Research Document 2021/056

Dissolved oxygen as a Marine Environmental Quality (MEQ) measure in upper estuaries of the southern Gulf of St. Lawrence: Implications for nutrient management and eelgrass (Zostera marina) coverage

By Coffin, M.R.S., Poirier, L.A., Clements, J.C., Dickson, E., Guyondet, T., van den Heuvel, M.R.

Abstract

Eutrophication, the biological response to nutrient enrichment, is a well-studied process that poses a threat to coastal ecosystems around the world. Despite an understanding of the mechanisms that contribute to nutrient impacts, there is no universal method for their quantification or monitoring. Herein, a monitoring indicator is presented and evaluated for estuaries that empty into the southern Gulf of St. Lawrence. Two monitoring indicators were assessed, the first evaluating responses of the regionally dominant seagrass, eelgrass (Zostera marina), and the second examining changes in dissolved oxygen (DO). Eelgrass coverage as a percentage of available habitat was previously quantified for each estuary and related to a suite of environmental variables. The only variable for which a significant relationship was found was nitrogen loading which explained 53.0% of the variability in eelgrass coverage using a non-linear response function. The relationship shows that ≈63% of available habitat occupied is the maximum for estuaries within the region. Interpolations using that relationship show eelgrass declines correspond to estuarine external nitrate-N. Evidence is also presented that DO is suited to be a primary indicator of nutrient impacts in systems with high levels of both benthic and pelagic primary production. DO loggers were deployed in the upper estuary, defined by a salinity range of 15-25 PSU and an average water depth of 1-2.5 m of 27 distinct estuaries from 2013-2020 across the southern Gulf of St. Lawrence. Time series data were parsed into biologically relevant metrics that captured symptoms of eutrophication: hypoxia, dissolved oxygen supersaturation, and coefficient of variability. These metrics were analyzed using multivariate statistics and distinct groups formed around habitat type, with algae-dominated habitat reflecting these eutrophication symptoms. The combination of hypoxia and dissolved oxygen supersaturation metrics led to a new metric termed “Eutrophic Time” (the percentage of time an estuary spends <4 mg/L and >10 mg/L dissolved oxygen) which effectively captured estuaries with a tendency towards hypoxia and those towards supersaturation. This new measure was then analyzed through multiple regression with nitrate-N loading and water residence time as predictor variables which were ultimately well correlated (R² = 0.81). This relationship can be exploited to provide estuary-specific nitrate-N loading targets for nutrient impacted systems at management’s discretion.

Research Document 2021/056 (PDF, 1,179 KB)

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