ENHANCEMENT OF CHLOROPHYLL-A PRODUCTION IN GULF-STREAM SURFACE SEAWATER BY SYNTHETIC VERSUS NATURAL RAIN

Rainwater concentrations of either ammonium or nitrate were sufficient to stimulate chlorophyll a (chl a) production in bioassay experiments using Gulf Stream surface water collected off North Carolina during t he summer of 1991. Previous studies primarily examined inshore waters and did not address the impact of rainwater ammonium. An increase in c hl a occurred within 1 d of the addition of synthetic rainwater (2 or 5 % rainwater, 98 or 95% seawater) containing up to 10 muM ammonium; t his increase was followed by a decrease in chl a the following day. A similar response to nitrate addition (5 % addition of 20 muM nitrate r ain) was observed. In separate experiments, natural rainwater having n itrate and ammonium concentrations less than those in the experimental synthetic rain yielded a greater chl a response than synthetic rain w hen added at similar dilutions (0.5 to 5.0% rain). The maximum dissolv ed inorganic nitrogen concentration in the enriched seawater in these bioassays was 1.8 muM; prior to enrichment the maximum was <0.4 muM. B ioassay experiments begun 2 d after a major storm event (sustained NE winds with gusts to 13 m s-1 and ca. 390 mumol m-2 inorganic nitrogen deposition from rain) showed a chl a increase in response to addition of natural rainwater, but not to synthetic rainwater with similar diss olved inorganic nitrogen concentration. These results suggest that phy toplankton stimulants, in addition to nitrate and ammonium, exist in n atural rain but not in the synthetic rain used in these experiments.