Microbiological nitrogen transformation in carbonate sediments of a coral-reef lagoon and associated seagrass beds

Nitrogen fixation (NF), denitrification (DN), microalgal uptake (MU) of dissolved inorganic nitrogen (DIN) and the net diffusive DIN flux (FL) between the sediment and the overlying water were measured in bare carbonate (sand) sediment and seagrass-bed sediment of the shallow (< 3 m) backreef lagoon of a fringing coral reef off Ishigaki Island in the western subtropical Pacific. Nitrogen metabolisms were generally more active in seagrass-bed sediment than in bare carbonate sediment, and in summer than in winter. Bottle-incubation experiments showed that the top 1 cm layer of sediments actively absorbed nitrate and ammonia through photosynthetic processes by benthic microalgae. Light dependence was observed for nitrate uptake, but not for ammonium uptake. MU had a much larger potential flux than NF, DN and FL, while the latter 3 fluxes displayed the same order of magnitude among sites and seasons. Dark incubation of intact cores for FL measurements resulted in nitrate uptake and ammonia release from the sediments, suggesting that the source of ammonia for the microalgae at the surface layer was the pore-water pool, while nitrate was mainly supplied from the overlying water. These results suggested that internal cycling between the surface MU and regeneration at deeper layers principally regulates nitrogen flux and distribution in these shallow carbonate sediments. The presence of microalgae effectively reduced the diffusive loss of nutrients from the sediments, enhancing accumulation of combined nitrogen in otherwise highly oligotrophic carbonate sediments. The sum of the external fluxes (NF, DN, FL) indicated net accumulation of combined nitrogen (i.e. inorganic nutrient and organic nitrogen) in the sediments. It is hypothesized that a significant amount of combined nitrogen is exported from the sediments by processes such as resuspension and grazing of organic detrital particles derived from microalgae and seagrasses in order to balance this accumulation.