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Establishing an Anthropogenic Nitrogen Baseline Using Native American Shell Middens
Citation:
Oczkowski, A., T. Gumbley, B. Carter, R. Carmichael, AND A. Humphries. Establishing an Anthropogenic Nitrogen Baseline Using Native American Shell Middens. Frontiers in Marine Science. Frontiers, Lausanne, Switzerland, , 79, (2016).
Impact/Purpose:
There has been great interest in understanding how the Narragansett Bay ecosystem will respond to recent nutrient reductions. As a result of concern over Bay water quality and in attempt to restore the estuary back to a healthier state, nutrient inputs (from sewage) have been reduced by more than half. Here we seek to better understand what we are "restoring back to." By measuring nitrogen stable isotopes in recent shells, and comparing them to isotope values in midden shells deposited by Native Americans, before European Contact, we increase our understanding of how the nitrogen dynamics in Narragansett Bay have changed over over the past 3000+ years.
Description:
Narragansett Bay, Rhode Island, has been heavily influenced by anthropogenic nutrients for more than 200 years. Recent efforts to improve water quality have cut sewage nitrogen (N) loads to this point source estuary by more than half. Given that the bay has been heavily fertilized for longer than monitoring programs have been in place, we seek to provide additional insight into how N dynamics have changed over thousands of years. To do this, we measured the N stable isotope (δ15N) values in clam shells from as early as 2700 BP to the present. Samples from Native American middens were compared with those collected, over a range of time periods, locally, from museums, an archaeological company, and graduate thesis projects. Overall, δ15N values in clam shells from Narragansett Bay have increased significantly over time, reflecting the pattern of anthropogenic nutrient enrichment. Pre-colonization midden shell δ15N values were significantly lower than those post-European contact. While there were no statistical differences among shells dated from the late 15th Century to 2005, there was a significant difference between 2005 and 2015 shells, which we attribute to the higher δ15N values in the effluent associated with recent sewage treatment upgrades. In contrast, the δ15N values of shells from the southern Rhode Island coast remained constant through time; while influenced by human activities, these areas are not directly influenced by point-source sewage discharge. Overall, our results suggest that this isotope technique for measuring δ15N values in clam shells may provide useful insight into how N dynamics in coastal ecosystems have changed over thousands of years, thus providing managers vital historical information when setting goals for N reduction.
