EFFECT OF SALMON CARCASS DECOMPOSITION ON LAKE-SUPERIOR TRIBUTARY STREAMS

We investigated the fate of organic matter and inorganic nutrients der ived from spawning runs of chinook salmon in tributary streams to Lake Superior during fall and winter 1990. Upstream-downstream comparisons and experimental introduction of carcasses were used to determine how salmon carcass decomposition influenced several stream eosystem compo nents, including total phosphorus, total nitrogen, soluble reactive ph osphorus (SRP), NO3-, NH4+, periphyton biomass, and fine particulate o rganic matter (FPOM) in transport. Total phosphorus, SRP, and periphyt on biomass were higher in a river reach that received a spawning run o f an estimated 1200 fish than in an upstream reach that lacked spawnin g salmon. No upstream-downstream gradient in these components occurred in a river that did not receive a spawning run. Total phosphorus, SRP , and periphyton also were elevated where we experimentally introduced salmon carcasses, in the absence of a natural salmon run. Stable isot ope analyses revealed that salmon-derived nitrogen was incorporated in to grazing mayflies, and to a lesser extent into filter-feeding caddis flies. Salmon-derived carbon was not incorporated into these macroinve rtebrates. These results show that salmon carcasses can be an importan t source of nutrients in streams even when runs are relatively small.