High organic carbon drives community shifts and biodiversity loss in benthic macroinvertebrates of Baiyangdian Lake, China

• Identified strong accumulation of tryptophan-like substance DOM (C2) linked to biodiversity loss in benthic communities. • Revealed dominance of pollution-tolerant taxa and severe degradation across over 80 % of sampling sites. • Demonstrated that internal macrophyte decomposition and terrestrial inputs jointly exacerbate habitat stress. • Proposed ecological restoration strategies targeting internal carbon loading and external organic inputs. • Suggested using DOM composition indicators as early warnings for benthic habitat deterioration. Dissolved organic carbon (DOC) plays a pivotal role in driving biogeochemical and ecological processes in aquatic ecosystems, yet its role in structuring benthic communities in shallow eutrophic lakes remains poorly understood. This study investigated the effects of DOC and its fluorescent components on the benthic macroinvertebrate community in Baiyangdian Lake, a representative floodplain lake in northern China. DOC concentrations varied markedly across the lake, peaking at 91.87 mg L⁻¹ in the northern region, suggestive of active internal carbon cycling. Parallel Factor Analysis identified three fluorescent dissolved organic matter (DOM) components: humic-like C1 (24.35% of total fluorescence) and protein-like C2 and C3 (75.6%). Benthic assemblages were dominated by pollution- and semi-tolerant taxa, including Chironomidae and Tubificidae, whereas sensitive taxa were scarce. Canonical correspondence and Mantel tests identified total phosphorus, total nitrogen, organic matter, and protein-like component C2 were the primary drivers of community structure. The significant correlation between C2 and species richness ( r = −0.462, p < 0.001) suggests that refractory humic DOM may contribute to biodiversity loss. These findings provide insights for ecological restoration efforts and carbon management strategies in shallow eutrophic lakes.