Document: ROS-3-71-10

The potamoplankton conundrum: Searching for regulatory factors in an advective reservoir.

BLACK, A.R.* and M.R.COOPER

Eastern Washington University, Cheney 99004, WA, USA ¹

Abstract:
Crustacean zooplankton dynamics and production were investigated over the 1998 and 1999 growing seasons in the Lake Roosevelt reservoir of the Columbia River (north-central Washington State). Use of ten-thousand liter nitex enclosures allowed us to corral ambient zooplankters and estimate demographic and productivity statistics over a 96 hour interval without the immigration and emmigration complications of the advective reservoir environment. Replicated production estimates were conducted weekly at multiple locations in the reservoir. The Lake Roosevelt reservoir has a remarkably low residence time (mean = 40 days) making it more like a large river than a lake. Seasonal flucuations in water flow, water levels, and turbidity were dramatic. Physical conditions during the spring and early summer suggest the reservoir is a large river. Typical taxa include cyclopoid copepods and the small bodied Bosmina. Late summer and autumn conditions resemble those of a large lake. Eventually, crustacean zooplankton taxa shift to successively larger bodied limnetic species (Daphnia schodleri and Leptodiaptomus ashlandi). While most species abundances were increasing, water residence time (WRT), temperature, and turbidity appear to regulate production. Temperature and WRT were positively correlated with production (measured as change in biomass or population growth rate). Turbidity was negatively correlated with the productivity of some taxa (D. schodleri and L. ashlandi). Alternatively, while species abundances were declining, production showed a positive correlation with chlorophyll densities. Other than this mild suggestion that reservoir zooplankton experienced food limitation, little evidence of important regulatory species interactions was detected.

Keywords: Secondary production, large rivers, potamoplankton, zooplankton

This abstract is being presented at: 3:30 PM in session:
Oral Session #47: Zooplankton Ecology.