Assessing Nitrogen Dynamics Throughout the Estuarine Landscape

Abstract

Assessing nitrogen dynamics in the estuarine landscape is challenging given the unique effects of individual habitats on nitrogen dynamics. We measured net N2 fluxes, sediment oxygen demand, and fluxes of ammonium and nitrate seasonally from five major estuarine habitats: salt marshes, seagrass beds (SAV), oyster reefs, and intertidal and subtidal flats. Net N2 fluxes ranged from 332 ± 116 μmol N-N2 m−2 h−1 from oyster reef sediments in the summer to −67 ± 4 μmol N-N2 m−2 h−1 from SAV in the winter. Oyster reef sediments had the highest rate of N2 production of all habitats. Dissimilatory nitrate reduction to ammonium (DNRA) was measured during the summer and winter. DNRA was low during the winter and ranged from 4.5 ± 3.0 in subtidal flats to 104 ± 34 μmol 15NH 4 + m−2 h−1 in oyster reefs during the summer. Annual denitrification, accounting for seasonal differences in inundation and light, ranged from 161.1 ± 19.2 mmol N-N2 m−2 year−1 for marsh sediments to 509.9 ± 122.7 mmol N-N2 m−2 year−1 for SAV sediments. Given the current habitat distribution in our study system, an estimated 28.3 × 106 mol of N are removed per year or 76 % of estimated watershed nitrogen load. These results indicate that changes in the area and distribution of habitats in the estuarine landscape will impact ecosystem function and services.