Mechanisms of Hypoxia Formation in the Ariake Sea, Japan, under Climate Change: Insights from Orthogonal Design

Seasonal hypoxia events pose significant threats to the environment and ecosystems of coastal areas. Temperature, river discharge, and nutrient loading are key drivers, but their relative importance is affected by spatial variations in the coastal topography and ecological structure. This study used the Taguchi method to evaluate the impacts of these factors on dissolved oxygen (DO) dynamics in the Ariake Sea with a particular focus on the summer flood period. Each factor was set at three levels to represent potential climate change scenarios, offering a key advantage in efficiency by enabling reliable assessment with fewer simulations than the 27 trials required in a full factorial design (3³). The results suggest that increased temperature and river discharge reduced DO throughout the year, while low nutrient loading enhanced DO in well-mixed nonsummer months. In summer, nutrient loading and river discharge were the primary drivers of hypoxia, contributing 51.5% and 41.2%, respectively (<i>p</i> < 0.05, at St. 3). A lagged autumnal response linked to summer floods showed that discharge explained 88.6% of DO decline (<i>p</i> < 0.05, at St. 1). Furthermore, experimental simulations indicated that doubling both discharge and nutrient loading expanded hypoxia to nearly two-thirds of the sea, threatening benthic fauna and ecosystem stability.