Bottom-up and top-down control of bacterioplankton from eutrophic to oligotrophic sites in the tropical northeastern Atlantic Ocean

Bacterioplankton biomass and production were determined over the whole water column of a eutrophic, a mesotrophic and an oligotrophic site (2300, 3200 and 4500 m deep) in the tropical NE Atlantic Ocean during a EUMELI cruise in May-June 1992. This resulted in an exceptionally large amplitude of data, with abundance, biomass and production ranging from 1.2 x 10(7) to 3.9 x 10(9) cell 1(-1), 1.2 x 10(-7) gC 1(-1) to 4.7 x 10(-5) gC 1(-1) and 2.0 x 10(-11) gC 1(-1) h(-1) to 9.6 x 10(-7) gC 1(-1) h(-1), respectively. These data were analyzed in order to determine whether bacterioplankton was controlled by bottom-up or top-down processes. Regressions of log-transformed bacterioplankton biomass versus log-transformed production were tested at the different sites and in different layers. Slopes of 0.40-0.55 suggest that bacterioplankton is moderately controlled by bottom-up processes increasing from the eutrophic coastal site to the open ocean oligotrophic site and from surface layers (<250 m) to deep waters. These conclusions are in agreement with other indices of bottom-up control like cell size, doubling times of the biomass, per cent of active cells, and relationships of bacterial biomass versus phytoplanktonic biomass and production. Copyright (C) 1996 Elsevier Science Ltd