Spatial subsidies in riverine food webs: consequences of disturbance and environmental change for stream fishes in Alaska and the Pacific Northwest

Freshwater food webs in Alaska and other parts of the Pacific Northwest rely heavily on nutrient, detritus and prey subsidies from marine and terrestrial ecosystems. Adult salmon provide tons of marine biomass to riverine ecosystems each year when they spawn and decompose; riparian forests provide terrestrial invertebrates to streams, which at times comprise over half of the food ingested by stream-resident salmonids; and up-slope, fish-less headwater streams are a year-round source of prey and detritus to fish-bearing food webs in valley bottoms. Disturbance and environmental change influence these resource subsidies. Fishing, dams, and ocean currents affect salmon returns to fresh water, and riparian forest management (fish-bearing and upslope fishless streams) ultimately affects the flow of prey to fishes. Following disturbances such as timber harvesting, regenerating red alder riparian forests elevate terrestrial invertebrate inputs to streams by up to four times compared to forests lacking alder. Because fish-bearing food webs receive resource subsidies from multiple sources, fish communities may be able to absorb short-term loss of subsidies from any given source. Understanding the effects of disturbance and environmental change on nutrient and food subsidies to freshwater food webs will broaden our knowledge of ecosystem function and improve resource management options.