Nutrient cycling in the water column of a subtropical seagrass meadow

The cycling of nutrients was studied over a 16 mo period to determine how processes occurring between the water column and benthos influenced nutrient dynamics in a Thalassia testudinum dominated seagrass meadow. Nutrient concentrations were low and ranged from below detection to 0.59 mu M ammonium (NH4+), 0.04 to 0.29 mu M nitrate plus nitrite (NO3- + NO2-), and below detection to 0.22 mu M soluble reactive phosphate (SRP). Water column and benthic fluxes of NO3- + NO2- and SRP were usually below detection. The benthic fluxes of NH4+ ranged from an uptake of -228 mu mol N m(-2) d(-1) to a release of 363 mu mol N m(-2) d(-1). Positive fluxes (i.e. directed out of the sediment) occurred primarily in light incubations and from seagrass-dominated sediments. Water column fluxes of NH4+ ranged from a net uptake of -145 mu mol N m(-2) d(-1) to a net regeneration of 643 mu mol N m(-2) d(-1). The net regeneration of NH4+ in the water column usually exceeded the release of NH4+ from the benthos. There was a significant correlation between the regeneration of NH4+ in the water column and the light-mediated release of dissolved organic matter (DOM) from the benthos, indicating that benthic-derived DOM supported the regeneration of NH4+ in the water column. Bacterioplankton growth efficiencies were significantly and positively correlated to the regeneration of NH4+ in the water column, possibly resulting from changes in the composition of DOM released from the benthos. The C:N ratios of the organic matter remineralized in the water column were variable and ranged from 14 to 81, with lowest values occurring in late summer and highest values in spring. The results of this study indicated that temporal variations in the source and composition of DOM significantly influenced the cycling of nutrients in the water column of this seagrass meadow.