Salmon and marine-derived nutrient effects on primary and secondary trophic levels

Graduation date: 2005The purpose of this study was to understand the influence of organic material and nutrients from spawning salmon and supplemented salmon carcasses on stream food webs. My study objectives were to examine 1) assimilation of salmon-derived nutrients (SDN) by producers and consumers in the food web, 2) epilithic biofilm productivity, 3) leaf-litter decomposition rates, and 4) benthic insect density and biomass, in areas with and without spawning salmon and also compare these responses downstream and upstream of salmon carcasses. My hypothesis was that production-related measures of organisms that assimilate SDN would increase in response to spawning salmon or added carcasses. Biofilm, leaf-litter, and macroinvertebrate responses to salmon were evaluated during two field studies in the Wind River basin of southwest Washington. The first study (July - November 2002) was observational and compared responses from a reach with spawning Chinook (Onchorhychus tshawytscha) to two reaches upstream of spawning salmon. In the second experiment (July - October 2003), Chinook carcasses were added and retained within three streams in which responses were measured at increasing distances downstream of the salmon (10m, 50m, 150m, and 250m) and compared to responses measured upstream of salmon. Analysis of stable carbon and nitrogen isotopes demonstrated that SDN from both naturally spawned salmon and manually added carcasses were incorporated into the stream food webs by epilithic biofilm, most benthic insects (scrapers, collectors, and predators), and juvenile steelhead. However, I was unable to detect changes in primary and secondary production-related measures in response to naturally spawned salmon. This observational study was limited in its design and the carcass-addition experiment in the second year provided greater resolution about secondary consumers and spatially explicit responses. Results from the carcass-addition study showed a non-significant increase in epilithic biofilm chlorophyll a levels in October, but no effect on biofilm ash-free-dry-mass. Leaf decomposition rates in September were significantly faster at one site downstream of added carcasses, but shredding insects did not increase in density or biomass, and shredders did not assimilate SDN. Of the nutrients measured (NH⁴-N, NO³-N, NO²-N, DON, SRP, DOC), only ammonium increased significantly downstream of added carcasses. Total benthic insect density significantly increased in September whereas total insect biomass was highly variable and no changes were detected. Densities and/or biomass of some scraping (Heptageniidae) and collecting (Chironomidae and Elmidae) benthic insects increased in September and/or October. Predatory insects did not increase in density or biomass, though they did assimilate SDN. These results suggest a potential bottom-up cascade in which increased primary production was reduced by an increase in secondary consumers. In general, benthic responses were highest within 50 m downstream of added carcasses. Salmon-derived nitrogen was observed in epilithic biofilm and some benthic insects collected 150 m downstream of carcasses. The timing of responses varied depending on the mode of consumption. In limnephiled caddis larvae colonizing carcasses, the SDN signal peaked just 2 weeks after carcasses were added. Among insects that indirectly consumed SDN, the signal peaked 2 months post-carcass addition. Benthic insect production peaked 1.5 months after carcasses were added, with most measures returning to background levels one month later. Augmenting streams with salmon carcasses may influence several ecosystem components, but responses may be spatially localized around carcasses and persist for only a short time after carcasses are added