Size structure of phytoplankton biomass in the equatorial Atlantic Ocean

Abstract-The size structure of chlorophyll a (Chl a) and phaeopigments ((20, (10, (3, <2, and <1 ym) has been studied along three transects (4O, 23", and 35OW) covering the entire equatorial Atlantic Ocean, with special attention to the small sizes (<3 pm). Everywhere in the studied area, even in the equatorial upwelling, the bulk of Chl a is within organisms which pass through a 3 pm Nuclepore filter. The vertical distribution of the (1 pm Chl a is closely related to the depth of the nitracline. In the nitrate-depleted mixed layer, the (1 ym Chl a always dominates, and represents 71% of the total Chl a on the average. At the top of the nitracline, the < 1 ym Chl a concentration is maximum but represents only 50% of the total Chl a. In the nitrate-rich waters, whatever the depth, the percentage of (1 pm Chl a is everywhere (50% and decreases downwards. At the bottom of the photic zone, the < 1 prn Chl a values are nearly zero. When integrated over the whole euphotic layer, the < 1 ym Chl a represents about 25% of the total Chl a when nitrate is present at surface; this percentage reaches 60% when the top of the nitracline deepens to 100 m depth. Preliminary measurements of photosynthetic activity (light gradient and time course experiments) indicate that the < 1 ym fraction contains actively photosynthetic organisms but also organisms which are able to fix CO1 in the darkness in a significant proportion. The roles of sinking, nutrients, and light in the vertical distribution of picoplankton are discussed. Our results indicate that the size distribution of Chl a (and especially the relationship between abundance of picoplankton and nitrate distribution) is the same throughout the whole equatorial (MALONE, 1980). Recent studies have focused attention on a smaller size class of autotrophic organisms-the picoplankton ((2 pm, according to SIEBURTH et al., 1978). WATERBURY et al. (1979) and JOHNSON and SIEBURTH (1 979) show the existence of small unicellular cyanobacteria (procaryotes) with a widespread geographical distribution and the ability to achieve considerable densities. Recently, LI et al. (1983) in the eastern tropical Pacific, and PLATT et al. (1983) in the tropical North Atlantic (west of Azores) concluded that autotrophic picoplankton (< 1 prn) are capable of supplying 60% (range 20 to 80%) of the total primary production in the open ecosystem. These two last results are extremely important because they modify our concepts on the biological structure of the pelagic ecosystem, but they are limited in space and time. We .I