Ecosystem Structure and Function Reflect Salmon-Related Enrichment and Disturbance in Southeast Alaska Streams Influenced by Timber Harvest

Pacific salmon (Oncorhynchus spp.) provide a pulsed resource subsidy to their natal freshwater ecosystems, but recent research has demonstrated that salmon also disturb stream bottoms via bioturbation. The relative magnitude of salmon-related enrichment or disturbance is related to stream geomorphology, including channel complexity and sediment size. Timber management, a common land-use throughout the range of salmon in the Pacific Northwest and Southeast Alaska, can alter stream geomorphology, potentially altering the net ecological role of salmon. Metrics of ecosystem structure, such as nutrient concentrations and biofilm chlorophyll-a, have been used to determine levels of salmon enrichment and disturbance. However, coupling these metrics with integrative functional metrics, such as whole-stream metabolism and channel transient storage, may provide a more complete understanding of the overall effect of salmon in streams.

We determined the influence of salmon on coupled metrics of ecosystem structure and function and whether differences in geomorphology related to timber harvest influenced the ecological role of salmon. We conducted our research on Prince of Wales Island, Southeast Alaska, which has a history of timber management in addition to annual salmon runs. Our seven streams represented a gradient in timber harvest, ranging from 5 to 70% of the watershed drainage area. In 2007 and 2008, we quantified changes in dissolved nutrients, chlorophyll-a, and whole-stream metabolism before and during the salmon run, as well as channel transient storage. Before the salmon run, dissolved nutrients, chlorophyll-a, and gross primary production (GPP) were similar across the streams. However, ecosystem respiration (ER) was negatively related to timber harvest intensity, with more complex channels having higher ER than simplified ones (3.5 versus 1.4 gO₂/m²/day). Also, nutrient export was negatively related to transient storage, and transient storage was higher in streams with less watershed timber harvest. During the salmon run, dissolved ammonium and phosphorus increased 8- and 3-fold, respectively. GPP was variable, increasing in most streams (e.g., from 0.89 to 5.07 gO₂/m²/day), but decreasing in the stream with the most timber harvest (0.48 to 0.26 gO₂/m²/day). Conversely, ER increased in all streams (e.g., from 0.80 to 3.96 gO₂/m²/day). Nutrient export was no longer related to transient storage, but rather was dependent primarily on salmon run size. In summary, we found that salmon had both an enrichment and disturbance effect on stream nutrients and whole-stream metabolism. Furthermore, timber harvest influenced channel transient storage and nutrient export and may mediate the net ecological role of salmon in their natal streams.