Rainfall stimulation of primary production in western Atlantic Ocean waters: roles of different nitrogen sources and co-limiting nutrients

Using shipboard bioassays, we examined the roles rainfall, individual and c ombined 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 ne t chlorophyll a (chl a) production were measured in replicated bioassays to assess individual and combined impacts of different constituents of atmosp heric deposition, including natural rainfall, a synthetic rain mix, dissolv ed inorganic nitrogen (DIN; NH4+, NO3-), dissolved organic nitrogen (DON; u rea), phosphorus (PO43-) and iron (as EDTA-chelated and unchelated FeCl3). Natural rainfall and DIN additions most often stimulated CO2 fixation and c hi a production, but frequencies and magnitudes of biostimulation, relative to controls. varied between these indicators. Spatial differences in the t ypes and magnitudes of stimulation were also observed. When added in equimo lar amounts, NH4+ was, at times, more stimulatory than NO3-. The NO3- stimu lation was significantly enhanced by Fe-EDTA. Urea was marginally stimulato ry at the coastal location. PO43- was never stimulatory. Fe-EDTA and EDTA b y themselves stimulated production only at the offshore locations, suggesti ng increased Fe limitation with increasing distance from land. Synthetic ra in, which contained both sources of DIN, but not Fe, generally proved less stimulatory per unit N than natural rainfall. Results indicate a broad sens itivity of these waters to N additions, which in the case of NO3- are enhan ced 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 DI N and co-limiting nutrients (e.g. Fe), contributing to the eutrophication p otential of waters downwind of urban, industrial and agricultural emissions .