Understanding the Mechanism and Variability of Chlorophyll a Concentration in the Waters Surrounding the Philippines

Phytoplankton plays a crucial role in marine life as it is the primary producer in food webs, providing energy for higher trophic levels. Understanding the seasonal variation of chlorophyll-a concentration in the Philippines can provide valuable information about the distribution patterns of phytoplankton. In this study, we aimed to comprehend the mechanism and variability of chlorophyll-a concentration in the waters surrounding the Philippines, including its seasonal changes, and how meteorological variables (such as sea surface temperature, rainfall, and wind) and oceanic circulations (such as Ekman Transport, Ocean Current, and ENSO) contribute to its variability. The results of this study showed that coastal waters generally have higher chl-a concentration, while open seas have lower chl-a concentration. Additionally, the highest seasonal chl-a concentration is found during the winter season (DJF) with the presence of an offshore chl-a patch in the north-western Philippine Sea (WPS). Conversely, seasonal chl-a concentration is at its lowest during spring (MAM). About 33% of the total chlorophyll-a variability showed a direct covariation in all seasons between the West Philippine Sea and the Philippine Sea that occurs every six years during winter and spring seasons, and every nine years during summer and autumn seasons. The concentration of chlorophyll-a is influenced by meteorological variables such as wind, rainfall, and sea surface temperature. Along the coasts, stronger winds and increasing rainfall result in higher chlorophyll-a concentration, while weaker winds and decreasing rainfall lead to lower concentration. Ekman Transport showed a direct relationship to chlorophyll-a in some parts of WPS and an inverse relationship in the northern West Philippine Sea wherein stronger (weaker) offshore Ekman Transport results in higher (lower) chlorophyll-a concentration. The El Niño Southern Oscillation has varying effects on the chlorophyll-a concentration across the seas of the Philippines. Lastly, the Kuroshio Current (KC) has a significant impact on the oceanic conditions in the western North Pacific.