Beam attenuation and micro-organisms: spatial and temporal variations in small particles along 140°W during the 1992 JGOFS EqPac transects

As part of the U.S. JGOFS EqPac program, transmissometer profiles were made from 12°N to 12°S at 140°W during February–March (cruise TT007) and August–September (cruise TT011) 1992. Meridional sections of cp (beam attenuation due to particles) were prepared by selecting profiles made at a specific time during the day (∼ 18:00 h) to reduce the influence of diel variability and to facilitate point-to-point comparisons with other variables (e.g. T, nitrate, bioabundance, etc.). A tight correlation between beam cp and total scattering cross-section of micro-organisms was observed, suggesting that heterotrophic bacteria, prochlorophytes, cyanobacteria and small autotrophic eukaryotes (all < 3 μm) were dominant contributors to beam cp. Size-filtration experiments also confirmed that small particles (< 8 μm) accounted for 41–89% of the cp signal in the equatorial Pacific. Contributions of the bacterial fraction and detrital material were assessed. biohydrographic regimes [northern (7°–12°N), equatorial (5°N–5°S) and southern (7°–12°S)] could be distinguished from characteristic profiles of cp and other variables. While the northern and southern regimes remained relatively constant in cp between El Niño (TT007) and cold surface water (TT011) conditions, the equatorial regime showed > 30% increase in surface beam cp and IBAC (integrated beam attenuation coefficient) during TT011 compared to TT007. This suggests that only the equatorial regime responds sensitively to the hydrodynamic factors (e.g. upwelling, currents, El Niño, tropical instability waves, etc.) regulating particle distributions. The cp:chlorophyll a ratio, a proxy for the C:chlorophyll a ratio, also was calculated to obtain insight into biogeochemical cycles in the upper waters of the equatorial Pacific.