Structure of the oligotrophic planktonic food web under low grazing of heterotrophic bacteria: Takapoto Atoll, French Polynesia

The study investigates the planktonic food web of an oligotrophic atoll lagoon (Takapoto, French Polynesia). The growth (k) and grazing mortality (g) rates of heterotrophic bacteria, cyanobacteria, and 1-3 pm and >3 mu m (large) algae were assessed, in the presence and absence of the large grazers (>35 pm protozoa), by serial dilution experiments. Taxa-specific feeding relationships were obtained from fluorescently labeled prey uptake experiments. The major consumers of 0.4 pm bacteria were aplastidic nanoflagellates (7 mu m Leucocryptos and 10 mu m Halosphaera), and those of 3-5 mu m chlorophyceae (Nannochloropis and Chlamydomonas) were heterotrophic dinoflagellates (32 mu m Achradina and 33 mu m Prorocentrum). The protozoa exerted a low grazing pressure on heterotrophic bacteria, even when the large grazers were removed (mean values: g = 0.26 d(-1)). In contrast, the grazing rates of less than or equal to 200 mu m protozoa on cyanobacteria (mean values: g = 0.43 d(-1)), which were the dominant primary producers, represented 74% of their growth rates. In absence of the >35 mu m grazers, the grazing on cyanobacteria increased, being 4 times higher than on heterotrophic bacteria. These results indicate that, in an oligotrophic planktonic food web where the consumption of bacteria is low, the grazing of protozoa on cyanobacteria is a major carbon pathway. Large algae were grazed at rates equivalent to their growth rates (mean values: g = 0.47 d(-1); k = 0.50 d(-1)), which implies a steady-state situation with rapid cycling of the algal biomass. In samples without the >35 pm grazers, the grazing mortality and growth rates of large phytoplankton increased significantly, indicating a close coupling between the growth of >3 mu m algae and grazing pressure under natural conditions. This coupling is likely mediated by increased nutrient regeneration. The results indicate that the large protozoan consumers controlled the grazing pressure and nutrient regeneration of the small protozoa, which in turn controlled the abundance of phytoplankton and growth of large algae. It follows that the structure of the oligotrophic planktonic food web under low grazing on bacteria is ultimately governed by protozoan microzooplankton.