Assessing Climate Change Impacts on the Distribution and Ecological Risks of Cryptophyta in the China Seas Using Ensemble Models

Cryptophyta are ecologically important microalgae in marine and brackish ecosystems, but their future habitat suitability and ballast-water-associated surveillance priorities under climate change remain insufficiently understood. Based on 136 georeferenced occurrence records from China’s coastal waters and 24 marine environmental variables, this study applied the Biomod2 ensemble modeling framework to predict the current and future potential distribution of Cryptophyta in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea. The ensemble model showed high predictive performance (Kappa = 0.94, TSS = 0.94, AUC = 0.997), outperforming most individual algorithms. Annual mean temperature (bio22) and maximum salinity (bio15) were identified as the dominant environmental constraints, indicating that thermal and salinity regimes jointly shape habitat suitability. Under present conditions, suitable habitats were broadly distributed across China’s coastal and shelf waters. Future projections indicated that suitable habitats generally remained extensive under most climate scenarios, suggesting that future environmental changes may maintain or reshape potential recipient environments for Cryptophyta. However, this response was not linear with increasing emission intensity; under the high-emission scenario, suitable habitats declined in some regions and showed stronger spatial reorganization, implying that excessive warming or altered salinity may reduce broad-scale suitability. These findings suggest that climate change may alter the spatial pattern of Cryptophyta habitat suitability under different emission pathways. Given the role of ship ballast water in transporting planktonic microalgae, persistent or newly suitable habitats, especially those located near ports, estuaries, semi-enclosed bays, and aquaculture areas, should be regarded as habitat-suitability-based surveillance priorities. This study provides a basis for climate-informed monitoring, ballast-water-associated screening, and adaptive management of coastal microalgae in the China Seas.