Mapping Hypoxia Response to Estuarine Nitrogen Loading Using Molybdenum in Sediments

Quantitative relationships between nitrogen loading and ecological effects such as hypoxia are critical to developing nitrogen (N) standards for coastal waters, but spatial and temporal variability within estuaries can make the determination of such relationships difficult. Accumulation of molybdenum (Mo) in surface sediments has been proposed as a quantitative indicator of the duration of hypoxia (defined as dissolved oxygen concentrations below 2.8 mg/L) in overlying waters, providing a metric to evaluate the relationship between varying N loads and the occurrence and duration of hypoxic conditions. Nitrogen loads were estimated for seven Rhode Island embayments based on watershed land use and normalized for embayment volume and local residence times (LRT) derived from hydrodynamic modeling. Mo was measured in surface sediments from sampling sites selected within and across the embayments to span the range of N loads. The spatial distribution of sediment Mo within the embayments closely followed that of normalized N loads, and Mo concentrations approximated a second-order relationship with normalized N loads. Sediment Mo concentrations were converted to mean annual duration of hypoxia using a previously derived linear relationship between Mo in surface sediments and annual duration of hypoxia in overlying water, and a quantitative relationship derived between normalized N loads and annual duration of hypoxia. Evaluation of that relationship provides an approach to develop standards for N loading in coastal waters.