Science Inventory

Molybdenum accumulation in sediments: a quantitative indicator of hypoxic water conditions in Narragansett Bay, RI

Citation:

Boothman, W., L. Coiro, AND S. Moran. Molybdenum accumulation in sediments: a quantitative indicator of hypoxic water conditions in Narragansett Bay, RI. ESTUARINE, COASTAL AND SHELF SCIENCE. Elsevier Science Ltd, New York, NY, 267:107778, (2022). https://doi.org/10.1016/j.ecss.2022.107778

Impact/Purpose:

While low dissolved oxygen (hypoxia) is a significant stressor of fish populations and is linked to excess input of anthropogenic nitrogen in marine ecosystems, determining the extent of hypoxia over large spatial and temporal scales remains challenging. Lab and field studies have indicated that molybdenum (Mo) concentrations in surficial marine sediments may be a quantitative surrogate for direct measurements of hypoxic conditions in overlying waters. We measured authigenic Mo in surface sediments from water quality monitoring sites in Narragansett Bay (RI) and compared the concentrations with mean annual duration of hypoxia in bottom waters at the sites. The strong linear relationship found between mean annual periods of hypoxia and concentrations of authigenic Mo in surface sediments demonstrates the utility of Mo as a quantitative indicator of hypoxia over a range of spatial and temporal scales in coastal water bodies. The relative ease of sampling and analysis enables widespread, detailed spatial analysis of recent occurrences of hypoxia as well as the ability to “look back” at hypoxia in places or times when monitoring instrumentation was not deployed. This approach can be used to relate the intensity and extent of hypoxia to factors such as nutrient loading, with the potential to help set nutrient management goals for coastal water bodies, as well as determination of historical trends of hypoxia, reflecting the impact of past nutrient management steps.

Description:

Authigenic molybdenum (Mo) accumulation in marine sediments has often been used as a qualitative indicator of hypoxic bottom water. To investigate its use as a quantitative indicator of hypoxic exposure, sediment cores were collected from water quality monitoring sites in Narragansett Bay (RI, USA) that experience varying periods of hypoxia. Total Mo concentrations in surficial (0–1 cm) sediments were determined by total digestion and ICP-MS analysis. Lithogenic contributions to total Mo concentrations were estimated by multiplying measured concentrations of aluminum (Al) by the mean crustal Mo:Al ratio and subtracting them from the total concentrations to yield the authigenic fraction. 210Pb dating was used to determine sediment accumulation rates at each site. Mean annual periods of hypoxia in bottom waters were determined from continuous monitoring data for the years coinciding with the top 1 cm of sediment. Results indicated a linear relationship between authigenic Mo concentrations and frequency of hypoxia, although the relationships differed between different sampling periods. These results demonstrate the potential of sedimentary Mo as a tool for assessing the spatial and temporal extent of hypoxia in coastal waters.