PARENT SESSION
Oral Session #56: Aquatic Ecology: Plankton. Presiding: N. Hairston.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas I.
Zooplankton in a large advective reservoir: composition and production through a riverine to lacustrine transition.
Black, Ross1, Earle, Jannell1, Barlow, George1, 1
ABSTRACT- The crustacean zooplankton in the Lake Roosevelt Reservoir of the Columbia River were studied from spring through autumn of 1998, 1999, and 2000. Lake Roosevelt is a long and advective storage reservoir in north central Washington. The reservoir's water elevation is reduced every winter by as much as 24 meters. As the reservoir is filled with mountain run-off in the spring, water residence times (WRT) are low, and lake water is cool and turbid. In contrast, after the reservoir has returned to full volume, WRT increases five fold, turbidity drops fifty fold, and temperatures exceed 20 C. These dramatic shifts in the physical environment suggest Lake Roosevelt is more typical of a large river than a lake. Concomitant with the physical dynamics are dramatic changes in the zooplankton. During spring and early summer, zooplankton include small-bodied individuals at low densities and biomass. Although chlorophyll is abundant at this time, zooplankton productivity and growth are negligible. In mid summer, composition shifts to large-bodied species. High productivity increases biomass and abundance to exceed spring values by more than 100 fold. Temperature, chlorophyll, WRT, and lake elevation are positively correlated, and turbidity is negatively correlated, with zooplankton abundance, biomass, and productivity. In situ cubitainer experiments further suggest increased turbidity has a negative effect on the late summer and autumn species and that these same species may be food limited.
KEY WORDS: zooplankton, productivity, reservoir, large river