MEPS 257:25-36 (2003) - doi:10.3354/meps257025
Sediment din fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary
Craig Tobias1,2,*, Anne Giblin1, James McClelland1, Jane Tucker1, Bruce Peterson1
ABSTRACT: Sediment-water fluxes of NH4 +, NO3 -, dissolved inorganic carbon, and O2 were measured in cores collected from the upper Rowley River estuary, Massachusetts, an organic nitrogen (N) mineralization, nitrification, and coupled and direct denitrification (DNF). The cores contained 15N label in benthic microalgae (BMA) and in NO3- in the overlying water as a result of an ongoing whole-estuary 15NO3- 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 δ15NO 3- in the overlying water of the cores. The isotope dilution seen in the cores was also detected as a deviation from conservative δ15NO3- 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 δ15NH4+ in the water of the cores indicated that the 15N labelled BMA were preferentially mineralized over bulk sediment organic N. Additional 15N enrichments in the sediment bacterial biomarker diaminopimelic acid showed a link among 15N labeled BMA, active bacteria, and 15NH4+released to the overlying water. Based on δ15NH4+ enrichments in the cores, BMA accounted for approximately 50 to 100% of the N mineralized. An isotopic enrichment of δ15NH4+ 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.
KEY WORDS: Nitrogen · Benthic microalgae · Microphytobenthos · Stable isotopes · Biomarkers · Nutrient flux · Denitrification
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