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

Abstract

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 (δ15N) of particulate N typically showed similar responses to the nitrogen additions. The δ15 decreased as the different DIN sources, having δ15N values near 0‰, were incorporated into cell biomass. The smallest changes (about 1‰) occurred in the Δ15N (δ15N(initial) - δ15N(final)) from nitrate additions. A greater shift of about 2‰ was observed with added DIN from rain, even though δ15N of total DIN was similar. Ammonium additions resulted in the largest difference from the control, about 6 to 7‰. This fractionation is indicative of isotopic fractionation during enzymetic incorporation and active transport of ammonium into the cells. In parallel incubations, 14C-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 incubation.