Effects of spawning salmon on dissolved nutrients and epilithon in coupled stream-estuary systems of southeastern Alaska

Spawning migrations of Pacific salmon (Oncorhynchus spp.) represent a significant nutrient pulse to freshwater ecosystems of southeastern Alaska. These salmon-derived nutrients (SDN) can be transported by streamflow from spawning reaches of streams to estuaries, where their fate and significance have not been studied. Such estuaries represent an important aquatic ecotone in southeastern Alaska, especially for juvenile salmonids. We sampled dissolved nutrients and epilithon (i.e., benthic algae attached to rocks) before, during, and after salmon runs in three Alaskan streams and their associated estuaries. Salmon runs increased streamwater fluxes of dissolved ammonium and soluble reactive phosphorus to estuaries by 30-350% and 14-130%, respectively, depending on the stream and period. Although epilithon chlorophyll a and biomass increased in streams in response to SDN, epilithon did not increase in associated estuaries. We suggest that salmon spawner disturbance during estuary reddbuilding, invertebrate grazing, physical transitions from summer to fall, and nutrient influx from nutrient-rich tidal flow may have limited epilithon growth responses to SDN in estuaries. Although estuaries receive a significant flux of SDN flushed downstream from freshwaters, epilithon does not measurably respond to this nutrient enrichment from spawning salmon.