Bacterioplankton dynamics in the equatorial Pacific during the 1992 El Niٌo

The stocks and dynamics of bacterioplankton in the upper 200 m were studied in detail providing daily temporal resolution and finescale vertical resolution during two ca 20-day occupations of a station on the equator at the U. S. JGOFS EgPac transect along 140°W longitude, in March–April and October 1992. Euphotic-zone bacterial biomass averaged 70% of phytoplankton carbon on the two cruises. This moderately high ratio was a consequence of low phytoplankton biomass, characteristic of high nutrient, low chlorophyll (HNLC) regimes, rather than high bacterial abundance, which averaged 6–8 × 108 cells l−1 in the upper 100 m. Turnover rates and thus production were low, with bacterial production (BP) averaging less than 20% of primary production (PP) for the two cruises. Biomass was higher and production was lower under the non-El Niño conditions prevailing in October, compared to the March–April observations made in El Niño conditions. Overall, bacterial production was a low fraction of primary production near the equator during both El Niño conditions. We suggest this might be the result of two possible but untested phenomena: (1) BP:PP is generally low (i.e. <20%) in oceanic waters, except during event-scale related disturbances of quasi-steady state conditions in trophic flow structure; and (2) bacteria respond more slowly in upwelling fields than phytoplankton, causing persistent uncoupling between the two groups. This latter condition might be relieved during El Niho when upwelling is reduced.