Distance-based mixing models of δ18NNO3− and δ18ONO3− in a marsh-lined estuary with multiple, distinct NO3− sources (Murderkill Estuary, Delaware, USA)

Abstract

The Murderkill Estuary (Delaware, USA) receives NO-3 principally from its upland watershed and from a wastewater treatment facility. Due to disparate NO-3 sources, one-dimensional salinity-based mixing models were inadequate for describing distributions of NO-3, δ15 NNO-3, and δ18 ONO-3. 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 NO-3. 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 NO-3 observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of NO-3, δ15NO-3, and δ18 ONO-3 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 NO-3 with low δ15NO-3, and δ18ONO3, low DO and high NH+4 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 NO-3 and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of NO-3 loss in the estuary. Nonetheless, distance-based models are useful tools for the study of NO-3, δ15 NNO-3 and δ18 ONO-3 distributions and cycling patterns in complex marsh-lined estuaries with multiple NO-3 inputs.