Growth rates and production of heterotrophic bacteria and phytoplankton in the North Pacific subtropical gyre

In field work conducted at 26 degrees N, 155 degrees W, in the North Pacific subtropical gyre, phytoplankton growth rates mu(p) estimated from C-14 labeling of chlorophyll a (chl a) averaged approximately one doubling per day in the euphotic zone (0-150 m). Microbial (microalgal plus heterotropic bacterial) growth rates mu(m) calculated from the incorporation of H-3-adenine into DNA were comparable to or exceeded phytoplankton growth rates at most depths in the euphotic zone. Photosynthetic rates averaged 727 mg C m(-2) day(-1). Phytoplankton carbon biomass, calculated from C-14 labeling of chl a, averaged 7.2 mg m(-3) in the euphotic zone. Vertical profiles of particulate DNA and ATP suggested that no more than 15% of particulate DNA was associated with actively growing cells. Heterotrophic bacterial carbon biomass was estimated from a two-year average at station ALOHA (22 degrees 45'N, 158 degrees W) of flow cytometric counts of unpigmented, bacteria-size particles which bound DAPI on the assumption that 15% of the particles were actively growing cells and that heterotrophic bacterial cells contained 20 fg C cell(-1). The heterotrophic bacterial carbon so calculated averaged 1.1 mg m(-3) in the euphotic zone. Heterotrophic bacterial production was estimated to be 164 mg C m(-2) day(-1), or 23% of the calculated photosynthetic rate. Estimated heterotrophic bacterial rates averaged 0.97 day(-1) in the euphotic zone and reached 4.7 day(-1) at a depth of 20 m. Most heterotrophic bacterial production occurred in the upper 40 m of the euphotic zone, suggesting that direct excretion by phytoplankton, perhaps due to photorespiration or ultraviolet light effects, was a significant source of dissolved organic carbon for the bacteria. Copyright (C) 1996 Elsevier Science Ltd