Caroline Cailliau

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

The variability in particle attenuation (cp) and in chlorophyll in situ fluorescence (Fis) was examined in November 1994 along 150°W in the Pacific Ocean. Two main sources of variation in cp and Fis profiles are identified by analyzing data from a 16°S–1°N transect, and from two 5 day stations (5°S and 16°S). The first source reflects changes in the trophic status resulting from prevailing hydrodynamical regimes at large scales. By using flow cytometric data and some assumptions about the size distribution of the different biological stocks, a decomposition of cp into its vegetal (cveg) and nonvegetal (cnveg) components is attempted. Within the euphotic layer, cveg accounts for 43% of the total cp signal at the equator and for only 20% in the South Pacific gyre. The nonvegetal component is then subdivided into heterotrophic organisms and detritus contributions. The detrital material is an important contributor with 43% of cp at 5°S and 55% at 16°S. A further decomposition of Fis and cveg into the three dominant phytoplanktonic groups (Prochlorococcus, Synechococcus, and picoeucaryotes) confirms that picoeucaryotes are important contributors of the vegetal biomass, especially within and below the deep chlorophyll maximum (DCM) (>50% of the algal stock) at 16°S. The second, and essentially local, source of variation is related to specific rhythms in biological and physiological processes. The prominent signals detected during the time series occur at the daily scale: besides the pronounced fluorescence depression at noon in upper layers, particle attenuation in all the layers examined and fluorescence in the DCM display conspicuous daily oscillations. They result from the balance between daytime accumulation and night removal of particles, of algal cells in particular. Finally, the estimation of cp-based growth rates points out the surprisingly rapid turnover time of the whole particulate matter stock in oligotrophic waters (16°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 corresponding growth rate at 5°S, within a quasi-mesotrophic regime, is 0.47 d−1 within the euphotic zone.