The dominant effect of the residence time on primary production and phytoplankton biomass has been demonstrated in other ecosystems (Smith 1984). In One Tree Island lagoon, the amount of primary production was related to the residence time of water (Kinsey 1985). Hamner & Wolansky (1988) demonstrated the dominant effect of hydrodynamic factors on biological processes in lagoons. Delesalle & Sournia (1992) observed a linear relationship between residence time and phytoplankton chlorophyll concentrations in coral reef lagoons. The increase of chlorophyll concentration with the degree of confinement of atoll lagoons may be related to a simple “ dilution ” by the low-chlorophyll oceanic waters in open atolls, but also to a difference in phytoplankton community structure.

Community structure

In spite of the large variations in structure, some general trends can be observed. All atoll lagoons except Taiaro and Tekokota were dominated by prokaryotic plankton. With the exception of the deeper lagoons, Kauehi (45 m) and Marokau (30 m), where growth of Prochlorococcus appears to have been promoted, the dominant group was Synechococcus. Prochlorococcus photosynthetically surpass Synechococcus in blue dominated light (Shimada et al 1996). Synechococcus are well known to be abundant only in the upper part of the photic layer (Blanchot et al 1992, Blanchot & Rodier 1996). The high chl a concentration observed in the Reka-Reka lagoon can be explained as a resuspension of benthic microphytes due to the shallow nature of the lagoon (1.5 m). However, these benthic organisms were not considered when flow cytometric measurements were made and therefore, the contribution of picoplankton groups to chl a could not be estimated. The large dominance of picoeukaryotes observed in Taiaro may be due to the high salinity of the lagoon (>40 PSU).  

Biotic factors can also affect the picoplankton biomass and community structure. There are a variety of macro-invertebrates that feed on ultra-plankton (Jörgensen et al 1984, Vacelet 1984, Vacelet & Boury-Esnault 1995). Reiswig (1971) found that coral reef sponges are a significant sink for plankton. Indeed, Pile et al (1996) observed that Prochlorococcus was filtered by sponges with the highest efficiency (93 %), followed by Synechococcus (89 %) and picoeukaryotes (72 %). In coral reef waters sponges significantly decreased concentrations of Prochlorococcus and Synechococcus while increasing autotrophic picoeukaryotes (Pile 1997). Sponges are thus susceptible to alter community structure. In atoll lagoons, sponges and other filter feeders are mainly located on hard substrates :coral reef pinnacles or fringing reefs. The density of hard substrate varies between lagoons. Nihiru and Tekokota lagoons have a high percentage of hard substrate per surface area while Tepoto Sud and Taiaro have a low percentage (Dufour & Harmelin 1997). The metabolism of cultivated pearl oysters, particularly abundant in Takapoto, can also affect the phytoplankton biomass (Charpy et al 1997). Indeed, waste products from the reared Pinctada margaritifera stock in Takapoto lagoon enhanced the growth rates of phytoplankton by decreasing the regeneration time of the nutrients (Vacelet et al 1996). Filtration experiments performed on Takapoto Pearl oysters demonstrated that Pinctada margaritifera feed only on picoeukaryotes (Blanchot pers. comm.). zooplankton can also have a strong effect on picoplankton abundance and community structure. In Tikehau lagoon, animals >35 µm grazed more than 60 % of primary production. Inorganic excretion was 32 and 18 % of the phytoplankton nitrogen and phosphorus requirements (Le Borgne et al 1989). Zooplankton biomass was studied in the prospected atolls but the data has not been processed yet.

References

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