Influence of algal deposition-decay on nitrification/denitrification in shallow eutrophic lakes: ecological interactions and mechanisms of nitrifiers and denitrifiers

Algal mats formed by wind-driven processes in eutrophic lakes significantly influence sediment nitrogen cycling through deposition and decay. However, the effects of varying intensities of algal deposition–decay on sediment nitrification, denitrification, and associated microbial communities remain poorly understood. To address this, we simulated algal deposition–decay at different intensities in microcosms and examined its impact on sediment nitrification, denitrification pathways, and microbial dynamics. Results revealed that dissolved oxygen (DO) levels initially declined and then recovered during algal decay, with high-biomass deposition markedly delaying DO recovery (0 mg/L DO persisted for 10 days). At the lower DO point, both nitrification (NR: 0.003 μmol/(g·h)) and denitrification rates (DNR: 0.005 μmol/(g·h)) were more suppressed in the high-algal (HA) group than in the low-algal (LA) group (NR: 0.013 μmol/(g·h); DNR: 0.007 μmol/(g·h)). During DO recovery and stabilization, nitrification rebounded more strongly in the HA group, whereas denitrification remained lower—a discrepancy likely attributable to prolonged anoxia and humic substance accumulation under high algal loading. High algal biomass deposition significantly altered sediment microbial community structure, reducing the abundance of key nitrifying (e.g., Bacterium_HR1) and denitrifying taxa (e.g., Dechloromonas sp. H13), along with shifts in carbon and nitrogen metabolic pathways, indicating substantial microbial restructuring. Community stability declined for hao-type nitrifiers (robustness: LA 0.32, HA 0.3) and nirS-type denitrifiers (robustness: LA 0.33, HA 0.3). Furthermore, inconsistent changes in the relative abundance of nitrification and denitrification pathways following high algal deposition–decay suggest that high algal biomass may partially decouple nitrification–denitrification coupling in sediments.