Abstract
Microphytobenthos (MPB) regulate nitrogen fluxes across the sediment-water interface in shallow coastal and estuarine environments, where the water-phase concentrations exhibit pronounced variations in time and space. The impact of MPB and N-availability on anaerobic ammonium oxidation (anammox) and denitrification was studied in experimental microcosms using a combination of 15 N isotope and microsensor techniques (NO 3 - + NO 2 - and O 2 ). The presence of MPB at low water-column NO 3 - concentrations led to an 85 % reduction in the capacity of the sediments for performing the anammox reaction within a 3 wk period, but did not affect the denitrification potential. The presence of MPB also had a significant impact on both O 2 and NO 3 - + NO 2 - (NO x - ) concentrations in the sediment. At low NO 3 - concentrations, NO x - was almost depleted in the alga-colonized sediments within 2 wk of incubation, due to assimilation and inhibition of nitrification. The depth distribution of O 2 displayed significant variations during the light-dark cycles, leading to periodical O 2 exposure of sediment strata that may harbor the anammox process. A sustained high anammox potential in sediments where MPB was allowed to colonize in the presence of 600 μM NO 3 - in the overlying water indicated that a steady supply of NO x - and not protection from O 2 exposure was the vital factor for maintenance of anammox capacity. In this case, NO x - penetrated approximately 9 mm into the sediment. We therefore suggest that a continuous supply of NO x - to the anoxic sediment layers is the key factor for the presence of anammox in marine sediments. On the basis of these results, we suggest that anammox is of very limited significance in environments that periodically experience N-limitation and that occurrence of high rates of anammox in coastal sediments is limited to estuaries with permanently high concentrations of NO x - in the water column.
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