MOLLUSCAN DEAD SHELL ASSEMBLAGES ARE ARCHIVES OF URBANIZATION: DISCOVERING HOW NITROGEN ISOTOPIC SIGNATURES VARY ALONG A GRADIENT OF SEPTIC POLLUTION AND AS A FUNCTION OF TROPHIC GROUP

Over the past century, increased coastal urbanization worldwide has resulted in increased nitrogen inputs to ecosystems, which can lead to eutrophication, anoxic conditions and other negative effects on marine wildlife and ecosystem services. Elevated nitrogen isotopic signatures (δ¹⁵N) in the soft tissues from live-collected animals is one established means of detecting eutrophication linked to wastewater inputs. Here, we used δ¹⁵N in molluscan shells from naturally occurring death assemblages to evaluate their potential to identify wastewater nitrogen inputs in two estuaries in Cape Cod, Massachusetts: Waquoit Bay, which currently receives pollution from private septic systems, and West Falmouth Harbor, which receives legacy nitrogen pollution associated with the groundwater plume from a municipal treatment plant. Using dead shells collected in these estuaries at ~0.5 m below low tide, we assessed the isotopic signatures of the nitrogen compounds present in those shells and their variations along the spatial wastewater pollution gradient. We also tested for variation in shell-nitrogen as a function of feeding type, comparing the shells of the filter-feeding mussel Geukensia demissa, the detritus-feeding gastropod Nassarius obsoletus, and the herbivorous gastropod Littorina littorea. The percent of nitrogen recovered from the shells varied widely among species, probably indicating differences in the total nitrogen content of shells, but also across sites, which may be due to local differences in the preservation of shell materials in these sites. We also found that, for all three species, nitrogen isotopic signatures varied significantly, and as expected, δ¹⁵N decreased with distance from the wastewater pollution source. Shells from filter-feeding bivalves had a lower variance in δ¹⁵N than both gastropod species. Dead shells are an effective archive of nitrogen eutrophication along spatial gradients of both ongoing and legacy wastewater pollution. Dead shells would thus provide a useful complement to conventional monitoring programs, providing a means of acquiring the spatial profile from a single sampling, without the need for living animals, and with potential to be used in historical records to evaluate longer-term trends.