δ13C, δ15N, AND δ34S AS SOURCE INDICATORS FOR PARTICULATE MATTER IN SALEM HARBOR, MA

Increasing turbidity levels in estuarine environments cause harmful changes to the ecosystems within them. Salem Harbor, Massachusetts, has experienced increasing turbidity levels in the past decade, and previous studies suggest a 70% depletion of Zostera marina in that time. The harbor is a mesotidal, well-mixed tidal dominated estuary, and is fed by two small rivers and a wastewater treatment plant. Early industrial impacts, including extensive tannery activity, on the harbor’s tributaries have resulted in significant legacy contamination in sediments, particularly chromium. With a densely populated mooring field, Salem Harbor houses over 1600 permitted recreational boats during the boating season.

In this study, a multi-facetted stable isotopic approach was utilized in order to constrain the origin of particulate matter responsible for turbid conditions. From May 2012 to present, samples have been collected in order to monitor suspended sediment (as point-sampled filter water samples and sediment traps), as well as potential sources of particulate matter (harbor surface samples and filtered water samples from rivers). Four hypothesized sources of this particulate matter have been examined; resuspended surface sediment associated with mooring activity, freshwater runoff from the watershed, phytoplankton blooms, and wastewater effluent. Elemental concentrations (CNS) and stable isotope ratios (δ¹³C, δ¹⁵N, δ³⁴S) of all samples were quantified using a continuous flow elemental analyzer/stable isotope ratio mass spectrometer.

Water filter and sediment trap δ¹³C and C/N ratio results, were consistent with phytoplankton signatures. River input samples were more depleted in δ¹³C than sediment trap samples. Surface sediment samples showed higher C/N ratios than water filter and sediment trap samples. Further, surface sediment samples had negative δ³⁴S values, while river and suspended sediments were positive, suggesting resuspended sediment is not a turbidity source. δ¹³C and C/N data were consistent with phytoplankton organic matter sources, and some river input mixing is likely. The approach used in this study has applications in turbidity source identification for other estuarine environments.