Environmental change alters the ecological role of Pacific salmon in southeast Alaska rivers

Background/Question/Methods Pacific salmon (Oncorhynchus spp.) return to their natal freshwater ecosystems to spawn and die. Salmon spawners can function as both nutrient resource subsidies and agents of benthic disturbance to freshwater ecosystems. Our studies of salmon spawner effects on aquatic ecosystems are beginning to reveal the potential interaction between the ecological effects of salmon spawners, and local and regional environmental change in Southeast Alaska. Timber harvest and climate change are two sources of environmental change in Southeast Alaska, the impacts of which are either evident or beginning to emerge. The Tongass National Forest includes most of Southeast Alaska and is managed for multiple uses, including timber harvest. Furthermore, areas in higher latitudes, such as Southeast Alaska, are considered most prone to the effects of climate change. Current climate projections, supported by long-term weather records, suggest that Southeast Alaska will experience warmer temperatures and increased precipitation, but with reduced snowpack. We used a combination of stream surveys across several watersheds and for multiple years, coupled with in-stream manipulations to study underlying mechanisms, to assess the impact of large-scale processes on the ecological influence of salmon spawners. Measurements taken include physical parameters (e.g., sediment size), and ecological measures of structure and function, such as dissolved nutrient concentrations (e.g., ammonium), and benthic algal abundance (e.g., chlorophyll a). Results/Conclusions Other studies have shown that timber harvest can alter watershed hydrology and erosion patterns, resulting in streams with finer sediments and less complex habitat. However, streamwater chemistry responses to salmon spawners did not differ between impacted and pristine streams. Regardless, benthic chlorophyll a was less abundant and more homogenous in impacted streams, corresponding to finer sediments that are more prone to disturbance. Few stream studies of salmon spawners have considered climate change impacts, but long-term temperature records in several of our study streams indicate small increases in water temperature. Our results suggest that environmental factors, including discharge as well as temperature, in combination with spawner abundance, explain the variable effects of salmon spawners among years. Critically, such factors may alter the balance between spawner disturbance and nutrient enrichment. Overall, our studies suggest that interactions between land use and climate change in Southeast Alaska have important consequences for the ecological role of salmon spawners in stream ecosystems. Ongoing studies over broader spatial and temporal scales are critical to understanding the impacts of environmental change, especially by allowing ecological effects to be detected against considerable background variation.