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

Nitrogen fixation (NF), denitrification (DN), microalgal uptake (MU) of dis solved 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 Pa cific. Nitrogen metabolisms were generally more active in seagrass-bed sedi ment 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 m icroalgae. Light dependence was observed for nitrate uptake, but not for am monium uptake. MU had a much larger potential flux than NF, DN and FL, whil e the latter 3 fluxes displayed the same order of magnitude among sites and seasons. Dark incubation of intact cores for FL measurements resulted in n itrate uptake and ammonia release from the sediments, suggesting that the s ource of ammonia for the microalgae at the surface layer was the pore-water pool, while nitrate was mainly supplied from the overlying water. These re sults suggested that internal cycling between the surface MU and regenerati on 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 accum ulation of combined nitrogen in otherwise highly oligotrophic carbonate sed iments. The sum of the external fluxes (NF, DN, FL) indicated net accumulat ion of combined nitrogen (i.e. inorganic nutrient and organic nitrogen) in the sediments. It is hypothesized that a significant amount of combined nit rogen is exported from the sediments by processes such as resuspension and grazing of organic detrital particles derived from microalgae and seagrasse s in order to balance this accumulation.