Distance-based mixing models of δ18 NNO3− and δ18 ONO3− in a marsh-lined estuary with multiple, distinct NO3− sources (Murderkill Estuary, Delaware, USA)

Abstract

The Murderkill Estuary (Delaware, USA) receives principally from its upland watershed and from a wastewater treatment facility. Due to disparate sources, one-dimensional salinity-based mixing models were inadequate for describing distributions of , δ15 , and δ18 . Distance-based mixing models with multiple, spatially-specified inputs were, therefore, applied to describe conservative mixing of these constituents and determine the extent to which biogeochemical reactions lead to non-conservative behavior of . These models closely matched Si observations in both winter and summer, consistent with high wastewater silicate loads and light limitation, and serve to validate modeling parameters for both seasons. A close fit of distance-based models to estuarine observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of , δ15 , and δ18 diverged from estuarine observations, particularly in the oligohaline and polyhaline regions, consistent with in situ nitrogen cycling or additional sources and sinks. Effluent from an adjacent marsh in the lower estuary contained with low δ15 and δ18ONO3, low DO and high concentrations in late summer. This data and previous studies of adjacent Delaware Bay suggest that reactions in marshes and Bay waters likely drove the non-conservative behavior of and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of loss in the estuary. Nonetheless, distance-based models are useful tools for the study of , δ15 and δ18 distributions and cycling patterns in complex marsh-lined estuaries with multiple inputs.