AMMONIUM UPTAKE AND REGENERATION RATES IN A COASTAL UPWELLING REGIME

Ammonium uptake and regeneration rates were measured in time course ex periments with N-15 as a tracer. Both ammonium uptake and regeneration rates measured over 12 to 18 h remained essentially constant. However , as the length of the incubations increased the amount of usable data decreased dramatically due to substrate depletion and recycling of N- 15. Mass balance calculations indicated that 22 to 51% of the ammonium removed from the dissolved pool was not recovered in the particulate fraction. This appeared to be a more serious problem at 0 and 8 m (47 %) than at 25 m (22 %). As a result, ammonium uptake rates were probab ly underestimated. At 0, 12, and 20 m uptake rates either balanced or exceeded regeneration rates, while at 8 and 25 m net regeneration occu rred. The fastest rates were measured during upwelling-induced phytopl ankton blooms, intermediate rates characterized post-bloom conditions and the lowest rates coincided with an active upwelling event. Ammoniu m uptake rates were highest during the upwelling season (11 to 17 mmol N m(-2) d(-1)) and lowest during the non-upwelling season (3 mmol Nm( -2) d(-1)), whereas regeneration rates did not differ significantly be tween seasons (11 to 20 mmol N m(-2) d(-1)).