Fischer, S. J., York, J. K., Voynova, Y. G. and Ullman, W. J. (2016): Distance-based mixing models of of δ15N NO3- and δ18O NO3- in a marsh-lined estuary with multiple, distinct sources (Murderkill Estuary, Delaware, USA). Limnol. Oceanogr., doi:10.1002/lno.10398



The Murderkill Estuary (Delaware, USA) receives math formula principally from its upland watershed and from a wastewater treatment facility. Due to disparate math formula sources, one-dimensional salinity-based mixing models were inadequate for describing distributions of math formula, δ15 math formula, and δ18 math formula. 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 math formula. 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 math formula observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of math formula, δ15 math formula, and δ18 math formula 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 math formula with low δ15 math formula and δ18ONO3, low DO and high math formula 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 math formula and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of math formula loss in the estuary. Nonetheless, distance-based models are useful tools for the study of math formula, δ15 math formula and δ18 math formula distributions and cycling patterns in complex marsh-lined estuaries with multiple math formula inputs.

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