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

Abstract

As a part of the US Joint Global Ocean Flux Studies, the microphytoplankton cell numbers, volumes and biomass from eight stations on a transect (12S-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 (j.m) 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 Niflo, 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 (Margalefs D > 10.4 at the maxima of each station), and low cell numbers (1.463.73 x cells nv) and low biomass (42.8-97.2 u.g C m ). 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 dinoflagellate numbers, coupled with a shallow mixed layer. Biomass had much the same latitudinal profile. During these El Niflo conditions, this integrated study across a total of 24° latitude shows an anomaly of low equatorial phytoplankton biomass. Introduction Phytoplankton supported by upwelling in the equatorial Pacific area contribute greatly to global new production, and hence this is an important area for the study of global carbon flux from the atmosphere to the sediment (Betzer et al., 1984; Chavez and Barber, 1987; Pefia et al., 1990). The US Joint Global Ocean Flux Study (US JGOFS) project is a component in an international effort to study the oceanic flux and atmospheric cycles of biologically active elements. One series of cruises has been in the vast expanse of the equatorial Pacific, designed to test different time and space scale variability, in part by utilizing transects across the equator. Phytoplankton of different size fractions take different routes in cycling nutrients and carbon (Moloney and Field, 1991). Most of the phytoplankton in this area are smaller than 15 u.m (Pefia et al., 1990) and belong to the size fraction of nanoor picophytoplankton. Microphytoplankton (>15 .m) are unlikely to be included in the nanoand picoplankton microbial loop, and some of their detritus has been found in the surface sediment of the ocean floor. Up to 50 x 10 diatom valves have © Oxford University Press 1167 D.Liu, G.A.Fryxell and I.Kaczmarska been found in a gram of surface sediment (Jouse' et al., 1971), an indication that microphytoplankton are an important part of sedimentation, at least on a long time scale. The microphytoplankton biomass, species composition and the time series study of microphytoplankton were given particular attention in recent US JGOFS research. Iriarte and Fryxell (1995) reported on results on Time Series I and II, cruises that concentrated on short-time-scale changes at the equator (18 and 21 days). Another microphytoplankton study was focused on the 140°W transect (Survey Cruise I, Kaczmarska and Fryxell, 1995), and this study can now contribute additional latitudinal variability in biomass in this nitrogen-rich water. More stations to the south of the equator are included, along with additional stations between previously reported stations. The variability within each microphytoplankton group suggested additional stations for clarification of consistent patterns. The goals of this paper are to determine the total microphytoplankton biomass and the most abundant taxonomic groups along an equatorial Pacific transect with stations from 12°S to 12°N and to observe the variability of each depth, as well as the unique features of this size fraction during an El Nino event.