In-lake transformations of dissolved organic matter composition in a subalpine lake do not change its biodegradability

In freshwater ecosystems, dissolved organic matter (DOM) originates from terrestrial and aquatic primary production, and its microbial biodegradability is expected to vary due to differences in its biogeochemical composition. However, the interaction of various DOM sources with the aquatic microbial population remains widely debated. Here, we performed laboratory bioassays, conducted in the dark for 20 d, to examine how changes in DOM composition influence its microbial biodegradability along a hydrological continuum (i.e., lake inlet, lake layers, and lake outlet) of a subalpine lake over four seasons. Despite a predominance of terrestrial DOM and lowest initial abundance of microorganisms, the highest microbial growth rates were observed at the lake inlet (0.0076 +/- 0.0011 cells mL(-1) h(-1)) across all seasons. DOM reactivity differed more with time (seasons) than with space (sampling sites) and DOM compositional differences showed higher degradability in upper lake layers, but not at the lake outlet. Additionally, gradual humification over incubation time was observed in almost all seasons and sites. Overall, an enhanced biodegradability of DOM was not detected even though lake sites were characterized by an autochthonous, presumably labile DOM signature. However, our results highlight the potential of thriving bacteria in terrestrial-dominated DOM environments. This observation challenges a previous hypothesis that DOM of terrestrial origin is less accessible and more recalcitrant, while emphasizing the flexible response of aquatic systems to seasonal fluctuations.