Ammonium distributions, production, and efflux from backreef sediments, St. Croix, US Virgin Islands

Sediments in < 3 m water depth on Tague Bay backreef, St. Croix, US Virgin Islands, were sampled for grain size, Eh, pH, organic carbon, pore-water ammonium and nitrate + nitrite concentrations, ammonium production, and ammonium and nitrate + nitrite benthic fluxes. Most sampling sites had nutrient-poor, oxidized, coarse sand-sized sediments. Ammonium and nitrate + nitrite concentrations were typically < 5 μM; organic carbon was < 0.6 % g dry weight. High spatial heterogeneity on the scale of cm² existed in all parameters measured except grain size, which decreased regularly away from the reef. We suggest that the nutrient-poor conditions were maintained by high turbulence in the environment, which would minimize organic input to the sediments and maximize resuspension and oxygenation. This view was supported by ammonification rates (mean = 70 μmol (1 sediment)⁻¹ d⁻¹) and benthic nitrogen fluxes which were low compared to rates reported from many temperate coastal areas. Benthic nitrogen fluxes were highly variable and averaged 71 ± 95 (mean ± SD) μmol ammonium m⁻² d⁻¹ and −5 ± 40 μmol nitrate + nitrite m⁻² d⁻¹. Local pockets of reducing sediments with ammonium concentrations > 30 μM occurred within the generally low-nutrient environment. Their location was correlated in part to stabilization of sediments by certain benthic microalgae. These areas may be zones of enhanced nitrogen remineralization, benthic nitrogen fluxes, and possibly nitrification coupled to denitrification. Comparison of the range of fluxes predicted from ammonification data to the range of measured fluxes showed both positive and negative deviations. Nitrification coupled to denitrification cannot be ruled out as an explanation for measured fluxes which were lower than the production-predicted fluxes because Eh and pH conditions were within the environmental limits of denitrifying organisms. Loss of nitrogen via denitrification would be an important process to verify in coral reef ecosystems where conservation of nitrogen apparently is otherwise maximized.