Carbon budget of the planktonic food web in an atoll lagoon (Takapoto, French Polynesia)

The structure and functioning of the planktonic food web were characterized in the lagoon of Takapoto Atoll (French Polynesia). Several variables were estimated over several weeks April 1996 and 1997: carbon stocks of different planktonic compartments, phytoplankton particulate net production (PPNP), release of dissolved organic carbon (DOC) from phytoplankton, bacterial production and metabolic activity, and sinking of particles. The carbon stocks and flows were used to build up a quantitative description of the planktonic food web. The heterotrophic bacterial community was characterized by high biomass, low production and a small proportion of actively respiring cells. This was consistent with the observed low consumption of bacteria by protozoa. The relatively high PPNP was mostly due to cyanobacteria <1 mu m in size. DOC release from phytoplankton represented, on average, 48% of the total photoassimilated carbon. The planktonic food web was characterized by a high biomass of small protozoa (82% were <= 14 mu m). The contribution of metazoa to the carbon stock of zooplankton was smaller than that of prtozoa, but their rates of production and ingestion were high. Detritus had a high sinking rate and were largely consumed by metazoan plankton. The carbon budget of the planktonic system shows that 70% of phytoplankton total net production (PTNP: particulate and dissolved) is lost through heterotrophic respiration. This moderate value for oligotrophic warm waters, is due to the, low activity of bacteria. The remaining 30% of the PTNP is exported from the planktonic system through food-web transfers and sinking of detritus (20 and 10% of PTNP, respectively). It follows that the lagoonal planktonic food web, despite the small size of primary producers, is quite efficient at exporting biogenic carbon. Protozoan grazers play a key role in carbon export, as they exert a strong grazing pressure on phytoplankton, and are themselves largely consumed by metazoan zooplankton.