Effects of Fine Sediment on the Productivity of Stream Ecosystems in the Pacific Northwest

Salmon are important to the culture, ecosystem and economy of the Pacific Northwest. However, populations have declined significantly since European colonization. Salmon require specific conditions for each life stage and are highly sensitive to environmental stressors. Excess fine sediment is a leading form of pollution as it degrades salmon spawning habitat and the benthos upon which their main food source, stream insects, live. To evaluate the effect of sediment pollution on food availability for salmon smolt, we conducted an experiment estimating the amount of stream insect biomass and calories lost due to sediment pollution. We counted insects on substrate samples with and without excessive fine sediment from Clear Creek in Clackamas County, Oregon. Insect biomass on each substrate was converted to kilocalories and extrapolated to the entire stream length to estimate the in-stream food available for salmon smolt and approximately how many kilocalories were lost due to sediment pollution. The stream insects living on unpolluted substrate represented 290,000 kilocalories in Clear Creek. Sediment-polluted substrate supported 52% less insects, representing a calorie loss of about 140,000 kilocalories, enough to feed approximately 16,000 salmon smolt for a day. As an energy estimation of the stream benthos alone, the results are an underestimate. This study is based on only one reach in one stream, but it illustrates the impact of sediment pollution on the stream food web. The findings have important implications for how fine sediment is managed, especially through efforts to restore stream habitat for salmon.