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Free keywords:
microbial food web; bacteria; mesozooplankton; trophic cascade; Daphnia; Appendicularians; Copepods; diversity
Abstract:
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.
