Structuring effects of mesozooplankton on freshwater and marine microbial food webs
Mesozooplankton (Copepods, Daphnia, Appendicularians) impact on microbial food webs was studied by experimental manipulation of its density and composition in five large-scale mesocosm experiments carried out in spring and summer in a mesotrophic lake (Schöhsee, Plön) and a fully marine site (Trondheim Fjord, Norway). Despite considerable differences in the biotic and abiotic start conditions, a general pattern of microbial food web structuring was found. The size-dependent food choice of copepods resulted in mostly community-level 3- to 4-link trophic cascades leading to a significant reduction of ciliate abundances and substantial increases in nanoplankton densities. Changes at mesozooplankton level cascaded down to bacterioplankton and triggered temporal and density-dependent changes in bacterial abundance, activity (production, single-cell DNA content), respiration (redox dye CTC), substrate turnover as well as phenotypic and genotypic community composition (Fluorescence In Situ Hybridization, Denaturing Gradient Gel Electrophoresis plus sequencing; Richness, Shannon´s diversity index H´) and feedback mechanisms against protist grazing. While the copepod-mediated predation cascade led to positive responses in freshwater bacterial communities with respect to biomass and activity, respective cascading effects caused negative trends in marine bacterioplankton. Filter-feeding Daphnia exerted strong top-down control on all microbial food web components in spring and summer and was the main reason for the termination of a bloom of filamentous bacteria. An induced blooming event of the appendicularian Oikopleura dioica substantially reduced bacterial abundance and production (3- to 5-fold), but caused only modest changes in bacterial community composition.