MEPS 121:239-248 (1995)  -  doi:10.3354/meps121239

Ammonium uptake and regeneration rates were measured in time course experiments with ¹⁵N 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 ¹⁵N. 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 probably underestimated. At 0, 12, and 20 m uptake rates either balanced or exceeded regeneration rates, while at 8 and 25 m net regeneration occurred. The fastest rates were measured during upwelling-induced phytoplankton blooms, intermediate rates characterized post-bloom conditions and the lowest rates coincided with an active upwelling event. Ammonium 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 N m^-2 d^-1), whereas regeneration rates did not differ significantly between seasons (11 to 20 mmol N m^-2 d^-1).

Ammonium uptake . Ammonium regeneration . Coastal upwelling