Biomass and productivity of bacterioplankton related to surface water divergence in the northeast equatorial Pacific Ocean

Abstract

Bacterial biomass and productivity together with physico-chemical parameters in the northeast equatorial Pacific Ocean (7 similar to 10.5 degrees N) were investigated during a KODOS (Korea Deep Ocean Study) 96-1 cruise. Surface water divergence occurred at the boundary (8 degrees N) of the north equatorial current and the north equatorial counter current, and largely determined the horizontal and vertical distribution of physico-chemical and microbiological parameters. The diverging area (7 similar to 9 degrees N) was characterized by a shallower thermocline depth, higher nutrient and chlorophyll-a (Chl-a) concentrations, and a higher cyanobacterial cell number than those of the KODOS area (10 similar to 10. 5 degrees N). Bacterial communities in the diverging area showed a high productivity but low biomass. Protozoa grazing seemed to be responsible for the low bacterial biomass. High bacterial turnover rate but low biomass suggested that bacteria in the diverging area may be a trophic link between a photosynthetic carbon source and protozoa, and thus accelerate the microbial loop. Relatively lower bacterial turnover but higher biomass in the KODOS area indicated that resources (organic carbon) limit the bacterial growth, and thus bacteria may play as a sink for photosynthetically fixed organic carbon. The results imply that major ecological role of heterotrophic bacteria in the microbial food web process in the two adjacent waters is different.