Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean -918 mu mol O-2 m(-2) h(-1)), ammonium (NH4 (+)), nitrate (NO3 (-)), dissolved organic nitrogen, dinitrogen gas (N-2), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH4 (+)), NO3 (-) and N-2 fluxes (annual means +14.6, +15.9 and 44.6 mu mol N m(-2) h(-1)) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification-denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (-1,000 mu mol O-2 m(-2) h(-1)), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH4 (+) flux which may have masked reductions in NH4 (+) flux associated with BMA assimilation during the light. Peaks in net N-2 fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO3 (-) fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification-denitrification may occur in sub-tropical systems at respiration rates exceeding -1,500 mu mol O-2 m(-2) h(-1).