LACK OF COUPLING BETWEEN HETEROTROPHIC NANOFLAGELLATES AND BACTERIA -A GENERAL PHENOMENON ACROSS AQUATIC SYSTEMS

The literature proposes that bacterial abundance (BA) and heterotrophi c nanoflagellates abundance (HNFA) are positively related across diffe rent aquatic systems, suggesting a strong resource control of HNFA and predatory control of BA. To test this proposed pattern, we collected published and unpublished data and analyzed trends across lakes, river s, marine, and benthic samples. BA varied between 10(5) and 10(11) cel ls ml-1, while HNFA ranged from 10(2) to 10(6) cells ml-1. The relatio nship was stronger in systems with low bacterial abundances and weaker in the most bacteria-rich systems. When data were averaged per study or per system we detected a trend with trophy, with highest HNFA in be nthic systems followed by lakes and coastal areas of the sea, and lowe st abundances in rivers and the open ocean. This relationship was impr oved in lakes, where cladoceran abundance was negatively related to HN FA. The effect of cladocerans was stronger in lakes where Daphnia spp. dominated. The weak pattern observed between BA and HNFA across syste ms suggests that several factors (resources, predators, environment, e tc.) are acting differently on BA and HNFA. Changes in population abun dances of bacteria and heterotrophic nanoflagellates are mainly determ ined by nutrient supply and grazing, and also by complex trophic inter actions with numerous feedbacks. Empirical relationships between BA an d HNFA across systems can only partially explain these changes in term s of predator-prey dependence.