PLANKTON PRODUCTIVITY AND BIOMASS IN THE WESTERN EQUATORIAL PACIFIC -BIOLOGICAL AND PHYSICAL CONTROLS

During October 1990 in the western equatorial Pacific, from 5 degrees S to 5 degrees N along 155 degrees E, the isothermal and isopycnal lin es (near 150 m) domed upwards, surface temperatures were high (29.5-30 .5 degrees C), surface salinites were low (33.8-34.2), SST anomalies w ere < 0.5 degrees C between 120 degrees E and 160 degrees E, and the w estward flowing SEC was well developed. These conditions are typical o f non-ENSO conditions. An eastward flowing surface current was observe d that was probably the result of a westerly wind burst. Between 5 deg rees S and 5 degrees N, the barrier layer was much thicker south of th e equator and the thickness of the barrier layer varied from 88 m at 2 degrees S to 6 m at 1 degrees N. Nitrate and nitrite were not detecte d in surface waters, but phosphate and silicate were present at concen trations of about 0.15 mu M and 1.5 mu M, respectively. Nitrate was al ways present at the deep chlorophyll maximum (DCM), which varied in de pth from about 74 to 96 m. The distribution of chlorophyll a (plus div inyl-chlorophyll alpha) about the equator was distinctly bimodal, with the highest values (0.4 mu g l(-1)) occurring 2 degrees either side o f the equator. Depth integrated values ranged from 23 to 32 mg m(-2), and despite the very different oceanographic conditions these values a re similar to those found in the nutrient-rich waters of the central a nd eastern equatorial Pacific. However, primary production estimates r anged from 96 to 241 mg C m(-2) day(-1) (8.0 to 20.1 mmol C m(-2) day( -1)), and these are much lower than previous rates reported for the ce ntral and eastern equatorial Pacific. The vertical distribution of cya nobacteria varied markedly from 5 degrees S to 5 degrees N. North of t he equator, the concentrations were fairly high and uniform in the upp er water column whereas, south of the equator, surface concentrations were low with a pronounced subsurface maximum. This distribution of cy anobacteria suggests that there was transient enrichment north of the equator but not to the south. The low salinity surface layer was highl y, oligotrophic and appeared to be effectively isolated from the subsu rface fluxes of nitrate or dissolved inorganic carbon (DIC). Productiv ity in the surface layer seemed to be limited by the supply of nutrien ts, although there were indications that transient nutrient enrichment of surface waters may have occurred north of the equator. In the DCM it appeared to be limited by light. Integrated primary production and cyanobacterial cell counts were elevated north of the equator where th e barrier layer was thin. The isolation of the surface waters from the deeper waters that were enriched in nutrients and DIC caused the pCO( 2) values in surface waters to be in near equilibrium with the atmosph ere. Despite a strong N-S gradient in the thickness of the barrier lay er, the nitracline and upper boundary of the deep chlorophyll maximum were independent of latitude and occurred at approximate to 60-70 m, c lose to the depth of 10% light penetration.