Distance‐based mixing models of δ¹⁸ and δ¹⁸ in a marsh‐lined estuary with multiple, distinct sources (Murderkill Estuary, Delaware, USA)

Abstract

The Murderkill Estuary (Delaware, USA) receives NO3− principally from its upland watershed and from a wastewater treatment facility. Due to disparate NO3− sources, one‐dimensional salinity‐based mixing models were inadequate for describing distributions of NO3−, δ15 NNO3−, and δ18 ONO3−. 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 NO3−. 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 NO3− observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of NO3−, δ15 NNO3−, and δ18 ONO3− 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 NO3− with low δ15 NNO3− and δ18ONO3, low DO and high NH4+ 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 NO3− and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of NO3− loss in the estuary. Nonetheless, distance‐based models are useful tools for the study of NO3−, δ15 NNO3− and δ18 ONO3− distributions and cycling patterns in complex marsh‐lined estuaries with multiple NO3− inputs.

Storm-driven hydrological, seasonal, and land use/land cover impact on dissolved organic matter dynamics in a mid-Atlantic, USA coastal plain river system characterized by 21 T FT-ICR mass spectrometry

“…The MR watershed is a mid-Atlantic coastal plain environment that covers approximately 280 km 2 within the greater DE Bay watershed and is predominantly composed of agricultural (52.4% of land cover), developed (15.5% of land cover), and forested (10.8% of land cover) lands (Rogerson et al, 2011). Soil biogeochemistry and riverine inorganic nutrient composition were found significantly different across different LULC units in the system (Fischer, 2014) and could be important factors observed in this study in context of LULC effects. Sampling sites were selected to sample waterways draining sub-watersheds disproportionately impacted by these agricultural (n = 3), developed (n = 1), and forested (n = 2) LULC units according to the most recent Department of Natural Resources and Environmental Control (DNREC) MR watershed report (Rogerson et al, 2011; Figure 1).…”

δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

“…In contrast to ON, relatively rapid turnover makes the isotopic composition of NH 4 + and NO 3 − useful indicators of spatially or temporally explicit changes in N sources and sinks. Specifically, NO 3 − dual isotopes can distinguish atmospheric from biogenic sources (Kendall ) and reveal pollution point sources (Fischer et al ), while decreasing δ 15 N‐NH 4 + values reveal when NH 4 + inputs (mineralization, external point sources) are higher than removal (nitrification, anammox) (Wells et al ). Here, the elevated dry season δ 18 O‐NO 3 − in MRE and NRE indicate atmospheric N inputs, although as a relatively minimal contributor to the estuary N pool given the low associated concentrations (< 1% TN).…”

“…): where S marine is the marine salinity (36) and S s is the salinity at the sampling location. To account for WWTP inputs, δ 15 N fw and N fw downstream from major WWTPs in BRE (Oxley Creek, Luggage Point) were further defined using a distance‐based mixing model (Fischer et al ) as: where the me...…”

Ecosystem scale nitrification and watershed support of tidal creek productivity revealed using whole system isotope tracer labeling

“…Nitrification supplied 5× more than the watershed under dominant base flow conditions. The importance of nitrification in tidal creeks has previously been inferred from natural abundance isotope distributions (Wankel et al ; Fischer et al ), but the added 15 N tracer provided direct quantification of its magnitude. The rate (2.1 mmol N m −2 d −1 ) was midway between mean and median nitrification rates compiled from 562 measurements in other coastal habitats (Fennel et al ), but 10‐fold smaller than whole system nitrification rates in other oligohaline and euryhaline marsh creeks determined with 15 N additions (Gribsholt et al ; Drake et al ).…”

“…3 load, was characterized by negligible assimilation of NO 2 3 by phytoplankton and a dominance of BMA and denitrification with limited transfer of 15 N tracer into benthic consumers (Tobias et al 2003a;Fry et al 2008). Neither study considered the role of nitrification as a contributing DIN source on a whole creek scale.…”