Active mineral cycling in a Jamaican seagrass sediment

There was an active cycling of C and N in the sediments underlying a Halodule beaudetti meadow. Intensive C cycling was inferred from high rates of O2 uptake (131 mm01 m-' d-l), CO2 production (199 mm01 m-2 d-l) and sulfate reduction (34 mm01 m-2 d-'). Subsurface input of organic matter was indicated by the high rates of sulfate reduction and the presence of a short-lived organic pool (5 to 10 h). We suggest that this organic pool, which was associated with whole plants, was due to root excretion. There was little accumulation of sulfide, which was probably oxidised in the deeper sediment strata by 02, secreted from roots. Nitrogen cycling was equally active. I5NH,' dilution indicated a net production rate for NHdt of ca 8 mm01 m-2 d' within the sediment, available for root uptake. Ammonium also accumulated in the sediment overnight (ca 7 mm01 m-2) and disappeared during the day, presumably due to greater uptake by the roots in hght compared to dark. Rates of nitrogen fixation were low (ca 1 mm01 m-2 dl ) and did not vary much during the day Dawn denitrification rates were high (ca 4 mm01 m-' dl ) , but they decreased during the day.