Stream ecosystem structure and function were studied in an acidic second-order Adirondack Mountain stream system with current beaver activity. Acid-neutralizing capacity, pH, dissolved organic carbon, Fe2+, and Mn2+ values were elevated and [Formula: see text], Aln−, and dissolved oxygen concentrations were decreased following water transport through the beaver impoundment. Upstream acidity was primarily ameliorated by [Formula: see text] and Fe retention in the impoundment during the low-flow summer period. High Fe and Al sediment concentrations were present during low-flow periods immediately downstream of the beaver dam. During the high-flow period, Fe and Al concentrations were highest 0.25 km downstream of the dam, owing to slow metal hydrolysis–oxidation kinetics during spring snowmelt. The immediate downstream site exhibited significantly lower invertebrate richness and diversity and collector–filterer, Plecoptera, and Trichoptera densities, but significantly higher total invertebrate, Diptera, Ephemeroptera, predator, and collector–gatherer densities. Significant differences were noted primarily during April and July. Our results indicate that beaver dams modify stream ecosystems longitudinally and temporally and ameliorate stream acidity. Current lotic ecosystem paradigms like the river continuum concept should incorporate "patch" occurrences such as those created by beaver.
Trophic Tangles through Time? Opposing Direct and Indirect Effects of an Invasive Omnivore on Stream Ecosystem Processes