Vertical and seasonal variations of bacterial abundance and production in the mesopelagic layer of the NW Mediterranean Sea: bottom-up and top-down controls

Bacterial abundance and production were measured in the mesopelagic layer (110–1000 m) at the French-JGOFS time-series station DYFAMED (NW Mediterranean) from June 2001 to October 2002. Abundance and production were seasonally variable down to 300 and 500 m, respectively. Depth-dependent decreases of abundance and production were well described by the log–log linear regression (p<0.05, t-test). The mean (± SD) magnitude of decrease (regression slope) was greater for production (−1.154±0.297) than for abundance (−0.662±0.133). Turnover time of bacterial biomass was estimated to be 55–141 days. Controlling factors on bacteria at 500 m were assessed by the comparison in changing rate of bacterial abundance in different treatments (whole water, predator-free water, predator-free water diluted by particle-free water from 500 m, predator-free water diluted by particle-free water from 110 m). The result suggested that bacteria at 500 m were controlled by both bottom-up and top-down controls and that the availability of dissolved organic matter was seasonally variable at 500 m. Since the study site is likely semi-enclosed, annual bacterial growth efficiency in the mesopelagic layer was constrained by replacing total amount of organic carbon (OC) assimilated by bacteria with the OC flux between 110 and 1000 m. It was suggested that bacterial growth efficiency in the mesopelagic layer is not so small as expected and that 19–39% of total OC assimilated by bacteria is used for biomass production, by which a portion of the mesopelagic food web is sustained. The mesopelagic respiration is estimated to be 0.9–2.3 mol-C m−2 yr−1.