SPECIFIC PHYTOPLANKTON BIOMASSES AND THEIR RELATION TO PRIMARY PRODUCTION IN THE TROPICAL NORTH-ATLANTIC

Phytoplankton pigment concentrations and primary production rates were measured in the North Tropical Atlantic Ocean (20 degrees N, 31 degre es W) in September-October 1991 and in May-June 1992 to provide new in sights into the phytoplankton biomass and dynamics of oligotrophic env ironments. The overall biomass standing stocks were remarkably constan t during both periods (around 23 mg chlorophyll a m(-2)), despite mark ed differences in the water column stratification. The structure of th e autotrophic community was also stable: prochlorophytes, cyanobacteri a and flagellates were the dominant autotrophic groups and contributed to 36, 30 and 34% of the chlorophyll a biomass in May-June and 43, 30 and 27% in September-October. The vertical distribution of these taxa was also stable with cyanobacteria dominating at the surface (100-10% of surface irradiance), prochlorophytes at intermediate depths (10-0. 1% of surface irradiance) and flagellates below the euphotic zone (0.1 -0.01% of surface irradiance). Despite this qualitative and quantitati ve stability of the phytoplankton biomass, primary production rates we re significantly higher (p < 0.05) in May-June (352 +/- 68 mg C m(-2) d(-1)) than in September-October (267 +/- 53 mg C m(-2) d(-1)). The cr oss-section for photosynthesis per unit chlorophyll a was constant dur ing both periods (0.063 m(2) g Chla(-1)) suggesting that differences i n production rates were mainly governed by variations in irradiance. T he photic zone accounted for more than 80% of the integrated productio n, but less than 50% of the chlorophyll a biomass. Analysis of the pho toadaptation characteristics of the dominant populations suggests that cyanobacteria and prochlorophyte distributions are mainly regulated b y light, whereas flagellate distribution is mainly linked to nutrient availability. The respective distributions of fucoxanthin, 19'-hexanoy loxyfucoxanthin and 19'-butanoyloxyfucoxanthin suggest that, in such o ligotrophic environments, a particular group of 19'-butanoyloxyfucoxan thin-containing flagellates, living close to the nitracline, is respon sible for the new production associated with the regular diffusion of nitrate, but that diatoms, generally present at background levels, can be responsible for spatio-temporal events of new production.