MEPS 257:25-36 (2003)  -  doi:10.3354/meps257025

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