Rethinking the food web impacts of migrating salmon

Posted on March 27, 2012

The key role played by migrating  salmon in transporting nutrients, organic matter and energy to upstream spawning areas has been well documented.   Several studies have described how nutrient subsidies derived from salmon have a strong impact on the structure of aquatic food webs,  through bottom-up increases in primary and then secondary productivity.  However, a new appraisal of salmon subsidies has emerged from the first ecosystem-level measurements of stream metabolism in salmon streams.  Data on dissolved  nutrients, oxygen concentrations, rock algae and returning sockeye salmon in a creek inWoodRiver, south-westAlaskawere collected over a seven year period.  In the salmon spawning season, dissolved nitrogen increased by 190% and phosphorus increased by 390%, but algal biomass and primary production declined by about 90%.  The non-response of algae to increased nutrient levels was explained by the large-scale disturbance – to 50-70% of the stream bed – caused by the actions of salmon in digging nests and displacing the bottom gravel and pebbles.  Sediment disturbance depressed the growth of surface algae but encouraged microbial production, the breakdown of organic matter, and the incorporation of salmon nutrients into the decomposer food chain rather than the photosynthetic pathway.  It seems that in streams where the bottom sediments are vulnerable to disturbance by salmon (i.e. where particles are smaller than about 65 mm across), the physical activities of salmon can have at least as great an influence on ecological processes as their role in introducing nutrients. 

Reference:  Holtgrieve, G.W. & Schindler, D.E.   2011.  Marine-derived nutrients, bioturbation, and ecosystem metabolism: reconsidering the role of salmon in streams.  Ecology 92(2), 373–385.