Stable Isotopic Evidence for Nutrient Cycling at Two Zostera marina Meadows in Cape Cod, MA

Pleasant Bay, MA is an estuary located on South-East Cape Cod. In 2007, the position of the inlet shifted and a resultant increase in eelgrass biomass and water quality occurred. This study analyzes nitrogen, carbon and sulfur isotopes in eelgrass from 2003 to 2016 at Pleasant Bay and Duck Harbor, MA. The goal is to determine changes in nutrient cycling at Pleasant Bay with a focus on the impact of the inlet formation in 2007; and to determine differences in nutrient cycling between the two sites, given that Pleasant Bay is more “contaminated” than Duck Harbor. The impact of the inlet formation in 2007 at Pleasant Bay is not reflected in the nitrogen and carbon isotopic composition of eelgrass. Shifts in δ34S occur after the inlet formation, and an increase in organic matter and sediment fine particulate appear to be responsible for this shift. These changes in sulfur represent a warning for eelgrass at Pleasant Bay. Temporally, interannual variation in δ15N at Pleasant Bay are observed, probably caused by pulses in anthropogenic or oceanic nitrogen. Spatially, the “contaminated” site Pleasant Bay (anthropogenic and oceanic inputs) has nitrogen isotopic values 1.5‰ more enriched on average than the “control” site Duck Harbor (oceanic input only)