The longitudinal influences of dam removal on river ecosystems are increasingly well described, but impacts to aquatic-terrestrial connectivity are largely unknown. Before and four successive years after dam removal, we quantified density and trophic metrics of nearshore tetragnathid spiders and riparian swallows and their potential emergent-aquatic insect prey at two experimental reaches located within the former impoundment (one unrestored, one with channel restoration) and one control reach above an intact low-head dam. We observed substantial annual variability in response variables; however, few patterns appear to be strongly linked to dam removal. We found negligible changes in emergent-insect biomass or community structure, although there were slight changes in the relative abundance of feeding modes (e.g., increased predators, decreased collector-gatherers). The most pronounced response was a ~9.9 times average decline in spider densities after dam removal at the restored reach. Stable-isotope analysis (13C, 15N) indicated that changes in aquatically-derived energy (i.e., nutritional subsidies derived from aquatic primary producers) to riparian consumers generally showed comparable patterns system-wide; swallow aquatically-derived energy declined more in the restored vs. control reach by the final year of the study. Trophic position (TP) of spiders showed little change post-dam removal, while swallow TP converged slightly between control and experimental reaches after dam removal. Collectively, we interpret our results as evidence that low-head dam removal can prompt relatively nuanced shifts in aquatic-terrestrial trophic dynamics, regardless of intensive channel-restoration activities. In contrast, we found that broader-scale forcings (e.g., discharge, regional temperature) may be more influential in governing cross-boundary trophic interactions than the perturbations related to low-head dam removal. This study furthers the current understanding of the effects of dam removal on integrative, food-web metrics of ecosystem structure and function.
Do the ecological impacts of dam removal extend across the aquatic–terrestrial boundary?
Year: 2018