Sediment DIN fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary

Sediment-water fluxes of NH4+, NO3-, dissolved inorganic carbon, and O-2 were measured in cores collected from the upper Rowley River estuary, Massachusetts, and used to calculate rates of organic nitrogen (N) mineralization, nitrification, and coupled and direct denitrification (DNF). The cores contained N-15 label in benthic microalgae (BMA) and in NO3- in the overlying water as a result of an ongoing whole-estuary (NO3-)-N-15 enrichment study (NISOTREX II). The tracer allowed for estimation of gross NO3- regeneration in sediments and the contribution of BMA derived N to total mineralization. The mean mineralization rate between sites was 16.0 +/- 2.0 mmol N m(-2) d(-1). Approximately 13 to 56% of the mineralized N was nitrified at rates ranging from 1.8 to 10.1 mmol N m(-2) d(-1). Total denitrification was dominated by direct DNF (3.6 mmol N m(-2) d(-1)) furthest upstream, where NO3- concentrations were highest. Coupled DNF was most important (8.0 mmol N m-2 d(-1)) in the sediments with high nitrification and low water column NO3-. A gross NO3- flux from sediments to water of 0.9 to 2.1 mmol N m(-2) d(-1) was estimated from the isotope dilution of delta(15)NO(3)(-) in the overlying water of the cores. The isotope dilution seen in the cores was also detected as a deviation from conservative delta(15)NO(3)(-) mixing along estuarine transects. Incorporation of this NO3- regeneration into the DNF calculations effectively increased the estimate of direct DNF by up to 50% and decreased the coupled DNF estimate by up to 220%. Increasing delta(15)NH(4)(+) in the water of the cores indicated that the N-15-labelled BMA were preferentially mineralized over bulk sediment organic N. Additional N-15 enrichments in the sediment bacterial biomarker diaminopimelic acid showed a link among N-15-labeled BMA, active bacteria, and (15)NH(4)(+)released to the overlying water. Based on delta(15)NH(4)(+) enrichments in the cores, BMA accounted for approximately 50 to 100% of the N mineralized. An isotopic enrichment of delta(15)NH(4)(+) above background in the estuary was observed at a magnitude consistent with the core-based rates of BMA mineralization. These results provide further evidence that BMA are not unidirectional sinks for water column-dissolved organic nitrogen, but instead act to turn over N between sediments and estuarine water on the scale of days.