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 NH,') 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 NH,' in 670 1 mesocosm experiments, performed In October 1993 and March and April 1994. Stable nitrogen isotope measurements ( 6 1 5 ~) of particulate N typically showed similar responses to the nitrogen additions. The S15N decreased as the different DIN sources, having 6"N values near O%, were incorporated into cell biomass. The smallest changes (about 1%) occurred in the A"N (S1SN,nifial-515Nlmal) from nitrate additions. A greater shift of about 2% was observed with added DIN from rain, even though 6'" of total DIN was similar. Ammonium additions resulted in the largest difference from the control, about 6 to 7%0. This fractionation is indicative of isotopic fractionation during enzymatic incorporation and active transport of ammonium into the cells. In parallel incubations, I4C-bicarbonate was added along with rain in addition to all N additions and controls. Subcellular 14C-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 mcubation.