Impact of rainwater hydrogen peroxide on chlorophyll a content of surface Gulf Stream seawater off North Carolina, USA

Bioassays indicate addition of hydrogen peroxide in concentrations similar to rain sometimes decreases chlorophyll a (chl a) production in surface Gul f Stream seawater. Bioassays were conducted on shipboard in the spring and autumn of 1993, 1994, and 1995, using surface Gulf Stream seawater collecte d off the coast of North Carolina. Chi a increases were observed after addi tion of FeCl3 (in 1 of 5 bioassays), iron (III) EDTA (6 of 6 bioassays), or EDTA alone (4 of 4 bioassays). The chi a increases were suppressed signifi cantly in 7 of 11 of these bioassays when the bioassay seawater was initial ly diluted by 1% with a 30 or 40 mu M solution of hydrogen peroxide (a conc entration similar to rainwater). Hydrogen peroxide induced inhibition of ch i a production was not observed in bioassays in which chi a increased in re sponse to addition of nitrate or ammonium, hence the growth inhibition was associated with added metal or complexing agent. Rainwater therefore plays a complex role in primary productivity in surface seawater, with the specif ic effect dependent upon rainwater concentrations of nitrate. ammonium, tra ce metals and hydrogen peroxide, as well as on the extent of nitrogen limit ation and the oxidant concentration in the surface seawater.