Sediment composition and nitrate availability control denitrification rates in the Saint Louis River Estuary

Background/Question/Methods

Humans have greatly increased availability of bioavailable nitrogen, which can affect the productivity and toxicity of aquatic ecosystems.  Nitrate concentrations in Lake Superior have increased fivefold in the past century, with minimal removal occurring in the open lake.  The Saint Louis River Estuary (SLRE) is the largest U.S. tributary input to Lake Superior, and is also a mixing zone of water from Lake Superior, the Saint Louis River, and the cities of Duluth and Superior.  Lake Superior supplies the lower portion of the SLRE with nitrate through seiche events; river inputs have high dissolved organic carbon concentrations; the cities contribute nutrient rich wastewater and stormwater. Freshwater estuaries such as the SLRE are poorly understood relative to saline estuaries, but they could be potential hotspots of aquatic nutrient processing.  We analyzed the SLRE mixing pattern and explored the ability of the estuary to remove bioavailable nitrogen during the 2012 ice-free season.  We performed seven longitudinal surveys of the estuary to determine nutrient concentrations, sediment properties, and sediment denitrification rates.  We used the acetylene block technique with nutrient amended site water to measure potential denitrification rates and varying input source waters to identify controls on this process.

Results/Conclusions

The greatest potential denitrification rates occurred in the lower estuary, and were one to two orders of magnitude greater than those reported in the open lake.   Sediment organic matter was highest in the lower estuary, and was strongly correlated with potential denitrification rates.  Sediment denitrification rates increased with increased nitrate concentrations in treatment water, suggesting nitrate control of denitrification rates.  These rates increased with warming spring temperatures and were highest before the Saint Louis River recorded a 100-year flood in late June, which greatly reduced sediment organic matter.  Collectively these results indicate that sediment composition, overlying nitrate concentrations, and temperature are the chief regulators of in situ denitrification in the Saint Louis River Estuary.  These results may apply to other estuaries with similar physical composition and nutrient availability, which can be identified as nitrogen sinks.  However, 2012 demonstrates a dynamic balance between these controls, reflecting the opposing effects of flooding on sediment organic matter versus the lake input effect on nitrate.