Particulate organic matter and chlorophyll in the surface layer of the equatorial Pacific Ocean along 135° W

Abstract

Vertical and horizontal distributions of chlorophyll (chl) a, phaeopigment, particulate organic carbon (POC) and nitrogen (PON) in the euphotic zone were studied on a transect across the equatorial Paclfic at ca 135' W from 12's to 15" N. POC. PON, chl, and nutrient concentrations were highest near the equator. Despite a coincidence of maximum POC, and PON levels with the subsurface chl maximum (SCM), photoadaptation appears to play the predominant role in the vertical distribution of chl along this equatorial transect. The size distribution of the above properties was examined within the mixed layer and SCM across the same transect. Small particles (< 10 pm) dominated at both depths. In general, phytoplankton cells constituted only a small proportion of the particulate organic matter, particularly in the < 1 pm fraction which had higher proportions of detritus than the other fractions. The residence time of POC outside the equatorial region was 14 d. In the equatorial upwelling region the residence time was shorter (6 to 7 d), indicating that the suspended particles were turning over more rapidly. Results suggest that particulate loss by grazing is the predominant removal mechanism rather than advection or sedimentation. This suggestion is supported by the distributions of phaeopigment to chl a ratios, and the observed particle size distributions.