Relative importance of phototrophic, heterotrophic, and mixotrophic nanoflagellates in the microbial food web of a river-influenced coastal upwelling area

The contribution of phototrophic nanoflagellates (PNF) as primary producers and het- erotrophic (HNF) and mixotrophic (MNF) nanoflagellates as major grazers of bacterioplankton was assessed during a 3 yr study in a highly productive, river-influenced coastal upwelling area under contrasting seasons (winter/non-upwelling vs. spring/upwelling). Sampling was conducted at 2 stations — around a river plume and at an intensive seasonal upwelling site — with contrasting en- vironmental gradients. The MNF were evaluated functionally for the possession of photo- pigments and experimentally for the ability to take up prey, specifically fluorescently labeled bacteria (FLB). During the short-term experiments, we estimated grazing rates over FLB for 2 size categories of HNF and MNF. Both bacterial production (BP) and nanophytoplankton primary production (PP) were higher in the river plume area. PNF abundance ranged from 6 × 10 9 to 411 × 10 9 cells m �2 , whereas HNF abundance fluctuated between 27 × 10 9 and 267 × 10 9 cells m �2 . In contrast, the abundance of MNF was usually low, with a maximum of ~7 × 10 9 cells m �2 . For MNF, ingestion rates were between 7.3 and 30.7 bacteria per flagellate and hour (bact flag �1 h �1 ), whereas HNF in- gestion ranged from 2 to 7.5 bact flag �1 h �1 . However, since HNF dominated in terms of abundance, they were the dominant grazers on bacterioplankton populations. Estimates of grazing pressure for the microbial food web showed that MNF were capable of removing 1 to 51% BP d �1 , whereas HNF could control BP, eliminating from 24% BP d �1 up to more than 100% BP d �1 . Given the area's relatively high nutrient condition, the elevated MNF biomass in the river plume and the greater bacterivory impact from MNF in winter, it seems that light and, thus, the energy/carbon limitation could be the main trigger for mixotrophy in this river-influenced coastal upwelling area.