Dynamics of atmospheric combined inorganic nitrogen utilization in the coastal waters off North Carolina

Phytoplankton in nitrogen-depleted coastal Atlantic waters off North Carolina, USA, had a positive response to nitrogen added as rain (DIN: NO3- and NH4+) or directly as NO3- or NH4+. Increases in primary production, photopigments, and cellular protein concentrations were observed when nitrogen Limitation was alleviated. NO3- concentrations decreased faster than those of NH4+ in 670 l mesocosm experiments, performed in October 1993 and March and April 1994. Stable nitrogen isotope measurements (delta(15)N) Of particulate N typically showed similar responses to the nitrogen additions. The delta(15)N decreased as the different DIN sources, having delta(15)N values near 0 parts per thousand, were incorporated into cell biomass. The smallest changes (about 1 parts per thousand) occurred in the Delta(15)N (delta(15)N(initial) - delta(15)N(final)) from nitrate additions. A greater shift of about 2 parts per thousand was observed with added DIN from rain, even though delta(15)N Of total DIN was similar. Ammonium additions resulted in the largest difference from the control, about 6 to 7 parts per thousand. This fractionation is indicative of isotopic fractionation during enzymatic incorporation and active transport of ammonium into the cells. In parallel incubations, C-14-bicarbonate was added along with rain in addition to all N additions and controls. Subcellular C-14-labeled fractions from these samples showed a short-term response to nitrogen additions and included an increase in the low molecular weight fraction after the first light incubation (from dawn to dusk). Carbon was allocated into protein after a 24 h period that encompassed the night incubation.