Biotic disturbance and benthic community dynamics in salmon-bearing streams

1. Organisms can impact ecosystems via multiple pathways, often with positive and negative impacts on inhabitants. Understanding the context dependency of these types of impacts remains challenging. For example, organisms may perform different functions at different densities. 2. Anadromous salmon accumulate > 99% of their lifetime growth in marine ecosystems, and then return to spawn, often at high densities, in relatively confined freshwaters. While previous research has focused on how salmon nutrients can fertilize benthic communities, we examined how an ecosystem engineer, sockeye salmon Oncorhynchus nerka, influences seasonal dynamics of stream benthic communities through their nest-digging activities in south-western Alaska, USA. Benthic invertebrate and algal abundance were quantified every 7-14 days during the open water seasons of 10 streams in riffle and run habitats across multiple years, leading to 25 different stream-year combinations that spanned a large gradient of salmon density. 3. In streams with few or no salmon, benthic algal and insect biomass were fairly constant throughout the season. However, in streams with more than 0.1 salmon m(-2), algal and insect biomass decreased by an average of 75-85% during salmon spawning. Algal biomass recovered quickly following salmon disturbance, occasionally reaching pre-salmon biomass. In contrast, in streams with more than 0.1 salmon m(-2), aquatic insect populations did not recover to pre-salmon levels within the same season. We observed no positive impacts of salmon on algae or insects via fertilization from carcass nutrients. 4. Salmon, when their populations exceed thresholds in spawning density, are an important component of stream disturbance regimes and influence seasonal dynamics of benthic communities. Human activities that drive salmon densities below threshold densities, as has likely happened in many streams, will lead to altered seasonal dynamics of stream communities. Human activities that alter animal populations that are sources of biogenic disturbance can result in shifts in community dynamics.