Temporal dynamics of carbon flow through the microbial plankton community in a coastal upwelling system off northern Baja California, Mexico

We investigated the temporal dynamics of carbon flow through the microbial food web of a coastal upwelling system (ENSENADA station) off northern Baja California during 6 cruises (September 2007 to November 2008). Carbon biomass assessments for major autotrophic size groups (pico- to micro-sized cells) and their microzooplankton grazers were based on analyses using flow cytometry, HPLC pigments and epifluorescence microscopy. Taxon-specific phytoplankton growth and microzooplankton grazing rates were determined from 24 h in situ incubations in the euphotic zone using an abbreviated 3-treatment dilution technique. Carbon biomass and instantaneous growth and grazing rate determinations were used to estimate daily rates of taxon- specific production and losses due to microzooplankton grazing. Overall, microbial biomass showed a close balance between autotrophic and heterotrophic components, except during a period of very strong upwelling (April 2008), which favored large phytoplankters and high primary production. Through- out a wide range of environmental conditions, the community primary production (PP) attributed both to small (mostly picophytoplankton and prasinophytes) and large (mostly diatoms and au- totrophic dinoflagellates) autotrophs was significantly grazed (78 ± 9% of PP) by small ( 20 µm) ciliates and flagellates (including mixotrophic dinoflagellates), respectively, showing complementary temporal shifts in protistan grazer types that matched the dominant phytoplankton. While large diatoms were strongly consumed by large ciliates during the 2 most productive periods (September 2007 and April 2008), pico- and nano-sized phytoplankton were grazed most by nanofla- gellates and small ciliates from November 2007 to January 2008. Consequently, biogenic carbon pro- duction in this ecosystem is transferred through a multivorous food web.