Abstract

This study describes the abundance and vertical distribution of microbial assemblages (from femto to nanoplankton) in the upwelling area off Concepción (~36oS), analyses their relationship to oceanographic conditions during the upwelling season (austral spring-summer: December 2002-March 2003), and explores microbial community inter-relationships; at one shelf station and at the shelf break off the Southeastern Pacific coast. Virioplankton (VP), bacterioplankton (BP), and autotrophic (ANF) and heterotrophic (HNF) nanoflagellates were enumerated by epifluorescence microscopy. Abundances of VP, BP, HNF and ANF ranged as follows: 0.04-13.9 × 107 virus like particles mL-1, 0.5-5.6 × 106, 0.016-0.4 × 103, and 0.08-0.6 × 103 cells mL-1, respectively. Variability in VP, BP and HNF abundance was explained in less than 27% by the oceanographic setting (i.e. temperature, salinity and dissolved oxygen). Upwelling conditions have a significant effect on distribution and abundance only of VP in both station examined. VP abundance was significant and positively related to the abundances of BP, ANF and HNF, suggesting that they may be possible hosts of marine viruses. Furthermore, the VP/BP ratio (mean = 9) obtained represented the upper limit of literature values, suggesting a high interaction between virioplankton and bacterioplankton.