Honea, Jon M. 2005. Effect of salmon spawning on seasonal changes in structure and function of stream macroinvertebrate communities. Ph.D.
Macroinvertebrates play a vital role in the uptake and retention of resources in riverine food webs; however, this function is mediated by processes of natural disturbance. I investigated the hypothesis that spawning salmon affect macroinvertebrates negatively and positively—the former due to the disturbance of redd excavation and the latter due to nutrients released by salmon during spawning and carcass decomposition. To test this hypothesis, macroinvertebrate density and biomass changes were monitored for five seasons in a small Puget Lowland stream with a large run of chum salmon (Oncorhyncus keta). Over the 1st 4 seasons, stable isotopes of carbon and nitrogen were measured in representative taxa of major functional feeding groups of macroinvertebrates to identify the relative contributions of base food sources to macroinvertebrates. Salmon-derived nutrients comprised a substantial quantity of macroinvertebrate biomass all year long in the salmon spawning reach, representing 22% of total macroinvertebrate biomass at its lowest level. Immediately after spawning, the proportion increased as macroinvertebrates fed on salmon carcasses and/or other material that had accumulated salmon nutrients; however, the total salmon-derived macroinvertebrate biomass decreased as a consequence of reduced density and biomass following the disturbance of redd excavation. The highest percentage (56%) and total biomass (2.71 g m-2) of salmon-derived tissue in macroinvertebrates was found in samples 3 months after spawning. Collector-filterers, scrapers, and predators were estimated to accumulate the greatest quantities of salmon nutrients among macroinvertebrates over the study period. Macroinvertebrates fed on by resident cutthroat trout increased in density and biomass in the spawning reach faster than they did upstream of a series of waterfalls, suggesting that macroinvertebrates are not a sink of salmon-derived nutrients and are likely active in the transfer of these nutrients to other consumers, including juvenile salmon. After 4.5 months the average proportion of salmon-derived biomass in macroinvertebrates was approaching the baseline level found before spawning. Macroinvertebrates consumed an estimated 2.2–5.6% of the salmon biomass from the 2000 escapement of chum in Kennedy Creek. As macroinvertebrates slow the export of salmon nutrients from spawning streams, they prolong its availability to higher consumers, including juvenile and resident salmonids.
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