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 P acific subtropical gyre, phytoplankton growth rates mu(p) estimated fr om C-14 labeling of chlorophyll a (chl a) averaged approximately one d oubling per day in the euphotic zone (0-150 m). Microbial (microalgal plus heterotropic bacterial) growth rates mu(m) calculated from the in corporation of H-3-adenine into DNA were comparable to or exceeded phy toplankton growth rates at most depths in the euphotic zone. Photosynt hetic rates averaged 727 mg C m(-2) day(-1). Phytoplankton carbon biom ass, 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 sugges ted that no more than 15% of particulate DNA was associated with activ ely growing cells. Heterotrophic bacterial carbon biomass was estimate d from a two-year average at station ALOHA (22 degrees 45'N, 158 degre es W) of flow cytometric counts of unpigmented, bacteria-size particle s which bound DAPI on the assumption that 15% of the particles were ac tively growing cells and that heterotrophic bacterial cells contained 20 fg C cell(-1). The heterotrophic bacterial carbon so calculated ave raged 1.1 mg m(-3) in the euphotic zone. Heterotrophic bacterial produ ction was estimated to be 164 mg C m(-2) day(-1), or 23% of the calcul ated photosynthetic rate. Estimated heterotrophic bacterial rates aver aged 0.97 day(-1) in the euphotic zone and reached 4.7 day(-1) at a de pth of 20 m. Most heterotrophic bacterial production occurred in the u pper 40 m of the euphotic zone, suggesting that direct excretion by ph ytoplankton, perhaps due to photorespiration or ultraviolet light effe cts, was a significant source of dissolved organic carbon for the bact eria. Copyright (C) 1996 Elsevier Science Ltd