Marine-Derived Nutrients, Bioturbation, and Ecosystem Function: Effects of Spawning Salmon on Stream Ecosystem Metabolism

Spawning anadromous salmon are the archetypal example of large-scale fish migrations that are common worldwide. In coastal areas of the North Pacific Ocean, annual returns of spawning salmon provide a substantial influx of nutrients and organic matter to streams and are generally believed to enhance the productivity of recipient ecosystems. Loss of this subsidy from areas with diminished salmon runs has been hypothesized to limit ecosystem productivity in juvenile salmon rearing habitats (lakes and streams), thereby reinforcing population declines. We measured changes in stream ecosystem metabolic properties, including gross primary productivity (GPP) and ecosystem respiration (ER), from three salmon streams by analyzing diel measurements of oxygen concentrations and stable isotopic ratios (d18O-O2) within a Bayesian statistical model of oxygen dynamics. A survey of primary producer abundance from 18 salmon spawning streams varying in substrate size places our ecosystem metabolism results in context of how geomorphic diversity regulates salmon effects on ecosystem function. We show that salmon predictably increased stream-water nutrient concentrations, which were on average 190% (nitrogen) and 390% (phosphorus) pre-salmon values, and that primary producers incorporated these nutrients into tissues. However, ecosystem metabolism results do not support a shift toward higher primary productivity with the return of salmon as is expected from a nutrient fertilization mechanism. Net ecosystem metabolism switched from approximately net autotrophic (GPP equal to or greater than ER) to a strongly net heterotrophic state (GPP << ER) in response to bioturbation of benthic habitats by salmon. Following the seasonal arrival of salmon, GPP declined to less than 12% of pre-salmon rates while ER increased by over three-fold. Metabolism by live salmon could not account for the observed increase in ER early in the salmon run, suggesting salmon nutrients and disturbance enhance in situ heterotrophic respiration. The ability of salmon to disturb benthic habitats is a function of substrate size; therefore, stream geomorphology in part regulates the effect of salmon on benthic communities and ecosystem function. We propose a conceptual model of factors controlling the effect salmon have on stream ecosystem function (nutrient cycling, ecosystem metabolism, physical structuring) that considers both salmon population biology and geomorphic parameters.