Multi-temporal evolution of surface chlorophyll-a in waters around South China Sea reefs: Differences and key...

Pelagic phytoplankton dynamics in reef-associated waters of the South China Sea (SCS) exhibit pronounced nonlinear and multi-scale characteristics. Using Chlorophyll-a (Chl-a) data from 1998-2024 in four major reef regions, this study applies the Hilbert-Huang Transform (HHT) and Liang-Kleeman Information Flow (LKIF) to examine temporal variability and statistical associations with environmental factors. Results reveal distinct regional patterns: Nansha is dominated by decadal variability, Xisha and Zhongsha show strong interannual fluctuations with overlapping high-frequency modes, and Dongsha maintains a relatively stable annual cycle. High-frequency Chl-a variability in Xisha and Zhongsha is likely associated with dynamic processes in the western SCS, including the Vietnam offshore jet and mesoscale eddies, while cross-scale coupling indicates interactions among temporal scales. Stable environmental conditions, particularly the dominance of a single forcing factor, correspond to reduced nonlinear variability. For example, the annual cycle in Dongsha is primarily linked to sea surface height variations. Long-term Chl-a trends are closely related to the stability of normalized information flow (NIF) from environmental drivers. Dongsha is characterized by predominantly unstable NIF, with photosynthetically active radiation exhibiting the highest value (0.95), coinciding with a nonlinear Chl-a trajectory of initial increase followed by decline. In contrast, Xisha and Zhongsha are mainly associated with stable NIF. These results provide a statistical framework for assessing environmental influences on phytoplankton dynamics in SCS reef-associated waters and support future mechanistic investigations. Plain Language Summary The pelagic phytoplankton dynamics in the South China Sea (SCS) are characterized by highly complex and interconnected interactions. Our study investigates the variation of Chlorophyll-a (Chl-a) concentration across four major reef regions in the SCS. Key findings reveal distinct temporal patterns: Nansha is dominated by long-term signals, while Xisha and Zhongsha display complex high-frequency fluctuations along with stable 3-6-year interannual cycles. Dongsha exhibits clear seasonal variations. The oceanic dynamic processes in the western SCS are strongly associated with these changes, and high-frequency environmental signals show cross-scale statistical coupling with longer-term processes. The presence of a single dominant environmental factor is linked to reduced complex nonlinear fluctuations. Long-term analysis indicates that "unstable information flow" inputs are strongly associated with distinct Chl-a trends. For instance, the Dongsha waters are predominantly characterized by unstable information flow, with the normalized information flow from photosynthetically active radiation reaching 0.95, coinciding with a nonlinear "rise-fall" pattern. Xisha shows a gradual decline under stable information flow, while Zhongsha experiences a slow upward trend. This study provides a statistical framework for evaluating the associations between marine environmental changes and pelagic phytoplankton dynamics in these reef-associated waters, and provides a basis for future mechanistic studies.