Anthropogenic influences on estuarine sedimentation and ecology: examples from the varved sediments of the Pettaquamscutt River Estuary,Rhode Island

Estuaries and lakes are undergoing anthropogenic alterations as development and industry intensify in the modern world. Assessing the ecological health of such water bodies is difficult because accurate accounts of pre-anthropogenic estuarine/lacustrine conditions do not exist. Sediments preserved in water bodies provide archives of environmental changes that can be used to understand both natural and anthropogenic forcings. Here, a high-resolution, multi-proxy approach is used to reconstruct environmental conditions of the Pettaquamscutt River Estuary, Rhode Island, over the last millennium. Two specific time periods reveal anthropogenic alterations to sediment supply and water-column productivity. First, European land clearance at the end of the seventeenth century caused two decades of increased sediment transport through the watershed and increased primary productivity in the water column. Turbidity increases associated with increased water column biomass likely limited green sulfur bacteria that reside below the oxycline. The second anthropogenic effect began in the 1950s with increased residential development in the watershed. Evidence of resulting cultural eutrophication is apparent in both stable nitrogen isotope values as well as in productivity proxies. This effect is likely related to residential development in the watershed and the use of septic systems to treat human waste. These results demonstrate that true base-line conditions of the estuary have not occurred for over three centuries, and that anthropogenic effects can last on the order of decades. The Pettaquamscutt River record serves as a model for using high-resolution sediment records to better understand anthropogenic forcing to natural estuarine/lacustrine systems.