Macrophyte Production, Fish Herbivory, and Water Quality in a Tailwater Reservoir- Lake Ogallala, Nebraska

Nebraska Public Power District (NPPD) began monitoring aquatic-macrophyte production in Lake Ogallala, Nebraska, in 1989 because the lake provides cooling water for a coal-fired electricity generating facility-Gerald Gentleman Station (GGS). Large mats of macrophytes in the cooling water caused several trips of the generating units at GGS, at considerable cost to NPPD. The plants also support macroinvertebrates that supply food for the trout fishery in and downstream of the lake. Ceratophyllum demersum, Potamogeton spp., Myriophyllum sibiricum, and Zannichellia palustris were the most commonly observed plants. In the early 1990s, macrophyte production declined to very low levels. Fresh-weight biomass averaged 6887 g/m2 in 1989 and declined to 20 g/m2 in 1995. June-July water temperatures also varied, increasing slightly through 1992, then decreasing 3°C through 1995. Fish activity in areas noted for previous high macrophyte production was suggested as a cause of decreased macrophyte production. A fish-exclosure study in 1994 and 1995 showed that herbivory and sediment-disturbance (probably by carp and white suckers) is partly responsible for the macrophyte decline, but productivity in the protected areas did not return to the high levels observed in the late 1980s.