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The metabolic activity of the microphytobenthos may effect inorganic nutrient exchange across the sediment-water interface. Ammonium exchange across the sediment-water interface was investigated in intact sediment cores and microphytobenthos isolated using lens tissue. Ammonium uptake rates of lens tissue extracts in darkness were 11.4 ± 5 μmol (mg chl a) -1 h -1 (mean ± SE). Uptake by lens tissue extracts was 2 to 5 times greater in the light than dark; however, uptake under illumination was not correlated with photosynthetically available radiation (PAR) intensity. Emersion under illumination and in the dark affected ammonium exchange after immersion in sediment cores. In dark-emersed cores there was net ammonium efflux from the sediment into the overlying water after immersion. An illuminated emersion period reduced the flux of ammonium to the overlying water or affected uptake after immersion. There was a significant positive relationship between the length of the period of emersion under illumination (and therefore photosynthesis) and the subsequent ammonium demand of the sediment after immersion. Porewater profiles revealed low ammonium concentrations in illuminated-emersed cores compared to dark-emersed and dark-immersed cores. Ammonium demand in illuminated-emersed cores was a result of direct and indirect effects by the microphytobenthos. As well as direct ammonium uptake in the light, the production of oxygen probably stimulated oxic bacterial processes such as nitrification (NH 4 + → NO 3 - ) in the surface of the sediment. These data show that the effect of the illumination and emersion period should be considered when nutrient budgets are calculated for intertidal, cohesive sediments dominated by microphytobenthos.

Effect of illumination and emersion period on the exchange of ammonium across the estuarine sediment-water interface