Ca/Fe-modified drinking water treatment residue with Vallisneria natans controls algal blooms via phosphorus transformation and smicrobial network shifts.

Internal phosphorus (P) loading is a key driver of algal blooms in shallow lakes, yet submerged macrophyte (SM) restoration alone often fails because clear water also favors epiphytic filamentous algae and regime shifts. We constructed a co-remediation system combining Ca/Fe-modified drinking water treatment residue (Ca/Fe-DWTR) with Vallisneria natans and ran a 183-day microcosm experiment to reveal how sediment P transformation coupled with macrophyte restoration controls water quality and suppresses two different algal blooms. Co-remediation markedly reduced sediment total-P and mobile-P while increasing stable-P forms, and no bloom occurred throughout cultivation, with chlorophyll a below 1.5 μg/L. The nitrogen (N) release risk associated with Ca/Fe-DWTR alone was also eliminated. Conversely, unremediated-sediment sustained planktonic blooms through sulfide-driven Fe bound P release coupled with nitrogen fixation, whereas the SM-only treatment stayed clear early but later developed massive epiphytic filamentous algae once photosynthetic pH rise mobilized Al bound P, confirming that success depends on P-stabilization rather than transient clarity. Metagenomics showed that co-remediation created low-P and oxidative niches that restructured microbial communities, enhancing nitrogen cycling potential in the phyllosphere while suppressing sulfate-reducing bacteria in sediment, thereby achieving coupled N and P removal within a resilient functional gene network. Mantel tests and PLS-SEM revealed that pH elevation by V. natans enhanced microbial P fixation and stable-P formation, while Ca/Fe-DWTR most strongly drove nitrogen transformation, and thus sediment P transformation indirectly governed water quality through microbial functional gene networks. Overall, the co-remediation showed potential for mitigating internal P loading and algal proliferation in shallow eutrophic lakes.