Deep-Sea Research I 51 (2004) 531–544 Vertical and seasonal variations of bacterial abundance and production in the mesopelagic layer of the NW Mediterranean Sea: bottom-up and top-down controls Tsuneo Tanaka*, Fereidoun Rassoulzadegan LOV-UMR7093, Station Zoologique, BP 28, F-06234 Villefranche-sur-Mer, France Received 28 April 2003; received in revised form 4 December 2003; accepted 4 December 2003 Abstract 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 500m, respectively. Depth-dependent decreases of abundance and production were well described by the log–log linear regression (po0.05, t-test). The mean (7 SD) magnitude of decrease (regression slope) was greater for production (1.15470.297) than for abundance (0.66270.133). 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 110m). 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 0.9–2.3mol-Cm 2 yr 1 . r 2004 Elsevier Ltd. All rights reserved. Keywords: Mesopelagic layer; Bottom-up control; Top-down control; Bacterial production; Bacterial growth efficiency; Mediterra- nean 1. Introduction In the ocean, dissolved and particulate organic carbon (DOC, POC) produced in the euphotic layer are exported to the mesopelagic layer, where most of organic carbon (OC) is considered remineralized to CO 2 . It was suggested that a large portion of sinking POC is consumed by bacteria in the mesopelagic layers of the North Pacific gyre and the Santa Monica basin (Cho and Azam, 1988), the subarctic Pacific (Smith et al., 1992), the Gulf of Oman (Ducklow, 1993) and the NE Atlantic (Turley and Mackie, 1994). On the ARTICLE IN PRESS *Corresponding author. Department of Microbiology, Uni- versity of Bergen, Jahnebakken 5, P.O. Box 7800, N-5020 Bergen, Norway. Tel.: +47-55582655; fax: +47-55589671. E-mail address: tsuneo.tanaka@im.uib.no (T. Tanaka). 0967-0637/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2003.12.001