Seasonally distinct taxonomic and functional shifts in macroinvertebrate communities following dam removal

Abstract

Dam removal is an increasingly popular restoration tool, but our understanding of ecological responses to dam removal over time is still in the early stages. We quantified seasonal benthic macroinvertebrate density, taxonomic composition, and functional traits for three years after lowhead dam removal in three reaches of the Olentangy River (Ohio, USA): two upstream of former dam (one restored, one unrestored), and one downstream of former dam. Macroinvertebrate community density, generic richness, and Shannon–Wiener diversity decreased between ∼9 and ∼15 months after dam removal; all three variables consistently increased thereafter. These threshold responses were dependent on reach location: density and richness increased ∼15 months after removal in upstream reaches versus ∼19 months downstream of the former dam. Initial macroinvertebrate density declines were likely related to seasonality or life-history characteristics, but density increased up to 2.27× from year to year in three out of four seasons (late autumn, early spring, summer) across all reaches. Macroinvertebrate community composition was similar among the three reaches, but differed seasonally based on non-metric multidimensional scaling (NMDS) and analysis of similarity (ANOSIM). Seasonal differences among communities tended to decrease after dam removal. We detected community-wide shifts in functional traits such as multivoltinism, depositional habitat use, burrowing, and collector-gatherer feeding mode. We observed that these traits were expressed most strongly with Chironomidae, which was the most abundant family. Our results suggest that seasonal environmental conditions can play a role in the response and recovery of macroinvertebrate communities—often used to monitor ecosystem condition—following dam removal. In particular, macroinvertebrate density and diversity can show recovery after dam removal, especially in seasons when macroinvertebrate density is typically lowest, with concomitant changes to functional trait abundance. Thus, we recommend scientists and managers consider responses to dam removal throughout the year. Further, similar density, generic richness, and functional traits among reaches suggest that channel restoration after dam removal may initially have equivocal effects on invertebrate communities.

Do the ecological impacts of dam removal extend across the aquatic–terrestrial boundary?

“…). Reduced swallow reliance on aquatically‐derived energy was most evident in the restored reach post‐dam removal in the final year of the study, suggesting that in‐stream changes to prey communities potentially played a role in shifting the energy basis of swallow diets (Sullivan and Manning ). In both cases, trends were evident to a greater degree in the restored reach compared to the unrestored reach, and, in the case of spiders, became more strongly aligned post‐dam removal.…”

“…For example, the reliance of aquatically‐derived energy to spider diets was negatively associated with discharge. There was a substantial increase in median discharge (~2 times), which may have affected benthic macroinvertebrates (Sullivan and Manning ), among other responses (geomorphic responses, nutrient availability, etc.). We found initial evidence that regional temperature and, to a smaller degree, precipitation may have also driven patterns of aquatic–terrestrial interactions, particularly for spiders, and these variables have also been linked to waterbird ecology in fluvial systems (Sullivan and Vierling ).…”

“…, Renöfält et al. , Sullivan and Manning ) and lead to shifts from lentic to lotic fish assemblages (Dorobek et al. ).…”

“…Based on findings by Kautza and Sullivan ()—who found that Chironomidae in dammed river reaches transported more aquatically‐derived energy to terrestrial consumers than in free‐flowing reaches—we predicted that dam removal would prompt a decrease in spider and swallow reliance on aquatically‐derived energy. Further, we expected that shifts in emergent‐insect community taxonomic or functional structure (e.g., functional feeding groups) following dam removal (Sullivan and Manning ) would modify food‐web pathways leading to alterations in tetragnathid spider and swallow TP.…”

Influence of Different Types of Small Hydropower Stations on Macroinvertebrate Communities in the Changjiang River Basin, China

“…Macroinvertebrates are often utilized as an indicator to evaluate the changes in a water environment [7][8][9]. Although many studies have investigated the impact of hydropower stations on macroinvertebrates [10][11][12][13][14][15][16][17], their results have been inconsistent. Different types of hydropower stations have different interception effects as well as ecological effects [18].…”