ACTIVE MINERAL CYCLING IN A JAMAICAN SEAGRASS SEDIMENT

There was an active cycling of C and N in the sediments underlying a H alodule beaudetti meadow. Intensive C cycling was inferred from high r ates of O2 uptake (131 mmol m-2 d-1), CO2 production (199 mmol m-2 d-1 ) and sulfate reduction (34 mmol m-2 d-1). Subsurface input of organic matter was indicated by the high rates of sulfate reduction and the p resence of a short-lived organic pool (5 to 10 h). We suggest that thi s organic pool, which was associated with whole plants, was due to roo t excretion. There was little accumulation of sulfide, which was proba bly oxidised in the deeper sediment strata by O2, secreted from roots. Nitrogen cycling was equally active. (NH4+)-N-15 dilution indicated a net production rate for NH4+ of ca 8 mmol m-2 d-1 within the sediment , available for root uptake. Ammonium also accumulated in the sediment overnight (ca 7 mmol m-2) and disappeared during the day, presumably due to greater uptake by the roots in light compared to dark. Rates of nitrogen fixation were low (ca 1 mmol m-2 d-1) and did not Vary much during the day. Dawn denitrification rates were high (ca 4 mmol m-2 d- 1), but they decreased during the day.