Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

<em>Abstract.</em>—The Upper and Lower Arrow lakes have undergone major anthropogenic changes. Dams were built below (Grand Coulee 1942), at the outlet (Keenleyside 1967), and above (Mica 1973 and Revelstoke 1983) the Arrow Lakes, and <em>Mysis relicta </em>were introduced in 1968. The reservoirs created behind the upstream dams act as nutrient traps, reducing the already naturally low levels of nutrients in the Arrow Lakes Reservoir. The objective of nutrient additions to the Arrow Lakes Reservoir was to replace nutrients trapped upstream and was driven by rapidly declining stocks of kokanee, a native landlocked sockeye salmon <em>Oncorhynchus nerka </em>and keystone species of this aquatic ecosystem. In the late 1980s and early 1990s, Upper and Lower Arrow tributaries supported between 600,000–800,000 kokanee salmon spawners, but the numbers declined steadily through the 1990s to a low of 97,000 in 1997. As the number of kokanee decreased, no increase in size was observed, consistent with nutrient-limited conditions. Unlike its neighbor, Kootenay Lake, which is one of the most studied in British Columbia, the Arrow Lakes Reservoir had received little limnological attention. After an initial study of the limnology and trophic status in 1997 and 1998, a 5-year fertilization experiment was initiated in 1999 with seasonally adjusted nutrient (phosphorus and nitrogen) additions to the Upper Arrow Reservoir, in an effort to restore historic kokanee populations. Preliminary data from the first two years of fertilization, 1999 and 2000, show positive and encouraging trends in primary productivity, phytoplankton succession, zooplankton biomass, and the number, size, and fecundity of kokanee spawners. No significant changes have been observed in the water quality parameters measured, consistent with immediate utilization of nutrients in an oligotrophic system.