Determining primary productivity of Lake Roosevelt with C-14

During two sampling trips in 1997, primary productivity was measured at 11 stations in Lake Roosevelt using a radioactive carbon tracer capable of det ermining subtle changes in water quality. Both spatial and temporal variati ons in productivity were observed. For the August 1997 field trip, producti vities on the mainstem reservoir ranged from 485 to 1,243 mgC/m(2)/d, with an average value of 783 mgC/m(2)/d. Temporal differences between the August and October trips varied by as much as 880 mgC/m(2)/d. The data are compar ed to measurements taken over a four-year period to determine subtle, long- term changes in the water quality. Productivity at Lake Roosevelt is heavil y influenced by tributary inflow, and generally increased in the reservoir reaches below the mouths of the tributaries. Mainstem sites between the tri butaries had lower productivities than the upstream and downstream tributar y sites, indicating that nutrients may be rapidly utilized in the immediate vicinity of the inflows. However, productivities in the tributary arms wer e consistently low, despite the fact that their physicochemical properties stimulated growth in the reservoir. The productivity data illustrate the di fficulty in characterizing a large water body as a homogeneous quantity in terms of assessing loading impacts. The results also demonstrate that long- term changes in water quality can be masked by short-term events. The longt erm trend is that Lake Roosevelt, a eutrophic/mesotrophic lake, is becoming an oligotrophic lake, but seasonal rainfall events influence the lake's sp ecific characterization.