Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

<em>Abstract.</em>—Big Silver Creek and the Adam River are oligotrophic (conductivity < 45 µΩ/cm; TDP < 2–5 µg/L; NO_{2 + 3}-N < 45 µg/L), mid-sized coastal rivers in southwestern British Columbia. They were treated with inorganic P (phosphorus) and N (nitrogen) to examine the feasibility of low-level inorganic fertilization as a method of increasing resident fish populations in rivers subject to habitat loss by historical logging practices. Both rivers have low numbers and sizes of resident salmonids (<20/ha, >20 cm fork length), despite extensive suitable habitat. Water temperatures in summer average 12°C and 14°C with summer discharge averaging 12 and 4 m3/s in Big Silver Creek and the Adam River, respectively. In 1992–1997, physical, chemical, and biological assessments took place from May to September in three reaches of each river. Liquid agricultural fertilizer was added to the lower reach(es) of each river from June to September of 1994–1997, while upstream reaches were monitored as controls. Fertilizer addition methods evolved from dripping through a hose and valve system, to a more dependable preprogrammable injection system, with the merits of each system discussed. In each river, chlorophyll-<em>a </em>accrual and benthic invertebrate biomass and density increased, on average, two to four-fold in the fertilized reaches. There was an average four-fold increase in rainbow trout abundance in each river following four summers of fertilization, with a large increase in mountain whitefish <em>Prosopium williamsoni </em>(Big Silver Creek) and a smaller increase in brown trout <em>Salmo trutta </em>(Adam River). The experimental treatments confirmed that low-level fertilization augmented productivity, resulting in a significant response of resident trout in two oligotrophic streams. The technique can be applied to aquatic systems with reduced fish populations resulting from habitat loss, overfishing, or to anadromous populations caught in the negative feedback loop of decreasing escapement and associated losses of marine-derived nutrients.