Seasonal patterns of delta15N and delta18O-NO3- in the Murderkill River Watershed and Estuary, DE

Fischer, Sarah Jane L.
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University of Delaware
The stable isotopes of dissolved nitrate δ15 N and δ 18 O-NO3- together with nitrogen speciation can be used to track sources and to determine the processes controlling nitrogen attenuation. Five seasonal surveys of δ15 N and δ 18 O-NO3- , NO3- , and related biogeochemical parameters were conducted to investigate nitrogen sources and cycling in surface waters across connected, but distinct, hydrodynamic settings of the Murderkill Watershed, DE. In the upper watershed, little systematic change in NO 3- concentrations and isotopic composition was observed in streams, but four to fifteen fold NO3- decreases and 2-4 / δ15 N and δ18 O-NO 3- enrichment occurred in the ponds during summer months. The ratio of δ15 N:δ18 O enrichment occurred in either a 2:1 or 1:1 ratio or somewhere in between, consistent with plant uptake and/or denitrification. During summer periods, chlorophyll a and dissolved oxygen levels also increased by several factors and therefore some of the first observations of δ 15 N and δ 18 O-NO3- dynamics are reported for freshwater algal blooms. Novel application of a distance-based multi-segmented estuarine model was used to propose δ15 N and δ18 O-NO 3- distribution in the marsh-lined estuary due to mixing of 3-end members: the upper watershed, Delaware Bay, and wastewater effluent of a distinctly enriched isotopic composition. Pronounced deviations between δ 15 N and δ18 O models and actual observations in the lower estuary during summer months reflect the un-modeled biogeochemically-altered discharge from marsh channels. Low dissolved oxygen and evidence of NH 4+ and DON inputs from marsh sources during summer reinforce that there is a seasonal effect of the marsh ecosystem on nitrogen cycling in the main stem of the Murderkill Estuary. Additional δ15 N, δ 18 O, and NO3- data on the outgoing discharge of one marsh channel supports the finding that the marsh discharges isotopically-altered NO3- to the estuary. In early winter, there is little evidence of NO3- uptake and fractionation in the estuary, indicated by the close fit of the isotopic model to isotopic observations and contrasting patterns in dissolved nutrients from summer months. Variable estimates of watershed discharge limited the accuracy of models for quantitative assessments of NO 3- loss or isotopic change, but the models did demonstrate the utility of spatially-precise mixing models in river-like estuaries with spatially and isotopically distinct inputs. The above findings suggest that isotopic signatures and mixing analyses can be used to determine the relative importance of different physical and biogeochemical processes controlling nutrient distributions in contrasting freshwater and estuarine settings on a watershed scale.