Abstract

The contribution of bacteria and nanoflagellates to marine carbon flux was studied in the upwelling area off Concepción, central Chile (361S), at coastal shelf and oceanic stations, during October 1998 (austral spring, upwelling season) and July1999 (austral winter, non-upwelling season). During the spring cruise, euphotic-zone integrated primaryproduction ranged from 1200 to 8740 and from 320 to 540 mg Cm-2 d-1 and bacterial production from 238 to 760 and from 91 to 102 mg Cm-2 d-1 at coastal shelf and adjacent oceanic stations, respectively. Bacterial growth in coastal shelf waters was higher (0.4–1.3 d-1) than at the oceanic stations (0.2 d-1). In the same station order, the abundance ranges for heterotrophic nanoflagellates were 0.7–33.9105 and 0.2–56.9105 cells L-1 and, for autotrophic nanoflagellates, 3.8–32.1105 and 0–30.4105 cells L-1, respectively. During the non-upwelling conditions, primaryproduction was lower at the coastal shelf stations (481–1710 mg Cm-2 d-1) and slightlyhigher (415–865 mg Cm2 d1) at the oceanic stations. Bacterial production (o100mg Cm2 d1) and bacterial growth rates (o0.2 d1) were also lower. Heterotrophic nanoflagellates (0.6–39.6105 and 1.9–59.9^105 cells L-1) and autotrophic nanoflagellates (0.4–22.6105 and 0–12.5105 cells L1) had similar abundances than those at coastal shelf and oceanic stations during the spring. Under upwelling conditions, bacterial production represented 4.3–22.4% (coastal) and 18.9–28.6% (oceanic) of the primaryproduction, whereas wintertime production was lower at the coastal stations (4.7–13.5%) but remained similar at the oceanic stations (11.9–26.8%). Significant correlations were found between primary and bacterial production for the spring cruise, and between bacterial abundance and (a) chlorophyll-a, (b) dissolved organic carbon (DOC), and (c) heterotrophic nanoflagellates (all data). This suggests that a significant fraction of the primaryproductio n is channeled through bacteria and, subsequently, through heterotrophic nanoflagellates, reflecting the importance of microbial food webs in carbon cycling in the upwelling system off central Chile.