MEPS 176:205-214 (1999)  -  doi:10.3354/meps176205

ABSTRACT: Using shipboard bioassays, we examined the roles rainfall, individual and combined nutrients play in accelerating primary production in coastal, Gulf Stream and pelagic (Sargasso Sea) locations in the North Atlantic Ocean off North Carolina, USA, from 1993 to 1995. Photosynthetic CO₂ fixation and net chlorophyll a (chl a) production were measured in replicated bioassays to assess individual and combined impacts of different constituents of atmospheric deposition, including natural rainfall, a synthetic rain mix, dissolved inorganic nitrogen (DIN; NH₄⁺, NO₃^-), dissolved organic nitrogen (DON; urea), phosphorus (PO₄^3-) and iron (as EDTA-chelated and unchelated FeCl₃). Natural rainfall and DIN additions most often stimulated CO₂ fixation and chl a production, but frequencies and magnitudes of biostimulation, relative to controls, varied between these indicators. Spatial differences in the types and magnitudes of stimulation were also observed. When added in equimolar amounts, NH₄⁺ was, at times, more stimulatory than NO₃^-. The NO₃^- stimulation was significantly enhanced by Fe-EDTA. Urea was marginally stimulatory at the coastal location. PO₄^3- was never stimulatory. Fe-EDTA and EDTA by themselves stimulated production only at the offshore locations, suggesting increased Fe limitation with increasing distance from land. Synthetic rain, which contained both sources of DIN, but not Fe, generally proved less stimulatory per unit N than natural rainfall. Results indicate a broad sensitivity of these waters to N additions, which in the case of NO₃^- are enhanced by Fe-EDTA. At all locations, the high level of stimulation of primary production attributable to natural rain may be due to the supply of both DIN and co-limiting nutrients (e.g. Fe), contributing to the eutrophication potential of waters downwind of urban, industrial and agricultural emissions.

KEY WORDS: Atmospheric deposition · Nitrogen · Iron · Primary production · Eutrophication · W. Atlantic Ocean