Plankton trophic structure and particulate organic carbon production during a coastal downwelling-upwelling cycle

We evaluated plankton trophic structure in a short-term temporal survey performed throughout a downwelling–upwelling cycle at the SE Brazilian coast. Size-fractioned phytoplankton biomass (PB), microzooplankton (MiZA) and mesozooplankton (MeZA) abundances along with primary (PP) and bacterial production (BP) were estimated for 5 consecutive days at a fixed station in the Cabo Frio upwelling core area. During the downwelling period, the dominance of pico- and nanoplankton in PB, and higher BP (0.1 μg C l–1h–1) and MiZA (170 ind. l–1), characterized a microbial food web structure. After the South Atlantic Central Water (SACW) upwelling, the decrease in BP (0.02 μg C l–1h–1), MiZA (50 ind. l–1) and picoplankton contribution to PB, along with the appearance of microplankton in PB and the increase in opportunistic herbivorous copepods densities (Paracalanus parvus: 2.0 ind. l–1) indicated the establishment of a herbivorous food web structure. Particulate organic carbon (POC) production was mainly due to phytoplankton (98%) and did not differ between periods. However, the observed variability in plankton trophic interactions should affect the magnitude of POC export from this dynamic system.