Modern history of hypoxia in Narragansett Bay: The geochemical record

Increased inputs of nitrogen from agricultural runoff, urbanization and suburbanization have resulted in degradation of water quality, including increased frequency and severity of hypoxia, in estuarine ecosystems. Much work has been conducted in recent years to characterize the spatial and temporal extent of hypoxia in coastal systems, but 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, an urbanized estuary in the northeastern U.S., through vertical profiles of geochemical markers in sediment cores. Concentrations of authigenic molybdenum indicate more frequent/ longer periods of hypoxia that are related to changes in population and anthropogenic inputs to the Bay from the surrounding watersheds. Cores from the urbanized upper bay, greatly affected by wastewater treatment facilities (WWTFs), indicate greater duration of hypoxia in the 20th century, with periods of hypoxia decreasing through mid-century and recurring thereafter. Trends of hypoxia are closely related to improvements and failures of WWTFs in surrounding communities. In Greenwich Bay, with a suburban watershed and only one WWTF, hypoxia increased substantially in parallel with growth of population in the surrounding watershed. 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 affect water quality.