EXPORT OF BIOGENIC CARBON TO FISH AND TO THE DEEP-OCEAN - THE ROLE OF LARGE PLANKTONIC MICROPHAGES

The turnover time of biogenic carbon in the ocean can be defined as the time elapsed between the photosynthetic uptake of dissolved inorganic carbon and the return of this carbon as CO2 in surface waters or the atmosphere. Three compartments can be defined: short-lived carbon (<10(-2) year), long-lived carbon (10(-2)-10(2) years) and carbon sequestered in deep water or sediments. Pelagic organisms that package small particles into larger ones lengthen the turnover time of biogenic carbon and, in some cases. transfer this carbon from a given compartment to a longer-lived one. The lengthening of turnover time should be a direct function of the ratio between the size of organisms and that of their food particles. Under some circumstances. large species of euphausiids and copepods, in the North Pacific and the Southern Ocean, contribute to the lengthening of carbon turnover time. However, the most efficient re-packagers of small particles into larger ones are salps, appendicularians, doliolids and thecosome pteropods. which feed on particles at least 3.5 orders of magnitude smaller than their own size. It is hypothesized that the capacity of these large microphages to swarm explosively allows them to control phytoplankton blooms and prevent the development of particle concentrations that could clog their filtering apparatus. Salps. appendicularians and, to some degree, thecosome pteropods are frequent in the diet of some larval and adult fish, and thus contribute to some extent to the direct transformation of short-lived microbial carbon into longer-lived harvestable resources. Yet, the main ecological role of large non-crustacean microphages may reside in the production of fast-sinking, resistant, faecal pellets (in particular those of oceanic salps) that often make up the bulk of the vertical flux of organic material into deep waters, where biogenic carbon can be sequestered for long periods.