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Evaluating the Impact of TROA Alternatives on Pyramid Lake Algal Production and Hypolimnetic Oxygen: Final Alternatives

Prepared For

U.S. Fish & Wildlife Service
Nevada State Office, Reno, Nevada 89502 (March 6, 2004)


The factors controlling nitrogen availability, and hence algal productivity, in Pyramid Lake differ from those in the Truckee River, and therefore, an assessment of the impacts of water management strategies must reflect those differences. As a lake, algal production in Pyramid is affected by total available nitrogen from external sources, internal sources, and the nitrogen concentration of lake waters. This dependence of production on a variety of nitrogen sources means that nitrogen availability for a given year depends on the supply of nitrogen to the lake over several years rather than simply during the current year. The Davis Limnology Group developed in 1994 a computer simulation modeling tool that predicts the eutrophication response of Pyramid Lake as a whole to nitrogen loadings. In the past, the U.C. Davis Tool was used to evaluate how different Truckee River Operating Agreement (TROA) alternatives may affect the coldwater fishery of Pyramid Lake for the report to the negotiators (1995) and the DEIS/EIR (1996) compared with current conditions and the No Action Alternative. This study evaluates the TROA alternative developed for the TROA EIS/EIR and compares predictions with current conditions, the No Action Alternative, and a Local Water Supply Alternative (LWS).

Simulated water quality for the lake under current conditions are similar to conditions reported during the 1970's and 1980's. Mean lake concentrations for dissolved inorganic (DIN) and dissolved organic (DON) nitrogen during the final 87 years of the simulation were 0.091 and 0.69 mg/l, respectively, while average algal production was 173 g C/m2?yr. Spikes in the simulated values for the DIN concentration in the lake and annual algal production were associated with years of high river inflow.

The impact of the Alternatives on food availability and habitat for the coldwater fish population of Pyramid Lake was evaluated by comparing values for No Action with values determined for current conditions and by comparing the TROA and LWS Alternatives with the No Action Alternative. Conditions for the No Action Alternative were similar to current conditions, with lower river inflow and corresponding coldwater fishery habitat. Under the TROA Alternative, Truckee River inflow to Pyramid Lake increased by 11,500 acre?ft/yr causing mean lake level for 1913-1999 to be 3.2 ft higher than under the No Action Alternative. This increase in river inflow for the TROA Alternative corresponded with higher predicted DIN loading (3.9 Mg N/yr) and DON loading (13.1 Mg N/yr). Differences in lake characteristics for TROA and the No Action Alternative were relatively small but generally benefited the coldwater fishery of Pyramid Lake. The LWS Alternative provided results similar to the No Action Alternative.

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