Date of Award:

5-1995

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Wayne A. Wurtsbaugh

Committee

Wayne A. Wurtsbaugh

Committee

Chris Luecke

Committee

Jack Schmidt

Abstract

I analyzed lake fertilization (with nitrogen and phosphorus) as a tool to assist in the recovery of the Snake River sockeye salmon (Oncorhynchus nerka) in the oligotrophic Sawtooth Valley Lakes in southcentral Idaho. These analyses involved monitoring, manipulating, and modelling several aspects of the lakes' primary producer, nutrient, and light parameters.

In Pettit Lake, I evaluated the effects of metalimnetic and epilimnetic fertilization in 330-m3 mesocosms. The metalimnetic treatment was equal to or more effective than the epilimnetic treatment in increasing chlorophyll a, phytoplankton biovolume, and primary productivity, yet caused smaller changes in periphyton growth and water clarity. Thus, metalirnnetic fertilization may provide a tool for increasing lake productivity while minimally impacting water clarity.

The Sawtooth Valley Lakes had deep chlorophyll maxima (DCM) with mean chlorophyll a peaks 240-1000% of mean epilimnetic concentrations. The DCM existed at low light levels and accounted for 36- 72% of the lakes' primary production. Epilimnetic fertilization of330-m3 mesocosms in Redfish Lake increased levels of primary productivity and chlorophyll a, but decreased Secchi depths and light available in the meta- and hypolimnion. I modelled the effects of increased chlorophyll (resulting from epilimnetic fertilization) and decreased light penetration on vertical primary productivity profiles. The simulations showed a large increase in epilimnetic primary productivity due to fertilization, and only a slight decrease in production in the deeper strata due to self-shading.

I also modelled the dependence of Red fish Lake's production on nutrients from the watershed, from lake fertilization, and from marine-derived nutrients from salmon. The model utilized our water budget and nutrient loading measurements. The model and empirical evidence indicated that even before hydropower dams were present in the migration corridor, marine-derived nutrients were not of major importance to lake production, contributing only ~3% of the lake's annual phosphorus load. This contribution was partially offset by the lake's quick flushing rate (3 yr) and phosphorus export by smolts. The model predicted annual adult salmon returns to be 3,800 under pre-dam conditions, 370 under modern conditions, 750 when doubling watershed nutrient loading (simulating lake fertilization), and 780 when doubling migration survival.

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