Seasonal Dynamics of Microbe–DOM Interactions in Water and Sediment of a Shallow Lake

This study investigated microorganism-dissolved organic matter interactions in Baiyangdian lake by analyzing water and sediments in summer and winter. The overlying water in winter showed nitrogen enrichment and phosphorus limitation, with phosphorus accumulating in the sediment. Humic-like dissolved organic matter (DOM) increased in the winter. Planktonic bacterial communities shifted from cyanobacteriota (18.19%) and actinomycetota (36.23%) in the summer to bacillota (40.23%) in the winter, while sediment bacteria remained stable. Environmental drivers indicated that planktonic bacteria were influenced by nutrients and humic-like DOM in summer and by NO 3 –– N in winter; sediment bacteria correlated with complex nitrogen and phosphorus forms in summer and bioavailable nitrogen in winter. Network analysis showed higher complexity (815 nodes, 1438 edges), tightness (0.462), and modularity (0.924) in winter, suggesting enhanced microbial cooperation under cold season. Structural equation modeling revealed that protein-like DOM promoted planktonic bacteria in summer (0.54–0.61) but inhibited them in winter (−0.30). Sediment bacteria were suppressed by nitrogen and phosphorus forms in summer (−0.40 to −0.41) and by protein-like DOM in winter (−0.30). Temperature acts as the core factor mediating seasonal transitions. This study revealed the seasonal adaptation mechanisms of the coupling between microorganisms and DOM in shallow lakes, providing a theoretical basis for water–carbon cycling and eutrophication control.