EL-NINO (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 microphytopla nkton cell numbers, volumes and biomass from eight stations on a trans ect (12 degrees S-12 degrees N) on or near 140 degrees W from the crui se of the RN 'Thomas G. Thompson' (Cruise TT007) February-March, 1992, are integrated with previously reported counts. Although these large cells (>15 mu m) were from a diverse population, with many species (81 -137) in this size range noted from each station, only a few (2-7) spe cies made up 50% of the cell abundance of the totals of the diatoms, d inoflagellates, coccolithophorids and other algal cells from discrete water samples taken in the upper 200 m. Even during the 1992 El Nino, reports indicate that surface nitrate was not depleted near the equato r, but the low numbers of cells in this size fraction indicate 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 m axima of each station), and low cell numbers (1.46-3.73 x 10(8) cells m(-2)) and low biomass (42.8-97.2 mu g C m(-2)). The integrated number s of larger coccolithophorids and diatoms showed some reduction near t he equator but the large reduction noted in the total phytoplankton fr om the equator to 2 degrees N was largely due to the dip in dinoflagel late numbers, coupled with a shallow mixed layer. Biomass had much the same latitudinal profile. During these Fl Nino conditions. this integ rated study across a total of 24 degrees latitude shows an anomaly of low equatorial phytoplankton biomass.