MEPS 180:65-79 (1999)  -  doi:10.3354/meps180065

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 δ¹⁵N decreased as the different DIN sources, having δ¹⁵N values near 0o/oo, were incorporated into cell biomass. The smallest changes (about 1o/oo) occurred in the Δ¹⁵N (δ¹⁵N_initial - δ¹⁵N_final) from nitrate additions. A greater shift of about 2o/oo 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 7o/oo. This fractionation is indicative of isotopic fractionation during enzymatic 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.

KEY WORDS: Atmospheric deposition · δ¹⁵N · Nitrogen cycling · Phytoplankton · Subcellular ¹⁴C-incorporation