BEAM ATTENUATION AND MICROORGANISMS - SPATIAL AND TEMPORAL VARIATIONSIN 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 Februa ry-March (cruise TT007) and August-September (cruise TT011) 1992. Meri dional sections of c(p) (beam attenuation due to particles) were prepa red by selecting profiles made at a specific time during the day (simi lar to 18:00 h) to reduce the influence of diel variability and to fac ilitate point-to-point comparisons with other variables (e.g. T, nitra te, bioabundance, etc.). A tight correlation between beam c(p) and tot al scattering cross-section of micro-organisms was observed, suggestin g that heterotrophic bacteria, prochlorophytes, cyanobacteria and smal l autotrophic eukaryotes (all < 3 mu m) were dominant contributors to beam c(p). Size-filtration experiments also confirmed that small parti cles (< 8 mu m) accounted for 41-89% of the c(p) signal in the equator ial Pacific. Contributions of the bacterial fraction and detrital mate rial were assessed. Three biohydrographic regimes [northern (7 degrees -12 degrees N), equatorial (5 degrees N-5 degrees S) and southern (7 d egrees-12 degrees S)] could be distinguished from characteristic profi les of c(p) and other variables. While the northern and southern regim es remained relatively constant in c(p) between El Nino (TT007) and co ld 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 t he 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 pro xy for the C:chlorophyll a ratio, also was calculated to obtain insigh t into biogeochemical cycles in the upper waters of the equatorial Pac ific. Copyright (C) 1996 Elsevier Science Ltd.