Quantifying assembly processes of dissolved organic matter pools in eutrophication using high-resolution mass spectrometry and ecological models.

Dissolved organic matter (DOM) represents a large, dynamic pool of carbon, playing a crucial role in eutrophic aquatic ecosystems through its continuous transport and transformation. However, the assembly mechanisms of DOM under different eutrophic conditions remain elusive, hindering the understanding of carbon dynamics and the prediction of carbon fate. Here we collected 72 lake water samples during two sampling events in Chaohu Lake, the fifth largest freshwater lake in China, and performed high-resolution mass spectrometry (HRMS) and ecological null modeling to quantify the assembly processes of DOM in eutrophication. We found that as eutrophic levels increased, the relative contribution of homogeneous selection rose, while the contributions of variable selection and dispersal limitation decreased. The influence of different assembly processes on the DOM pool across sites, although estimated solely from HRMS data, exhibited reasonable consistency with the spatiotemporal variations. Several environmental parameters, including total phosphorus, Secchi disk depth, trophic state index, pH, temperature, and fluorescence index, were significantly correlated with one or more DOM assembly processes (p < 0.05), and assembly mechanisms also shaped the compound composition of DOM. Our findings reveal a shift in DOM assembly from variable selection to homogeneous selection in eutrophication, highlighting the importance of DOM dynamics and environmental homogenization in the management and restoration of eutrophic lakes.