Impacts of Bioturbation by Spawning Salmon on the Community Dynamics and Ecosystem Processes of Alaskan Streams

Ecosystem processes and community dynamics are often controlled by a few dominant species. Species can have large impacts via trophic impacts, such as keystone predators, or via non-trophic impacts, such as ecosystem engineers. For example, it is widely recognized that migrations of Pacific salmon transport marine-derived nutrients and energy to coastal ecosystems. However, while Pacific salmon often spawn at high densities and dig large nests, the impacts of this ecosystem engineering are poorly understood. We collected data every two weeks for four summers on a suite of abiotic and biotic variables in six streams in southwestern Alaska that span a gradient of salmon densities. In streams and years with high densities of salmon, disturbance from spawning salmon impacted virtually all aspects of stream ecology. For example, disturbance by salmon leads to the export of silt and nutrients from streams. This bioturbation often exported more nutrients and matter than were imported by migrating salmon. In addition, there was a severe seasonal decline in periphyton and benthic invertebrate abundance associated with disturbance from salmon nest-digging. Thus, salmon nest-digging is an important component of stream disturbance regimes that controls benthic community dynamics and ecosystem processes of streams.