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

Abstract

We examined the influence of diurnal cycle, depth, exchange and proximity to land on the dynamics of microbial populations (bacteria, cyanobacteria, 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 106 bacteria ml-l, 1.1 to 4.0 X 105 cyanobacteria ml-l, 200 to 1090 eukaryotic phytoplankton ml-', 380 to 1500 phagotrophic nanoflagellates ml-l, 0.3 to 4.0 ciliates ml-l, and 11 to 195 nanodinoflagellates rnl-l.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 bactena 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 T~kehau lagoon, as well as the role of grazers and dinoflagellates, are analyzed.

Heterotrophic protists as a trophic link between picocyanobacteria and the pearl oyster Pinctada margaritifera in the Takapoto lagoon (Tuamotu Archipelago, French Polynesia)

“…Additionally, ciliate density increased during the day; Sakka (1999) observed a decrease in the > 35 µm protists at night and related this to predation by zooplankton, which migrates to the surface at night (Renon 1977). The abundance of heterotrophic dinoflagellates we reported in the Takapoto lagoon was 6-fold lower than in Tikehau Atoll, where it reached 25 000 cells l -1 (González et al 1998). In our study, tintinnids were as abundant as Oligotrichida (102 vs 86 cells l -1 ), and their density was much higher than those reported in Tikehau Atoll: 5 to 30 cells l -1 (Blanchot et al 1989).…”

“…The protist communities in atoll lagoons have been rarely investigated (Blanchot et al 1989, González et al 1998, Sakka 1999. The aim of this study was: (1) to evaluate the importance of hetero/mixotrophic protists as a potential energy resource in the lagoon, and (2) to estimate their contribution to the diet of pearl oysters.…”

Importance of bacterivory by pigmented and heterotrophic nanoflagellates during the warm season in a subtropical western Pacific coastal ecosystem

“…data), indicates that alternative sources of bacterial mortality, such as lysis by viruses and bacterivory by other organisms (Sanders et al 1989, Vaque & Pace 1992, Fuhrman & Noble 1995, Hwang & Heath 1999, Almeida et al 2001, Taira et al 2009) may also be important in this environment. Grazing on bacteria by HNF has been estimated using the FLB method in numerous ocean and coastal sea environments (James et al 1996, González et al 1998, Safi & Hall 1999, Sime-Ngando et al 1999, Ichinotsuka et al 2006. However, only a few studies have made comparisons between bacterivory rates by HNF and PNF, and information on the importance of these groups of grazers in oceans and coastal seas is still limited (Epstein & Shiaris 1992, Hall et al 1993, Safi & Hall 1999, Sanders et al 2000, Unrein et al 2007, Moorthi et al 2009.…”

Spatial and temporal variability of the pico- and nanophytoplankton and bacterioplankton in a deep Polynesian atoll lagoon

“…They respond quickly to temporal and spatial variations in their environment; the scale and source of such variability are likely to play a significant role in the long-term equilibrium of an ecosystem (Haury et al 1978, González et al 1998. Therefore, despite the environmental homogeneity commonly assumed in atoll lagoon waters, numerous studies highlight transitory variations in hydrobiological parameters at small scales, such as in day-to-day, diel or vertical distributions of temperature, salinity, chl a or microbial community abundance (Sournia & Ricard 1976, Moriarty et al 1985, Charpy-Roubaud et al 1988, Charpy & Blanchot 1998, González et al 1998, Buestel & Pouvreau 2000. However, most of these studies were done in shallow lagoons, which were usually considered as well mixed and thus homogeneous (Torréton & Dufour 1996, González et al 1998, Charpy & Blanchot 1999.…”