Subfossil Chironomid Assemblages as Indicators of Remedial Efficacy in the Historically Contaminated St. Lawrence River at Cornwall, Ontario

Abstract

Long-term data are required to quantify the impacts of historic industrial pollution and subsequent remedial action on the nearshore benthic community in the St. Lawrence River Area of Concern at Cornwall, Ontario. Speci cally, high-quality temporal records are needed to understand changes in benthic invertebrate assemblages in response to multiple possible drivers including industrial pollution, environmental heterogeneity, and climate warming. We compare long-term records of subfossil chironomid assemblages and geochemical variables among sediment cores from two Cornwall sites with differing pollution histories and a minimally disturbed downstream reference site. Chironomids were functionally absent from the Cornwall sediment cores when mercury and zinc concentrations were elevated. As metal concentrations decreased in more recent sediment intervals, chironomid abundance and the relative abundance of pollution-sensitive taxa increased. Recently deposited sediment in all three sediment cores display increased relative abundance of warm-water, macrophyte-associated taxa. We conclude that these temporal changes in chironomid assemblages provide evidence for ecological recovery for both of the impacted sites, consistent with the objectives of the current management strategy. These ndings advance our understanding of industrial impacts on uvial chironomid ecology, directly inform local management strategies, and further develop the application of chironomids as bioindicators for contaminated sediments.

Response of Chironomidae (Diptera) to DDT, Mercury, and Arsenic Legacy Pollution in Sediments of the Toce River (Northern Italy)

“…Regarding water quality analysis, data on total alkalinity, nitrates (N-NO 3 ), ammonia (N-NH 4 ), total phosphorous and dissolved DDT (sum of DDT, DDE, and DDE), and trace elements were provided by ARPA Piemonte [ 14 ], with site and sampling season used as criteria to match chemical data with chironomid samplings ( Table 1 ). Those data were included to test the potential influence of water parameters on chironomids.…”

“…Conversely, field studies at the community level have rarely shown clear relationships between species and toxicants, as other variables may act as confounding factors. For example, long-term shifts in taxa assemblages were described along sediment cores collected from historically contaminated sites, yet variations in contaminant concentrations have often been associated with trends of other significant drivers, such as nutrient loads or water temperature [ 13 , 14 ]. Studies conducted in contaminated rivers have frequently revealed community structures strongly correlated with variables expressing the longitudinal axis of the river, such as distance from the source, altitude, and water temperature [ 15 ].…”