Field-Scale Suppression of Cyanobacterial Blooms by Cationic Polymer-Modified Local Soil: Selective Inactivation Mechanism and Ecological Responses

Harmful cyanobacterial blooms require control strategies that suppress bloom-forming taxa while limiting effects on non-target primary producers and toxin-related risks. We developed a cationic polyquaternium (P126)-engineered local red-soil composite and evaluated its electrokinetic properties, taxon-dependent photophysiological effects, activity against colonial Microcystis, robustness to water chemistry, short-term Danio rerio responses, and field-scale performance. P126 modification reversed soil surface charge to the range of +20 to +33 mV across a pH range of 6.0–10.0, generating a strong electrostatic contrast with negatively charged cyanobacteria. At 10 mg L−1, the composite suppressed maximum photosystem II quantum yield (Fv/Fm) in Microcystis aeruginosa and Dolichospermum sp., with no statistically detectable effects on the four tested chlorophyte and diatom strains. For Microcystis colonies ≥ 500 μm, 20 mg L−1 of composite achieved >95% MTT-based inhibition within 24 h, versus < 15% for CuSO4. Suppression persisted across the tested pH and nutrient ranges but was attenuated by high humate concentrations. The maximum unfractionated-sample ELISA microcystin-equivalent signal was approximately 25% lower than with CuSO4; however, dissolved and particulate fractions were not resolved. No statistically detectable treatment-related differences in zebrafish hatching or larval length were observed up to 10.5 mg L−1. In a physically isolated sub-lake, composite application preceded a decline of more than three orders of magnitude in algal/cyanobacterial density and an increase in Secchi depth from approximately 0.25 to 1.2 m; Vallisneria natans was planted subsequently. These results provide laboratory evidence and field case-study observations consistent with electrostatically favored, selective bloom suppression, while residual-polymer fate, sediment effects, chronic toxicity, and multi-trophic responses require further evaluation.