The significance of transparent exopolymeric particles in the vertical distribution of bacteria and heterotrophic nanoflagellates in Lake Pavin

The abundance, size distribution, and bacterial colonization of Transparent Exopolymeric Particles (TEP) were examined in two consecutive years during the spring diatom development throughout the water column of the deep meromictic Lake Pavin, France. TEP concentrations ranged from 1.9 to 13.4 × 10⁵ particles l⁻¹ and their distribution and size spectra indicated that these particles are the main factor in governing the transport of diatoms to the deep hypolimnion of the lake. The majority of TEP was colonized by bacteria that constituted 0.4–8.9% of total DAPI-stained bacteria. The intensity of bacterial colonization was strongly related to temperature and decreased with particle size. A more important colonization of small particles in the hypolimnion during thermal stratification suggested that bacterial colonisation also increased with the age of the particle. The abundance of heterotrophic nanoflagellates (HNF) was more significantly related to the density of particles than to the density of total bacteria and the intensity of bacterial colonization of TEP. Our results therefore suggest that TEP are a more important factor for HNF development than attached and free bacteria. We conclude that TEP are involved not only in sedimentation processes but also in the dynamics of bacteria and protozoa in freshwater pelagic environments.