Understanding the Causes of High Organic Matter with Low Bioavailability in Cold-Zone Lake Water: A Case in Hulun Lake.

In cold-region lakes, high organic matter concentrations with low bioavailability are common, yet the underlying causes and stabilisation mechanisms remain unclear. This study conducted a 60-day microbial treatment experiment in Hulun Lake using algae (DOMa), grass (DOMg), and manure (DOMm) as DOM sources. Fourier transform ion cyclotron resonance mass spectrometry and 16S rRNA analysis were employed to characterise DOM composition and bacterial communities. The bioavailability of DOMa, DOMg, and DOMm was 86.1%, 84.08%, and 70.9%, respectively. Differences in degradation cycles were mainly associated with the slowly biodegradable fraction; the half-lives of DOMa, DOMg, and DOMm were 49.51 days, 77.02 days, and 198.04 days, respectively. At the molecular level, proteins and lipids were rapidly utilised by microorganisms, leading to the generation of lignin, condensed aromatic hydrocarbons, and tannins, with many new molecules falling within the carboxylic acid-rich alicyclic molecule (CRAM) region. The overall community succession patterns of different DOM sources were highly similar, with initial DOM composition differences leading to variations in microbial communities during intermediate degradation stages (5~10 days). Moreover, microbiological processes facilitated the convergence of DOM source compositions and the accumulation of refractory organic matter. It is hypothesised that the regional climatic characteristics of the freeze-thaw cycle exacerbate organic matter accumulation by compressing the "effective degradation time". These findings elucidate the causes of high organic matter and low bioavailability in cold-region lakes.