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

Hervé Claustre (1) , Jc Marty (1)

1 LPCM - Laboratoire de Physique et Chimie Marines

Phytoplankton pigment concentrations and primary production rates were measured in the North Tropical Atlantic Ocean (20 degrees N, 31 degrees W) in September-October 1991 and in May-June 1992 to provide new insights into the phytoplankton biomass and dynamics of oligotrophic environments. The overall biomass standing stocks were remarkably constant during both periods (around 23 mg chlorophyll a m(-2)), despite marked differences in the water column stratification. The structure of the autotrophic community was also stable: prochlorophytes, cyanobacteria 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 quantitative stability of the phytoplankton biomass, primary production rates were 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 cross-section for photosynthesis per unit chlorophyll a was constant during both periods (0.063 m(2) g Chla(-1)) suggesting that differences in production rates were mainly governed by variations in irradiance. The photic zone accounted for more than 80% of the integrated production, but less than 50% of the chlorophyll a biomass. Analysis of the photoadaptation characteristics of the dominant populations suggests that cyanobacteria and prochlorophyte distributions are mainly regulated by light, whereas flagellate distribution is mainly linked to nutrient availability. The respective distributions of fucoxanthin, 19'-hexanoyloxyfucoxanthin and 19'-butanoyloxyfucoxanthin suggest that, in such oligotrophic environments, a particular group of 19'-butanoyloxyfucoxanthin-containing flagellates, living close to the nitracline, is responsible 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.

Domaines: Océanographie

Hervé Claustre, Jc Marty. SPECIFIC PHYTOPLANKTON BIOMASSES AND THEIR RELATION TO PRIMARY PRODUCTION IN THE TROPICAL NORTH-ATLANTIC. Deep Sea Research Part I: Oceanographic Research Papers, 1995, 42 (8), pp.1475-1493. ⟨10.1016/0967-0637(95)00053-9⟩. ⟨hal-03479864⟩