Synergistic effects of nutrient competition, biotic interactions, and hydrological regulation drive green tide outbreaks in the South Yellow Sea

Green tides serve as indicators of coastal ecosystem imbalance, yet their interannual variability and driving mechanisms remain poorly characterized. This study develops the Comprehensive Green Tide Magnitude Index (GTMI) to quantitatively classify South Yellow Sea green tide events (2008–2023) into large- and small-magnitude years. By integrating Seasonal-Trend decomposition (STL) and Random Forest Regression (RFR), we systematically decompose the trend and seasonal components of the time series while preserving its integrity, significantly enhancing the ability to interpret key environmental factors. The results identify silicate, salinity, nitrate, and sea surface temperature as key controlling factors influencing both the magnitude and periodic variability of green tide outbreaks. A partial least squares path model (PLS-PM) reveals a potential mechanism involving nutrient competition, biological interaction, and hydrological regulation. Nutrient overlap between Ulva and phytoplankton triggers resource exclusion, enabling Ulva to dominate through niche preemption and suppress phytoplankton growth, while hydrological forces amplify Ulva expansion and inhibitory effects. The GTMI provides a quantitative assessment tool, and the STL-RF-PLS framework advances mechanistic understanding of multifactorial drivers, offering critical insights for regional green tide mitigation and coastal ecosystem management.