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

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

Carmen Aguilar1,*, Marilyn L. Fogel2, Hans W. Paerl3

1Carnegie Institution of Washington, and University of North Carolina, Institute of Marine Sciences, Morehead City, North Carolina 28557, USA
2Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC 20015, USA
3University of North Carolina, Institute of Marine Sciences, Morehead City, North Carolina 28557, USA
*Present address: University of Wisconsin-Milwaukee, Great Lakes WATER Institute, 600 East Greenfield Avenue, Milwaukee, Wisconsin 53204-2944, USA. E-mail:

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 (δ15Ninitial - δ15Nfinal) 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

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