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Modern history of hypoxia in Narragansett Bay: the geochemical record
Citation:
Boothman, W. AND L. Coiro. Modern history of hypoxia in Narragansett Bay: the geochemical record. Coastal and Estuarine Research Federation (CERF) 24th bienial conference, Providence, Rhode Island, November 05 - 09, 2017.
Impact/Purpose:
This abstract describes the results of a field study determining concentrations of molybdenum (Mo) as a function of depth in sediment cores. Prior research shows that concentrations of Mo in sediments reflect the length of time overlying waters are hypoxic (concentrations of dissolved oxygen < 2.8 mg O2/L). Using profiles of lead-210 and other data to determine a chronology for each core site to convert depth to a date of deposition, profiles of Mo in the cores will provide a history of the frequency of hypoxia at each site. Comparison of such history with the development of anthropogenic influences in the surrounding watershed, such as population growth and implementation of waste treatment facilities may provide insight into the potential effectiveness of reductions of nutrient loadings to urban estuarine waters.
Description:
Increased inputs of nitrogen and other nutrients to estuarine and marine ecosystems from agricultural runoff, urbanization and suburbanization have resulted in degradation of water quality, including increased frequency and severity of hypoxia. While much work has been conducted in recent years to characterize the spatial and temporal extent of hypoxia in coastal systems, the historical record of hypoxia in such systems is much less well known. The current work examines the history of hypoxia in upper Narragansett Bay and nearby Greenwich Bay, an urbanized estuary in the northeastern U.S., through vertical profiles of geochemical markers in sediment cores. Concentrations of molybdenum (Mo), a quantitative surrogate for direct measurement of hypoxic conditions in overlying waters, reveal periods of more frequent hypoxia that vary spatially and temporally throughout the Bay. Variations in the degree and extent of hypoxia are related to changes in the population and anthropogenic inputs to the Bay from the surrounding watersheds, as well as estuarine circulation patterns within the Bay. The more urbanized upper bay shows greater frequency of hypoxia at the turn of the 20th century with conditions improving through mid-century and recurrent hypoxia thereafter. in Greenwich Bay, which has a more suburban watershed, hypoxia was less prevalent in the early 20th century and increased substantially from ~1950 to the present. Carbon and nitrogen concentrations and isotope values reflect increased nitrogen enrichment and productivity in the Bay in the 2nd half of the 20th century. These results can help inform study of the environmental responses to societal activities that may degrade or improve water quality.