Assessing Impacts of Historical Pulp Mill Effluent on Coastal Food Web Structure and Identifying Suitable Bioindicators for Wastewater: a Stable Isotope (δ13C and δ15N) Analysis

Abstract

Boat Harbour, Nova Scotia was a tidal estuary that was converted into a wastewater treatment facility for pulp mill effluent in 1967. Treated effluent from Boat Harbour was discharged through a dam into the coastal Northumberland Strait. Effluent release ceased in January 2020 and remediation of Boat Harbour will soon commence with a goal to restore Boat Harbour to its pre-industrial state tidal estuary. To ensure successful completion of this remediation goal, effective characterization of the ecosystem in Boat Harbour and Northumberland Strait is required before and after remediation. Discharge of effluent contributed significant nutrient and freshwater inputs into the coastal environment, potentially impacting local biogeochemistry and ecosystem structure. This study used stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) of Northumberland Strait taxa to assess spatial variability in nutrient sources and trophic dynamics. Results identified stable isotope variation along a gradient of historical effluent exposure, with depleted δ13C and δ15N values in taxa near Boat Harbour. Additionally, differences in trophic dynamics were observed with stable isotope values suggesting many organisms occupy lower overall trophic positions near the former effluent outflow. Findings of this study may be governed by residual pulp mill effluent-derived nutrients or differences in marine versus terrestrial nutrient sources, with a pronounced coastal salinity gradient. Certain taxa exhibited pronounced spatial variation in δ13C and δ15N. Blue mussel (Mytilus edulis) and mummichog (Fundulus heteroclitus) were the most suitable bioindicators within the coastal environment. This finding contributed to a review of bioindicators for wastewater in stable isotope studies and an assessment of their perceived suitability. Suitable bioindicators exhibited low mobility, a well-defined life history, and limited diet complexity. Stable isotope analysis was effective at delineating nutrient sources and elucidating trophic dynamics and is recommended as a monitoring technique following remediation of Boat Harbour.