Investigating the Causes of Low Dissolved Oxygen in the St. Louis River Estuary

Dissolved oxygen (DO) is important for aquatic organisms and affects aquatic biogeochemistry. Anecdotal reports suggested that areas of low DO existed under the winter ice. In 2013 and 2014 we conducted under-ice surveys in the St. Louis River Estuary to search for areas with low DO, targeting both the main river channel and bays dissociated from the main river channel. We hypothesized the DO would be lower in these embayments because ice cover would reduce oxygen replenishment and primary productivity would be low, while decomposition would continue to reduce oxygen concentrations. We found large areas with severe oxygen depletion (\u3c1mg/L) in Pokegama Bay, a tributary of the St. Louis River Estuary. Smaller bays also exhibited lower DO concentrations (5-10mg/L) than the main river channel (\u3e10mg/L). Many of these low DO zones were near the sediment surface and associated with higher temperatures and higher specific conductivity suggesting an influence of groundwater upwelling zones. In 2014 we conducted surveys in the early and late summer in these areas. Surprisingly, we found extremely low dissolved oxygen (\u3c0.5mg/L) over the full diurnal cycle, even when primary productivity was high and with an open air-water interface, indicating rapid removal of dissolved oxygen. Our study indicates that both groundwater upwelling and decomposition processes likely act to reduce DO concentrations in these bays both under ice and during open water. Additional research is needed to determine whether this phenomenon is natural or anthropogenic and examine the impact on organisms and biogeochemical processes