Sediment DIN fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary
Sediment -water fluxes of NH 4 + , NO 3 - , 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 15 N label in benthic microalgae (BMA) and in NO 3 - in the overlying water as a result of an ongoing whole- estuary 15 NO 3 - enrichment study (NISOTREX II). The tracer allowed for estimation of gross NO 3 - 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 NO 3 - 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 NO 3 - . A gross NO 3 - flux from sediments to water of 0.9 to 2.1 mmol N m -2 d -1 was estimated from the isotope dilution of δ 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 δ 15 NO 3 - mixing along estuarine transects. Incorporation of this NO 3 - 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 δ 15 NH 4 + in the water of the cores indicated that the 15 N-labelled BMA were preferentially mineralized over bulk sediment organic N. Additional 15 N enrichments in the sediment bacterial biomarker diaminopimelic acid showed a link among 15 N-labeled BMA, active bacteria, and 15 NH 4 + released to the overlying water. Based on δ 15 NH 4 + enrichments in the cores, BMA accounted for approximately 50 to 100% of the N mineralized. An isotopic enrichment of δ 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.