Importance of terrestrial subsidies for native brook trout in Appalachian intermittent streams

Abstract

Summary Recent studies of perennial streams have shown that changes in riparian vegetation can reduce terrestrial invertebrate subsidies to streams and can cause trophic cascades through aquatic food webs. Intermittent stream food webs have received less attention but may be even more dependent on terrestrial invertebrate subsidies because of limited aquatic invertebrate resources. The objectives of this research were: (i) to quantify the abundance and biomass of aquatic, adult aquatic and terrestrial invertebrates in and entering two Appalachian intermittent streams to determine how these resources vary with environmental factors such as stream flow and (ii) to determine the effects of experimental reductions in terrestrial invertebrate subsidies on brook trout diet. Stream flow was the main factor driving total invertebrate abundance and biomass, and these resources decreased by 71% as the summer dry season progressed. Terrestrial invertebrates represented only 7% of total resource abundance, but made up 54% of brook trout diet by abundance. Experimental reductions in terrestrial invertebrate subsidies resulted in a 49% decrease in terrestrial invertebrate abundance, which resulted in a 55% decrease in terrestrial invertebrates in brook trout diet samples. In contrast to studies of other salmonids in perennial streams, brook trout in these intermittent streams did not switch to consuming more aquatic invertebrates when terrestrial invertebrates were experimentally reduced. Therefore, land use or vegetation changes that cause reductions in terrestrial invertebrate resources may adversely affect brook trout populations in intermittent streams by reducing caloric intake as fish prepare for autumn spawning.