Oxygen and nutrient dynamics within mats of the filamentous macroalga Chaetomorpha linum

Concentration profiles of O-2, NH4+, NO3- and PO43 were measured at high sp atial resolution in a 12-cm thick benthic mat of the filamentous macroalga Chaetomorpha linum. Oxygen and nutrient concentration profiles varied depen ding on algal activity and water turbulence. High surface irradiance stimul ated O-2 production in the surface layers and introduced O-2 to deeper part s of the mat while the bottom layers of the mat and the underlying sediment were anoxic. Nutrient concentrations were highest in the bottom layers of the mat directly above the sediment nutrient source and decreased towards t he surface layers due to algal assimilation and enhanced mixing with the ov erlying water column. Increased turbulence during windy periods resulted in more homogeneous oxygen and nutrient concentration profiles and shifted th e oxic-anoxic interface downward. Denitrification within the mat, as measur ed by the isotope pairing technique on addition of (NO3-)-N-15, was found t o take place directly below the oxic-anoxic interface. Denitrification acti vity was always due to coupled nitrification-denitrification, whereby nitri fiers in the mat utilize NH4+ diffusing from below and O-2 diffusing from a bove. The denitrification rate in the mat ranged from 22 mu mol m(-2) h(-1) to 28 mu mol m(-2) h(-1), approximately equivalent to that measured in the surrounding nonvegetated sediment. Although sediment denitrification is su ppressed when the sediment surface is covered by a dense macroalgal mat, th e denitrification zone may migrate up into the mat. In eutrophic estuaries with a large area of macroalgal cover, the physical structure and growth st age of algal mats may thus play an important role in the regulation of nitr ogen removal by denitrification.