Temporal and spatial dynamics of the pelagic microbial food web in an atoll lagoon

We examined the influence of diurnal cycle, depth, exchange and proximity to land on the dynamics of microbial populations (bacteria, cynaobacteria, nanoplankton, phagotrophic nanoflagellates, ciliates and dinoflagellates) in Tikehau atoll lagoon (French Polynesia). Microbial populations increased over this 15 d study. Their abundances in the lagoon ranged from 1.2 to 2.6 x 10(exp 6) bacteria/ml, 1.1 to 4.0 x 10(exp 5) cyanobacteria/ml, 200 to 1090 eukaryotic phytoplankton/ml, 380 to 1500 phagotrophic nanoflagellates/ml, 0.3 to 4.0 ciliates/ml, and 11 to 195 nanodinoflagellates/ml. Microbial abundances revealed that the intrusion of oceanic water into the lagoon only affected bacterial and cyanobacterial distributions in a narrow band around the lagoon. Grazing rate estimates and population dynamics showed that phagotrophic nanoflagellates were the major grazers of picoplankton (mainly bacteria and cyanobacteria). Cyanobacteria contributed about twice as much as bacteria to this grazed biomass and are therefore supposed to have a higher contribution to the transfer of organic carbon to the upper trophic levels. Hetero- and autotrophic nanoflagellate abundance appeared to be regulated primarily by predators. Ciliates, and perhaps heterotrophic dinoflagellates, appeared to be grazing mostly on nanoplankton, both autotrophic and heterotrophic cells. Autotrophic and heterotrophic dinoflagellate populations increased at net rates comparable to, or even more rapidly than other microbial communities ; this suggests that dinoflagellates also play a significant role in the lagoon microbial foodweb. Additional aspects of the spatial and temporal variability of the microbial foodweb in Tikehau lagoon, as well as the role of grazers and dinoflagellates, are analyzed. (Résumé d'auteur