Observed Water Mass and Chlorophyll-a Distribution in the Western Maluku Sea: From BUDEE Cruise 2022

Abstract

Banggai Waters is in the western Maluku Sea, where the secondary pathway of Indonesian Throughflow passes and seasonal coastal upwelling takes place in this region, providing a high marine productivity and potential fishing ground. This study examines the vertical stratification and water mass distribution in the western Banggai region of the northern Maluku Sea, using hydrographic data collected during the Banggai Upwelling Dynamics and Ecosystem Experiment (BUDEE) in September 2022. Eight CTD stations were selected along two transects that cross both upwelling and non-upwelling zones. The water column is characterized by a thin mixed layer and a relatively thick thermocline, with spatial variability observed between stations. Salinity profiles indicate fresher surface waters, a subsurface salinity maximum (~100 m), and a deeper minimum (~300 m), reflecting the presence of multiple water masses. The pycnocline is shallower and more pronounced in the northern transect, suggesting differences in stratification and mixing. Dissolved oxygen (DO) is highest near the surface and decreases with depth, forming an oxygen minimum zone below 400 m. Chlorophyll-a (Chl-a) profiles show two patterns: surface maxima and subsurface peaks (40–60 m), particularly at Stations 14 and 36, associated with upwelling centers. Five distinct water masses were identified: surface water, North Pacific Subtropical Water (NPSW), North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), South Pacific Subtropical Lower Thermocline Water (SPSLTW). These findings reveal the influence of upwelling, circulation, and vertical mixing in shaping the hydrographic structure of the western Maluku Sea.