Plankton productivity and biomass in the western equatorial Pacific: Biological and physical controls

Abstract During October 1990 in the western equatorial Pacific, from 5°S to 5°N along 155°E, the isothermal and isopycnal lines (near 150 m) domed upwards, surface temperatures were high (29.5–30.5°C), surface salinites were low (33.8–34.2), SST anomalies were The distribution of chlorophyll a (plus divinyl-chlorophyll a ) about the equator was distinctly bimodal, with the highest values (0.4 μg 1 −1 ) occurring 2 degrees either side of the equator. Depth integrated values ranged from 23 to 32 mg m −2 , and despite the very different oceanographic conditions these values are similar to those found in the nutrient-rich waters of the central and eastern equatorial Pacific. However, primary production estimates ranged 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 central and eastern equatorial Pacific. The vertical distribution of cyanobacteria varied markedly from 5°S to 5°N. North of the equator, the concentrations were fairly high and uniform in the upper water column whereas, south of the equator, surface concentrations were low with a pronounced subsurface maximum. This distribution of cyanobacteria suggests that there was transient enrichment north of the equator but not to the south. The low salinity surface layer was highly oligotrophic and appeared to be effectively isolated from the subsurface fluxes of nitrate or dissolved inorganic carbon (DIC). Productivity in the surface layer seemed to be limited by the supply of nutrients, 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 the barrier layer was thin. The isolation of the surface waters from the deeper waters that were enriched in nutrients and DIC caused the p CO 2 values in surface waters to be in near equilibrium with the atmosphere. Despite a strong N-S gradient in the thickness of the barrier layer, the nitracline and upper boundary of the deep chlorophyll maximum were independent of latitude and occurred at ≈60–70 m, close to the depth of 10% light penetration.