Variability in particle attenuation and chlorophyll fluorescence in the tropical Pacific: Scales, patterns, and biogeochemical implications

The variability in particle attenuation (c(p)) and in chlorophyll in situ f luorescence (F-is) was examined in November 1994 along 150 degrees W in the Pacific Ocean. Two main sources of variation in c(p) and F-is profiles are identified by analyzing data from a 16 degrees S-1 degrees N transect, and from two 5 day stations (5 degrees S and 16 degrees S). The first source r eflects changes in the trophic status resulting from prevailing hydrodynami cal regimes at large scales. By using flow cytometric data and some assumpt ions about the size distribution of the different biological stocks, a deco mposition of c(p) into its vegetal (c(veg)) and nonvegetal (c(nveg)) compon ents is attempted. Within the euphotic layer, c(veg) accounts for 43% of th e total c(p) Signal at the equator and for only 20% in the South Pacific gy re. The nonvegetal component is then subdivided into heterotrophic organism s and detritus contributions. The detrital material is an important contrib utor with 43% of c(p) at 5 degrees S and 55% at 16 degrees S. A further dec omposition of F-is and c(veg) into the three dominant phytoplanktonic group s (Prochlorococcus, Synechococcus, and picoeucaryotes) confirms that picoeu caryotes are important contributors of the vegetal biomass, especially with in and below the deep chlorophyll maximum (DCM) (>50% of the algal stock) a t 16 degrees S. The second, and essentially local, source of variation is r elated to specific rhythms in biological and physiological processes. The p rominent signals detected during the time series occur at the daily scale: besides the pronounced fluorescence depression at noon in upper layers, par ticle attenuation in all the layers examined and fluorescence in the DCM di splay conspicuous daily oscillations. They result from the balance between daytime accumulation and night removal of particles, of algal cells in part icular. Finally, the estimation of cp-based growth rates points out the sur prisingly rapid turnover time of the whole particulate matter stock in olig otrophic waters (16 degrees S), not only in the euphotic zone (0.63 d(-1)) but also within the dimly lit layers of the DCM (0.36 d(-1)). The correspon ding growth rate at 5 degrees S, within a quasi-mesotrophic regime, is 0.47 d(-1) within the euphotic zone.