Nitrogen-phosphorus stoichiometry and Cladophora growth affected by grass-sourced dissolved organic matter in the littoral zone of the Qinghai Lake, China.

Climate-driven lake expansion on the Qinghai-Tibet Plateau has triggered the extensive release of organic compounds through inundation-induced vegetation decomposition. This process generates substantial dissolved organic matter (DOM), potentially threatening alpine aquatic systems. In this work, the effects of grass-sourced DOM inputs on the nitrogen-to-phosphorus (N/P) ratio and Cladophora growth were studied. Field observation conducted in the lakeshore zone of Qinghai Lake showed that water column N/P ratios were closely related to Cladophora chlorophyll a (Chl.a) content. Microcosm experiment indicated that grass decomposition released DOM dominated by humic and fulvic acids. This DOM input enhanced sediment phosphorus (P) release and denitrification activity, reducing the dissolved inorganic nitrogen-to-soluble reactive phosphorus (DIN/SRP) ratio in pore water. The altered overlying water stoichiometry further affected Cladophora growth. High-throughput sequencing showed that grass-sourced DOM significantly altered sediment microbial community composition. Notably, the relative abundance of denitrifying and phosphate-solubilizing bacteria, such as Acinetobacter and Pseudomonas exhibited a remarkable increase. Furthermore, a more complex co-occurrence network between DOM-degrading microbes was observed. Easily biodegradable DOM fractions, particularly tryptophan-like (P2), fulvic acid-like (P3), and microbial by-products (P4), were closely associated with an increased abundance of denitrifying and phosphate-solubilizing bacteria. Herein, we investigated the DOM-driven ecological cascade effects of N/P ratio changes, microbial community succession, and algal proliferation, which could provide critical insights for assessing emerging environmental hazards in vulnerable high-altitude aquatic ecosystems under global change scenarios.