Microbial abundance and activity in the seasonal upwelling area off Concepción (∼36°S), central Chile: a comparison of upwelling and non-upwelling conditions

The contribution of bacteria and nanoflagellates to marine carbon flux was studied in the upwelling area off Concepción, central Chile (∼36°S), at coastal shelf and oceanic stations, during October 1998 (austral spring, upwelling season) and July 1999 (austral winter, non-upwelling season). During the spring cruise, euphotic-zone integrated primary production ranged from 1200 to 8740 and from 320 to 540 mg C m−2 d−1 and bacterial production from 238 to 760 and from 91 to 102 mg C m−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 0.7–33.9×105 and 0.2–56.9×105 cells L−1 and, for autotrophic nanoflagellates, 3.8–32.1×105 and 0–30.4×105 cells L−1, respectively. During the non-upwelling conditions, primary production was lower at the coastal shelf stations (481–1710 mg C m−2 d−1) and slightly higher (415–865 mg C m−2 d−1) at the oceanic stations. Bacterial production (<100 mg C m−2 d−1) and bacterial growth rates (<0.2 d−1) were also lower. Heterotrophic nanoflagellates (0.6–39.6×105 and 1.9–59.9×105 cells L−1) and autotrophic nanoflagellates (0.4–22.6×105 and 0–12.5×105 cells L−1) 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 primary production, 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 primary production 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.