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.