JOURNAL OF PLANKTON RESEARCH | VOLUME 18 | NUMBER 7 | PAGES 1167-1184 | 1996
© Oxford University Press
research-article |
El Niņo (1992) in the equatorial Pacific: low biomass with a few dominating species in the microphytoplankton
Department of Oceanography, Texas A&M University College Station, TX 77843-3146, USA 1Present address: Department of Biology, Texas A&M University College Station, TX 77843-3258, USA 2Present address: Department of Botany, University of Texas Austin, TX 78713-7640, USA 3Present address: Department of Biology, Mount Allison University Sackville, New Brunswick, EOA 3CO, Canada
Received on July 25, 1995; accepted on February 9, 1996 As a part of the US Joint Global Ocean Flux Studies, the microphytoplankton cell numbers. volumes and biomass from eight stations on a transect (12°S–12°N) on or near 140°W from the cruise of the R/V ‘Thomas G.Thompson’ (Cruise TT007) February-March, 1992, are integrated with previously reported counts. Although these large cells (>15 µm) were from a diverse population, with many species (81–137) in this size range noted from each station, only a few (2–7) species made up 50% of the cell abundance of the totals of the diatoms, dinoflagellates, coccolithophorids and other algal cells from discrete water samples taken in the upper 200 m. Even during the 1992 El Niņo, reports indicate that surface nitrate was not depleted near the equator, 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 maxima of each station), and low cell numbers (1.4.6–3.73 × 108 cells m–2) and low biomass (42.8–97.2 µg C m–2). The integrated numbers of larger coccolithophorids and diatoms showed some reduction near the equator, but the large reduction noted in the total phytoplankton from the equator to 2°N was largely due to the dip in dinofiagellate numbers, coupled with a shallow mixed layer. Biomass had much the same latitudinal profile. During these El Niņo conditions, this integrated study across a total of 24° latitude shows an anomaly of low equatorial phytoplankton biomass.