Abstract
Macroalgal accumulation and decomposition in shallow water environments typically result in an increase in the organic matter content of the sediment, affecting both benthic metabolism and nutrient dynamics. The present study investigates how a pulse addition of Ulva detritus to estuarine sediment influences the micro-distribution of O2, NO3 and NO2 within the sediment, as well as the rates of oxygen consumption, nitrification and nitrate reduction. The micro-distributions of O2, NO3 and NO2 were monitored with microsensors in Ulva-amended sediment microcosms and in unamended controls during a 1 mo incubation experiment. Process rates were obtained from numerical modeling of the concentration profiles. Oxygen consumption and nitrate reduction were enhanced by a factor of 2 in Ulva-amended sediment compared to the control. This led to a significant reduction in the penetration depths of oxygen and nitrate. Nitrification increased significantly in response to enhanced NH4 supply from decomposition of the Ulva detritus. Aerobic ammonia oxidation exceeded rates of nitrite oxidation, leading to accumulation of NO2 in the oxic zone of the sediment. Nitrite and NO3 produced via nitrification diffused up to the sediment surface, inducing a net efflux to the water column, and downwards, supporting a high rate of denitrification coupled to nitrification. The present study shows that organic enrichment with Ulva detritus enhances sediment oxygen uptake, nitrification and denitrification, the net result being loss of nitrogen from the system. This might constitute a compensating or self-restoration mechanism counteracting an increase in N in intertidal sediment affected by eutrophication-induced macroalgal blooms.
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