Allochthonous versus Autochthonous Organic Matter Contributions to the Trophic Support of Fish Populations in Clear-Cut and Old-Growth Forested Streams

Abstract

Annual organic matter inputs and production of stocked coho salmon (Oncorhynchus kisutch), coastal cutthroat trout (O. clarki clarki), and shorthead sculpin (Cottus confusus) from spring through early autumn were monitored for 2 yr in two headwater tributaries of the Deschutes River, Washington. One site was bordered by old-growth coniferous forest; the other was an area clear-cut without buffer strips 7 yr before the study. Allochthonous organic matter (terrestrial origin) dominated inputs to the old-growth site and contributed ~300 g∙m−2∙yr−1, while autochthonous organic matter totaled ~100 g∙m−2∙yr−1. In the clear-cut site, autochthonous inputs contributed ~175 g∙m−2∙yr−1, but allochthonous inputs contributed only ~60 g∙m−2∙yr−1 owing to loss of riparian vegetation. Although combined allochthonous and autochthonous inputs were almost twofold greater in the old-growth site, fish production was greater in the clear-cut site. Production of coho salmon and shorthead sculpin during early summer was largely responsible for differences between sites. Fish populations appeared to depend upon food derived from autotrophic pathways during spring and summer in the presence or absence of forest canopy, a hypothesis supported by analysis of coho stomach contents and the similar ratios of autochthonous inputs and fish production between the two streams.