Primary productivity and size structure of phytoplankton biomass on a transect of the equator at 135°W in the Pacific Ocean

Abstract

The distribution of total and size-fractioned phytoplankton biomass was studied in a transect across the equatorial Pacific at ca 135°W from 15°S to 15°N in April 1988. Cold, nutrientrich surface waters characterized the equatorial region, whereas north and south of this region warmer temperatures prevailed and nitrate was low or undetectable in surface waters. Highest primary productivity and chlorophyll concentration were found in the equatorial region. A subsurface chlorophyll maximum (SCM) was a consistent feature along the transect. The depth of the SCM and integrated production were not related to the depth of the nitracline as has been found in other oceanic regions. An analysis of variance of the chlorophyll concentrations along the transect showed a significant spatial heterogeneity in both the horizontal and vertical. Picoplankton (cells < 1 μm in size), nanoplankton (1–10 gmm) and netplankton (> 10 μm) accounted on average for ca 50, 40 and 10% of the total chlorophyll, respectively. Although a slight increase in the contribution of netplankton was observed at the equator, at all the stations along the transect more than 75% of the chlorophyll was within organisms which passed through a 10 μm Nucleopore ® filter. No significant differences were observed in the relative size distribution of chlorophyll between the surface layer and the chlorophyll maximum. However, below the maximum a significant increase in the contribution of nanoplankton and a decrease in picoplankton was observed. The horizontal pattern observed in the size distribution of phytoplankton along the transect seems to be better explained by the effect of circulation rather than nitrate availability. The lack of correlation of primary productivity and chlorophyll concentration with the vertical nitrate distribution suggests that an important proportion of the nutrient supply may be due to meridional advection of rich nitrate water from the equatorial upwelling region.