Full-depth profile (0–2000 m) of bacteria, heterotrophic nanoflagellates and ciliates in the NW Mediterranean Sea: Vertical partitioning of microbial trophic structures

Abundance and biomass of free-living bacteria, heterotrophic nanoflagellates (HNF) and ciliates were measured monthly throughout the water column at the French-JGOFS time-series station DYFAMED in the NW Mediterranean Sea from May 1999 to March 2000. These microbial heterotrophs decreased by one, two and three orders of magnitude, respectively, over depth (5–2000 m). Vertical profiles were well described by the log–log linear regression (p<0.0001, t-test). Regardless of marked seasonality of environmental variables in the upper layer, the regression slope values (the index of magnitude of depth-dependent decrease) were relatively constant for each group. For 6 of the 9 months studied, the depth-dependent decreases of abundance and biomass were significantly smaller for bacteria than for protists (HNF and ciliates). This demonstrates that the balance between growth and loss processes is less variable for bacteria than for protists over depth. The magnitude of decrease was greater in ciliates than in HNF over depth (6 months for abundance and 5 months for biomass out of the 9 studied). The result suggests that the consortium of strict heterotrophic microbes becomes more dominant with increasing depth down to 2000 m. In the aphotic layer, the depth-dependent decrease for each group of microbial heterotrophs was significantly greater in the bathypelagic layer (500–2000 m) than the mesopelagic layer (110–500 m). A significant difference between groups was found only in the 500–2000 m layer: bacteria <HNF and ciliates for abundance and bacteria <HNF <ciliates for biomass. The ecological possible implication of microbial heterotrophs in these two layers is discussed.