Dynamic bio-regionalization of a highly variable oceanic system: Insights from the Southern South China Sea

Abstract

Classifying ecologically distinct biological regions with similar phytoplankton dynamics is crucial for marine resource management. The southern South China Sea (SSCS) is one of the world's most ecologically important and productive regions. It is a highly dynamic system, characterized by the extensive shallow Sunda Shelf (<200 m) in the south and a deep basin (> 1000 m) in the north. While the entire basin is influenced by reversing monsoonal winds, coastal regions are shaped by both monsoonal winds and riverine inputs. Clustering such a variable system remains challenging, and no study has systematically classified its bio-regions. Using empirical orthogonal function analysis and fuzzy-c means clustering on 24 years of satellite-derived chlorophyll-a (chl-a) data, we identified three distinct regions: (1) the eastern boundary of the SSCS, (2) the central SSCS and off the coast of Vietnam, and (3) the western region from the Karimata Straits to the Mekong River mouth. The eastern region exhibits unimodal seasonality, while the central and western regions display bimodal seasonality. These regions are temporally dynamic, shifting in size and location on interannual timescales. During the 1998 El Ni & ntilde;o, chl-a variability in the deep basin changed drastically under intense warming. In contrast, during the 2015 El Ni & ntilde;o, despite strong warming, chl-a variability remained statistically similar to climatology. While monsoonal modulation by the El Ni & ntilde;o Southern Oscillation influences these patterns, our results show that interannual variability is complex and diverse, warranting further validation for improved modelling and fisheries management.