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 × 107 to 3.9 × 109 cell 1−1, 1.2 × 10−7 gC1−1 to 4.7 × 10−5 gC1−1 and 2.0 × 10−11 gC1−1 h−1 to 9.6 × 10−7 gC1−1 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.