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

doi:10.1016/0967-0645(95)00038-R

Deep Sea Research Part II: Topical Studies in Oceanography

Volume 42, Issues 2–3, 1995, Pages 499-533

https://doi.org/10.1016/0967-0645(95)00038-R Get rights and content

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 <0.5°C between 120°E and 160°E, and the westward flowing SEC was well developed. These conditions are typical of non-ENSO conditions. An eastward flowing surface current was observed that was probably the result of a westerly wind burst. Between 5°S and 5°N, the barrier layer was much thicker south of the equator and the thickness of the barrier layer varied from 88 m at 2°S to 6 m at 1°N. Nitrate and nitrite were not detected in surface waters, but phosphate and silicate were present at concentrations of about 0.15 μM and 1.5 μM, respectively. Nitrate was always present at the deep chlorophyll maximum (DCM), which varied in depth from about 74 to 96 m.

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 pCO₂ 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.

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

Abstract

Deep-Sea Research

Science of the Total Environment

Deep-Sea Research

Deep-Sea Research

Deep-Sea Research

Progress in Oceanography

Deep-Sea Research

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Deep-Sea Research

Deep-Sea Research

Biochemie and Physiologie der Pflanzen

Deep-Sea Research

Marine Chemistry

Journal of Chromatography

Marine Chemistry

Regulation of primary productivity rate in the equatorial Pacific

Limnology and Oceanography

ENSO events and consequences on the nutrients, planktonic biomass and production in the Western Pacific Tropical Ocean

Effect of El Niño Southern Oscillation events on the distribution and abundance of phytoplankton in the western Pacific tropical ocean along 165°E

Journal of Plankton Research

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