Distinct mechanisms of chlorophyll-a blooms occur in the Northern Maluku Sea and Sulu Sill revealed by satellite data

Chlorophyll-a is the predominant phytoplankton pigment responsible for determining primary productivity. In the present study, we used satellite-based data of chlorophyll-a, surface wind, and precipitation from 2003 to 2019 to investigate the variability of chlorophyll-a in the northern Maluku Sea and the Sulu Sill and examine its generating mechanism. We found that the chlorophyll-a bloom in the northern Maluku Sea occurs during the southeast monsoon season, while in the Sulu Sill, the chlorophyll-a concentration is higher than that in the northern Maluku Sea and occurs throughout the year. In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling. The maximum southerly wind during the southeast monsoon generates the strongest offshore Ekman Mass Transport (EMT) in the northern Maluku Sea trig-gering coastal upwelling. However, the power spectra analysis of satellite-derived chlorophyll-a shows strong peaks and amplitudes at both fortnightly (MSf) and monthly (Mm) frequencies, indicating that tidal mixing is an important generating mechanism for chlorophyll-a blooms in the Sulu Sill. Shallow bathymetry in the Sulu Sill may aid tidal mixing in effectively transporting nutrients from the near bottom to the sea surface, increasing chlorophyll-a concentration.