Effect of Salmon Carcass Decomposition on Lake Superior Tributary Streams

Abstract

We investigated the fate of organic matter and inorganic nutrients derived 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 ecosystem components, including total phosphorus, total nitrogen, soluble reactive phosphorus (SRP), NO₃^-, NH₄⁺, periphyton biomass, and fine particulate organic matter (FPOM) in transport. Total phosphorus, SRP, and periphyton biomass were higher in a river reach that received a spawning run of an estimated 1200 fish than in an upstream reach that lacked spawning 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 isotope analyses revealed that salmon-derived nitrogen was incorporated into grazing mayflies, and to a lesser extent into filter-feeding caddisflies. Salmon-derived carbon was not incorporated into these macroinvertebrates. These results show that salmon carcasses can be an important source of nutrients in streams even when runs are relatively small.