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

The metabolic activity of the microphytobenthos may effect inorganic nutrie nt exchange across the sediment-wafer interface. Ammonium exchange across t he sediment-water interface was investigated in intact sediment cores and m icrophytobenthos isolated using lens tissue. Ammonium uptake rates of lens tissue extracts in darkness were 11.4 +/- 5 mu mol (mg chl a)(-1) h(-1) (me an +/- 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 ill umination and in the dark affected ammonium exchange after immersion in sed iment cores. In dark-emersed cores there was net ammonium efflux from the s ediment into the overlying water after immersion. An illuminated emersion p eriod reduced the flux of ammonium to the overlying water or affected uptak e after immersion. There was a significant positive relationship between th e length of the period of emersion under illumination (and therefore photos ynthesis) and the subsequent ammonium demand of the sediment after immersio n. Porewater profiles revealed low ammonium concentrations in illuminated-e mersed cores compared to dark-emersed and dark-immersed cores. Ammonium dem and in illuminated-emersed cores was a result of direct and indirect effect s by the microphytobenthos. As well as direct ammonium uptake in the light, the production of oxygen probably stimulated oxic bacterial processes such as nitrification (NH4+ --> NO3-) in the surface of the sediment. These dat a show that the effect of the illumination and emersion period should be co nsidered when nutrient budgets are calculated for intertidal, cohesive sedi ments dominated by microphytobenthos.