Dense macrophyte cover has significant structural and functional influence on planktonic microbial communities leading to bacterial success.

We intend to assess how macrophyte cover affects planktonic microbial communities by changing the physical and chemical environment, and how macrophyte-derived DOC affects the balance between autotrophy and heterotrophy/chemoorganotrophy in a shallow lake. The structure and production of phytoplankton and bacterioplankton in the open water of a large shallow lake and in the littoral zone were compared at two sampling stations with different macrophyte cover. According to the obtained results, uncoupling between bacterioplankton and phytoplankton was observed due to the high content of organic carbon of emergent macrophyte origin. While phytoplankton were regulated by TSS, bacterioplankton (in both heterotrophic and photoheterotrophic forms) were determined by dissolved organic carbon. As a result of these processes, the littoral and pelagic zones in the lake are completely separated from each other. In open water the autotrophic processes dominated, but at the sampling stations inside the reed belt, the metabolic processes shifted in the direction of chemoorganotrophy. Our results suggest that increase of macrophyte cover in shallow water bodies will increase the significance of microbe-based carbon pathways and weakens the efficiency of carbon transport from primary producers to higher trophic levels through the planktonic food chain.