Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

Abstract

<em>Abstract.</em>—Snake River sockeye salmon <em>Oncorhynchus nerka </em>once inhabited five prealpine lakes in the Sawtooth Valley, Idaho, but are presently reduced to the Redfish Lake stock. Declining returns to Redfish Lake in the 1980s prompted the National Marine Fisheries Service to list Snake River sockeye salmon as endangered under the Endangered Species Act, and a multi-agency effort was initiated in 1991 to prevent their extinction. The recovery effort focused on the development of a captive broodstock coupled with evaluation and enhancement of nursery lake habitats. Large populations of nonendemic kokanee salmon <em>O. nerka </em>and the oligotrophic conditions of these lakes raised concerns about overstocking sockeye salmon and causing the collapse of macrozooplankton populations. To minimize these risks and to improve sockeye salmon forage production, the Shoshone- Bannock Tribes initiated a 4-year nutrient enrichment program in Redfish Lake. Liquid fertilizer (20:1, N:P by wt) was added weekly during the growing season from 1995 to 1998 to the surface of Redfish Lake with Stanley Lake (unfertilized) acting as a control. During the fertilization of Redfish Lake, Secchi depth decreased by 13% and compensation depth by 24%, while increases were observed for surface chlorophyll <em>a </em>(106%) and primary production (117%). Uniformity of phytoplankton communities throughout the experiment indicated that the Redfish Lake food web was efficient (without major carbon sinks) and improved forage conditions for macrozooplankton. Total macrozooplankton biomass increased 31%, and <em>Daphnia </em>spp<em>. </em>biomass increased by 225%, simultaneous to a 26% increase in <em>O. nerka </em>density. Also, during fertilization, overwinter survival of supplemented sockeye salmon increased 192% in Redfish Lake. However, meteorological conditions were partly responsible for these changes. In unfertilized Stanley Lake, during the same time periods, Secchi depth declined 27%, and compensation depth was reduced by 28%; chlorophyll <em>a </em>increased 16%, primary production increased 14%, and zooplankton biomass was stable. These changes highlight the importance of climate (meteorological forcing) and the need for a control when attempting to identify impacts from lake fertilization. Disproportionately larger increases in Redfish Lake chlorophyll <em>a</em>, primary productivity, and zooplankton biomass relative to observed changes in Stanley Lake provide evidence for the efficacy of nutrient supplementation in Redfish Lake.