Spatial and temporal scales of variability in phyto-and bacterioplankton abundance were investigated in a deep atoll lagoon (Ahe, French Polynesia). Ahe Lagoon showed a classical phytoplankton composition, but its picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes) concentrations are higher than the mean for Polynesian atoll lagoons. Phyto-and bacterioplankton descriptors were highly variable, with mean CVs of > 20% in most cases: daynight, day-to-day, vertical, lagoonal and seasonal. Phytoplankton biomass, which was estimated by in vivo fluorescence, was higher in the deep layer than in the upper layer. This pattern was detected on a day-night scale, showing transitory vertical gradients, and through long-term events that were recorded during day-to-day measurements, especially during low wind conditions. The main factors potentially driving this vertical structure of the phytoplankton appeared to be photoinhibition in upper layers and a bottom effect providing more favourable nutrient conditions in deeper layers. Day-to-day fluctuation was the major source of temporal variation and appeared to be highly driven by vertical patterns and mixing events showing phytoplankton supply from deep layers. At the lagoon scale, the picophytoplankton exhibited a patchy structure that is closely linked to the geomorphology of the rim, lagoon depth and proximity of the pass. Stations close to the pass were characterized by consistently lower Synechococcus and picoeukaryote concentrations. Lastly, warm periods showed higher Synechococcus, nanophytoplankton and bacterial concentrations. Prochlorococcus and picoeukaryotes showed no seasonal trend, but had an annual variability that could be related to variations in exchanges between the lagoon and the ocean.KEY WORDS: Flow cytometry · Picophytoplankton · Nanophytoplankton · Bacterioplankton · Vertical distribution · Day-to-day dynamics · Large-scale heterogeneity · Deep atoll lagoon · French Polynesia
Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 59: [89][90][91][92][93][94][95][96][97][98][99][100][101] 2010 Microbes play major roles in pelagic food webs and biogeochemical cycles (Pomeroy 1974, Cho & Azam 1990. 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...