The ciliate-copepod link in marine ecosystems

We show the results of a comparative, cross-ecosystem, analysis of the relative importance of ciliates as carbon sources for copepod, and, specifically, evaluated the strength of the ciliate-copepod trophic link. Although phytoplankton represent, globally, a far higher biomass than ciliates (>1 order of magnitude), the consumption of the latter nevertheless comprises, on average, 30% of copepod daily carbon rations (ciliates + phytoplankton). The relative importance of ciliate consumption by copepods clearly depends on the trophic state of the system. In regions where phytoplankton concentrations are low (<50 mugC l(-1)), ciliate and phytoplankton contribution to the diet are equivalent. In richer environments this value declines, with ciliates accounting for only 25 and 22% of diet in environments characterized by phytoplankton concentrations of 50-500 and >500 mugC l(-1), respectively. From a biogeochemical point of view, our carbon budget estimates indicate that copepods process, on a global scale, 5.5 to 8.1 gigaton (Gt) phytoplankton-derived C yr(-1); from the heterotrophic side, this flux would represent 2.0 to 2.4 Gt ciliate-derived C yr-1 and, probably, a higher value would be obtained if other microheterotrophs, like heterotrophic dinoflagellates, were included. Hence, the flux of carbon from ciliates, and other microzooplankton, towards upper trophic levels should definitely be considered in oceanic biogeochemical cycles and pelagic food web models.