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

Phytoplankton in nitrogen-depleted coastal Atlantic waters off North Caroli na, USA, had a positive response to nitrogen added as rain (DIN: NO3- and N H4+) or directly as NO3- or NH4+. Increases in primary production, photopig ments, and cellular protein concentrations were observed when nitrogen Limi tation was alleviated. NO3- concentrations decreased faster than those of N H4+ in 670 l mesocosm experiments, performed in October 1993 and March and April 1994. Stable nitrogen isotope measurements (delta(15)N) Of particulat e 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 ch anges (about 1 parts per thousand) occurred in the Delta(15)N (delta(15)N(i nitial) - delta(15)N(final)) from nitrate additions. A greater shift of abo ut 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 lar gest difference from the control, about 6 to 7 parts per thousand. This fra ctionation is indicative of isotopic fractionation during enzymatic incorpo ration and active transport of ammonium into the cells. In parallel incubat ions, C-14-bicarbonate was added along with rain in addition to all N addit ions and controls. Subcellular C-14-labeled fractions from these samples sh owed a short-term response to nitrogen additions and included an increase i n 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 e ncompassed the night incubation.