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

As part of the U.S. JGOFS EqPac program, transmissometer profiles were made from 12 degrees N to 12 degrees S at 140 degrees W during February-March (cruise TT007) and August-September (cruise TT011) 1992. Meridional sections of c(p) (beam attenuation due to particles) were prepared by selecting profiles made at a specific time during the day (similar to 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 c(p) and total scattering cross-section of micro-organisms was observed, suggesting that heterotrophic bacteria, prochlorophytes, cyanobacteria and small autotrophic eukaryotes (all < 3 mu m) were dominant contributors to beam c(p). Size-filtration experiments also confirmed that small particles (< 8 mu m) accounted for 41-89% of the c(p) signal in the equatorial Pacific. Contributions of the bacterial fraction and detrital material were assessed. Three biohydrographic regimes [northern (7 degrees-12 degrees N), equatorial (5 degrees N-5 degrees S) and southern (7 degrees-12 degrees S)] could be distinguished from characteristic profiles of c(p) and other variables. While the northern and southern regimes remained relatively constant in c(p) between El Nino (TT007) and cold surface water (TT001) conditions, the equatorial regime showed > 30% increase in surface beam c(p) 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 Nino. tropical instability waves, etc.) regulating particle distributions. The c(p):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. Copyright (C) 1996 Elsevier Science Ltd.