Biomass and biomass production of heterotrophic bacteria along 140°W in the equatorial Pacific: Effect of temperature on the microbial loop

Biomass and biomass production of heterotrophic bacteria were measured from 12°N to 12°S along 140°W during the El Nifio-affected months of February and March and also in August and September 1992. Although particulate primary production was highest in equatorial waters (2°N to 2°S) even during El Nifio, bacterial production was not significantly higher in these waters compared to elsewhere along the transect. Consequently, the ratio of bacterial production to primary production (BP: PP) varied from a maximum of 0.3 at 12°N and 12°S to a minimum of about 0.12 in equatorial waters. Relatively low bacterial production at the equator seemed to be due to low bacterial biomass, not low growth rates. In August-September, bacterial growth rates were significantly higher in equatorial waters than elsewhere along the transect whereas bacterial numbers and biovolumes were somewhat lower. In contrast to the lack of correlation between bacteria and phytoplankton along the transect, bacterial production increased with primary production, even though water temperatures decreased when the 1992 El Nifio momentarily waned in August and September. Temperature affected bacterial production in short-term bottle experiments, and temperature effects on production should have been measurable in situ, but increases in primary production, not temperature, accounted for the increase in bacterial production from February-March to August-September. These data suggest that large scale changes in bacterial production are controlled by the supply of dissolved organic matter which is coupled somehow to primary production and that primary production, not temperature, is most important in determining the movement of carbon and energy through the microbial loop.