# Predicting Ecological Risks of Alexandrium spp. Under Climate Change: An Ensemble Modeling Approach

## TL;DR

This study predicts how climate change will shift harmful algal blooms caused by Alexandrium spp. in Chinese coastal waters, emphasizing the role of salinity and temperature.

## Contribution

The study introduces an ensemble modeling approach to predict Alexandrium spp. bloom risks under climate change scenarios in Chinese coastal waters.

## Key findings

- Salinity is the most important factor controlling Alexandrium spp. presence, while temperature influences bloom timing and location.
- Future climate change is expected to shift high-risk areas for blooms southward, especially under high-emission scenarios.
- Ensemble models outperformed single models in predicting Alexandrium spp. distribution (AUC = 0.998, TSS = 0.977, Kappa = 0.978).

## Abstract

Harmful algal blooms (HABs) caused by Alexandrium spp. are a growing threat to marine ecosystems, fisheries, and human health. This study used computer-based ensemble models to predict where these algae might occur in Chinese coastal waters now and in the future under climate change. We found that salinity is the most important factor controlling their presence, while temperature influences when and where blooms may happen. At present, the highest-risk areas are in Bohai Bay, the Yangtze River estuary, and along the Fujian and Guangdong coasts. In the future, suitable habitats are expected to shift southward, especially under high-emission scenarios, with new risk zones emerging in the South China Sea. These findings suggest that climate change will restructure regional bloom risks and highlight the need for proactive monitoring and management to protect fisheries and public health.

Alexandrium spp., globally recognized as harmful algal bloom (HAB) species, pose severe threats to marine ecosystems, fisheries, and public health. Based on 469 occurrence records and 24 marine environmental variables, this study employed the Biomod2 ensemble modeling framework to predict the potential distribution of Alexandrium spp. under current and future climate scenarios, and to assess the role of key environmental factors and the spatiotemporal dynamics of habitat centroid shifts. The results revealed that (1) the ensemble model outperformed single models (AUC = 0.998, TSS = 0.977, Kappa = 0.978), providing higher robustness and reliability in prediction; (2) salinity range (bio18, 19.1%) and mean salinity (bio16, 5.8%) were the dominant factors, while minimum temperature (bio23) also showed strong constraints, indicating that salinity determines “whether persistence is possible,” while temperature influences “whether blooms occur”; (3) under present conditions, high-suitability habitats are concentrated in Bohai Bay, the Yangtze River estuary to the Fujian coast, and parts of Guangdong; (4) climate change is predicted to drive a southward shift of suitable habitats, with the most pronounced expansion under the high-emission scenario (RCP8.5), leading to the emergence of new high-risk areas in the South China coast and adjacent South China Sea; (5) centroid analysis further indicated a pronounced southward migration under RCP8.5 by 2100, highlighting a regional reconfiguration of ecological risks. Collectively, salinity and temperature are identified as the core drivers shaping the ecological niche of Alexandrium spp., and future warming is likely to exacerbate HAB risks in southern China. This study delineates key prevention regions and proposes a shift from reactive to proactive management strategies, providing scientific support for HAB monitoring and marine ecological security in China’s coastal waters.