Mapping Hypoxia Response to Estuarine Nitrogen Loading Using Molybdenum in Sediments.

Area of Science:

• Estuarine Ecology
• Environmental Chemistry
• Water Quality Management

Background:

• Nitrogen loading impacts coastal water quality, leading to hypoxia.
• Spatial and temporal variability in estuaries complicates nitrogen-hypoxia relationship assessment.
• Molybdenum (Mo) in sediments is a proposed quantitative indicator for hypoxia duration.

Purpose of the Study:

• To evaluate the relationship between nitrogen loads and hypoxia duration in Rhode Island embayments.
• To use sediment Mo as a metric for assessing nitrogen impacts.
• To develop a quantitative approach for setting nitrogen loading standards.

Main Methods:

• Estimated nitrogen loads based on watershed land use, normalized for embayment volume and local residence times (LRT).
• Measured Mo concentrations in surface sediments across embayments.
• Applied hydrodynamic modeling to derive LRT.
• Converted sediment Mo to hypoxia duration using established relationships.

Main Results:

• Sediment Mo distribution correlated with normalized nitrogen loads.
• A second-order relationship was observed between Mo concentrations and normalized N loads.
• Quantitative relationships were derived between N loads, sediment Mo, and hypoxia duration.

Conclusions:

• Sediment Mo effectively quantifies hypoxia duration linked to nitrogen loading.
• The derived relationships provide a framework for developing nitrogen standards in coastal waters.
• This approach addresses estuarine variability in assessing nitrogen impacts.