Science Advisory Report 2021/023

Validation of dissolved oxygen (DO) as Marine Environmental Quality (MEQ) measure of nutrient loading status of estuaries

Summary

• The southern Gulf of St. Lawrence is a region characterized by multiple estuaries experiencing similar weather (e.g. freezes over in winter), geography, and lithology, with relatively small watersheds (10s-100s km²) that have low freshwater input that ultimately empty into either the Northumberland Strait or the Gulf of St. Lawrence.
• Estuaries and bays of the southern Gulf of St. Lawrence, like many places around the world, are sensitive to impacts from excess nutrients, including loss of eelgrass and the occurrence of hypoxia/anoxia.
• The monitoring indicator proposed here, termed “Eutrophic Time”, is defined as the percent of time an upper estuary spends <4 mg/L (hypoxia/anoxia) and >10 mg/L (supersaturation), and is the latest iteration using dissolved oxygen as the endpoint of interest.
• Eutrophic Time reflects the relative trophic status of upper estuaries and was effective at discriminating between those dominated by eelgrass (Zostera marina) or macroalgae (Ulva spp.). Further, a threshold for Eutrophic Time was interpolated at 35%, based on an apparent inflection point in habitat type from eelgrass to macroalgae.
• Nitrate-N loading and Water Residence Time were highly predictive (R² = 0.81) of Eutrophic Time but less predictive of hypoxia/anoxia (R² = 0.63).
• This relationship was exploited for Ulva-dominated estuaries to develop nitrate-N loading targets to move towards a Eutrophic Time threshold (0.35) consistent with a habitat shift toward eelgrass and an improvement in water quality.
• Estimated eelgrass coverage in the entire estuary was inversely correlated with nitrate-N loading (though not Water Residence Time), with eelgrass coverage declining as nitrate-N loading increased.
• Eutrophic Time can be used to identify estuaries that should benefit ecologically, including improved water quality and eelgrass recovery and conservation, and coarse nitrogen reduction targets are provided to do so.
• There was insufficient long-term data to provide a recommendation towards an effective monitoring cycle. Monitoring cycles should reflect the needs of the monitoring program to inform effective decision making and adaptive management.

This Science Advisory Report is from the Regional Advisory Meeting of February 23-24, 2021 on the Validation of DO as MEQ measure of nutrient loading status of estuaries. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/023 (PDF, 1,259 KB)

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