TEMPORAL AND SPATIAL DYNAMICS OF THE PELAGIC MICROBIAL FOOD-WEB IN ANATOLL LAGOON

We examined the influence of diurnal cycle, depth, exchange and proxim ity to rand on the dynamics of microbial populations (bacteria, cyanob acteria, nanoplankton, phagotrophic nanoflagellates, ciliates and dino flagellates) in Tikehau atoll lagoon (French Polynesia). Microbial pop ulations increased over this 15 d study. Their abundances in the lagoo n ranged from 1.2 to 2.6 x 10(6) bacteria ml(-1), 1.1 to 4.0 x 10(5) c yanobacteria ml(-1), 200 to 1090 eukaryotic phytoplankton ml(-1), 380 to 1500 phagotrophic nanoflagellates ml(-1), 0.3 to 4.0 ciliates ml(-1 ), and 11 to 195 nanodinonagellates ml(-1) Microbial abundances reveal ed that the intrusion of oceanic water into the lagoon only affected b acterial and cyanobacterial distributions in a narrow band around the lagoon. Grazing rate estimates and population dynamics showed that pha gotrophic nanoflagellates were the major grazers of picoplankton (main ly 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 abundanc e appeared to be regulated primarily by predators. Ciliates, and perha ps heterotrophic dinoflagellates, appeared to be grazing mostly on nan oplankton, both autotrophic and heterotrophic cells. Autotrophic and h eterotrophic dinoflagellate populations increased at net rates compara ble to, or even more rapidly than, other microbial communities; this s uggests that dinoflagellates also play a significant role in the lagoo n microbial foodweb. Additional aspects of the spatial and temporal va riability of the microbial foodweb in Tikehau lagoon, as well as the r ole of grazers and dinoflagellates, are analyzed.