooplankton, while Daphnia spp. made up only 9.9%. Correlational analyses showed a positive relationship between total zooplankton, copepods and copepod nauplii and chlorophyll a and turbidity during normal reservoir conditions. Daphnia species showed a negative relationship with chlorophyll a during normal conditions and a negative relationship with turbidity in drought conditions. Our results document the dynamics of lower trophic levels during fluctuating water availability; such information will likely improve conservation and management of aquatic ecosystems during future episodes of changing environmental conditions.