Oceanic upwelling results in the intermittent intrusion of cold ocean water enriched in nitrate, and to a lesser extent soluble reactive phosphorus (SRP), into the Kariega Estuary (South Africa). Laboratory mesocosm experiments were conducted to determine the effects of such changes on fluxes of dissolved nutrients across the surface of a salt marsh within the estuary. When replicate mesocosms of the tidal creek and salt marsh were inundated with nonupwelled water (at 25-degrees-C and nitrate concentrations of 4.5-mu-mol 1(-1)), nitrate fluxes in both regions were small, and the tidal creek exhibited net uptake (negative value) of nitrate from the water column (-85-mu-mol m-2 tide-1), and the marsh, net release (positive values; 113-mu-mol m-2 tide-1). When the mesocosms were inundated with upwelled water, at 16-degrees-C and with nitrate concentrations of 24.2-mu-mol 1(-1), both regions exhibited large net uptakes of nitrate (-514-mu-mol m-2 tide-1 and -226-mu-mol m-2 tide-1 for the tidal creek and salt marsh, respectively). In contrast to nitrate, the fluxes of nitrite, ammonium, and SRP were not significantly different under upwelling and nonupwelling conditions, probably because initial concentrations in the two water types were similar. To determine the extent to which the nitrate uptakes were caused by decreased water temperatures or increased concentrations of nitrate, experiments were conducted in which mesocosms were inundated with water with a range of nitrate concentrations (1.8-25-mu-mol 1(-1)), at two temperatures representative of summer upwelling (16-degrees-C) and nonupwelling conditions (25-degrees-C). In both regions, the net fluxes of nitrate were positively correlated with initial concentrations of nitrate in the water column. For any given concentration, the fluxes at 16-degrees-C fell within the range of values at 25-degrees-C, indicating that the shifts in fluxes caused by upwelling occurred in response to increased concentrations in the water column and not reduced temperatures.
