Citation:

Boothman, Warren S. AND L. Coiro. Mapping Hypoxia Response to Estuarine Nitrogen Loading Using Molybdenum in Sediments. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 46(5):1363–1374, (2023). https://doi.org/10.1007/s12237-023-01215-9

Impact/Purpose:

Hypoxia linked to excess input of anthropogenic nitrogen is a significant stressor in marine ecosystems, yet the spatial and temporal extent of hypoxia and its quantitative relationship to inputs of nitrogen in many coastal embayments is often unknown. We measured Mo, a quantitative geochemical surrogate for direct measurement of hypoxic conditions, to assess the spatial and temporal distribution of hypoxia in 7 embayments within Narragansett Bay (RI, USA) and relate the distributions to estimates of nitrogen loads derived from a land-use model and normalized for hydrodynamic characteristics of the water bodies. The results showed a strongly correlated load-response relationship, although they also suggested that high concentrations of boats in nearby mooring fields or marinas could contribute to some of the observed effects. Load-response relationships similarly derived may be used by those responsible for managing the condition of coastal waterbodies to derive water quality standards protective of coastal water bodies or portions within.

Description:

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.

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