Influence of marine‐derived nutrients from spawning salmon on aquatic insect communities in southeast Alaskan streams

Abstract

The objective of this study was to evaluate the influence of the nutrient transfer system between anadromous salmon and aquatic insect communities across multiple, natural stream systems. Between 2000 and 2002, we sampled seven streams in southeast Alaska, seasonally. Of the seven study streams, four received large annual salmon runs (high‐run streams), and three were no‐run streams. All the streams selected had a natural waterfall barrier to salmon, providing an upstream control reach for each study stream. Insect density, biomass, richness, diversity and functional feeding groups were analyzed before, during and after the fall salmon run in each stream section (i.e. above and below the barrier) of the seven study streams between 2001 and 2002. Results showed that diversity and richness were similar across stream sections and run size within each period, except for during the run when both were significantly lower in downstream sections of high‐run streams. Functional feeding group patterns showed higher abundance and biomass of collector–gatherers and shredders during the post spawning, carcass decomposition period. High‐run streams had upstream sections with greater abundance and biomass of mayflies (dominated by Baetidae, Heptageniidae and Ephemerellidae) during the run, and downstream sections with greater abundance and biomass of dipterans (dominated by Chironomidae). This study suggests that the often published positive relationship between MDN and stream insect abundance and biomass may only exist for certain taxa, primarily chironomid midges.