El Nio (1992) in the equatorial Pacific: low biomass with a few dominating species in the microphytoplankton

As a part of the US Joint Global Ocean Flux Studies, the microphytoplanktoncell numbers. volumes and biomass from eight stations on a transect(12S–12N) on or near 140W from the cruise of the R/V‘Thomas G.Thompson’ (Cruise TT007) February-March,1992, are integrated with previously reported counts. Althoughthese large cells (>15 µm) were from a diverse population,with many species (81–137) in this size range noted fromeach station, only a few (2–7) species made up 50% ofthe cell abundance of the totals of the diatoms, dinoflagellates,coccolithophorids and other algal cells from discrete watersamples taken in the upper 200 m. Even during the 1992 El Nio,reports indicate that surface nitrate was not depleted nearthe equator, but the low numbers of cells in this size fractionindicate that an unknown factor (other than nitrate or light)limited the growth. This synthetic analysis shows high diversity(Margalef's D > 10.4 at the maxima of each station), andlow cell numbers (1.4.6–3.73 10⁸ cells m^–2) andlow biomass (42.8–97.2 µg C m^–2). The integratednumbers of larger coccolithophorids and diatoms showed somereduction near the equator, but the large reduction noted inthe total phytoplankton from the equator to 2N was largelydue to the dip in dinofiagellate numbers, coupled with a shallowmixed layer. Biomass had much the same latitudinal profile.During these El Nio conditions, this integrated study acrossa total of 24 latitude shows an anomaly of low equatorial phytoplanktonbiomass.