Variability in dissolved oxygen off Eastern Luzon, Philippines

The eastern coast and shelf of Luzon is a unique area encompassed by the bifurcation of the western boundary North Equatorial Current (NEC) into the Kuroshio and Mindanao Currents. This region is also productive and has become a rich fishing ground. Of interest is how biogeochemistry in this area is influenced by variability in the bifurcation driven by ENSO events, as well as by production and remineralization processes. Results from 2011 and 2012 oceanographic cruises show changes in dissolved oxygen (DO) off Eastern Luzon in both spatial and temporal scales. Between 2011 and 2012, there was a southern shift of the bifurcation latitude. Water masses from the NEC and the Kuroshio Recirculation Gyre (KRG) east of Luzon have inherent low and higher DO concentrations, respectively. A subsurface oxygen minimum layer was seen at 150-200m. Waters with this low dissolved oxygen signature comes from a 400m-deep sill basin (Lamon Deep) off Eastern Luzon. Apart from low ventilation rates, organic matter decomposition contributes to depletion of DO. Proximity of the basin to the coast is evident in the high particulate organic carbon concentration that is delivered from land through run-off and the nearby river. The low DO water is advected offshore and contributes to the spatial variability of DO in the area. Linear regression of particulate organic carbon, dissolved organic carbon, dissolved inorganic carbon, and nutrients with AOU strongly correlate organic matter remineralization to the change in DO with depth. The variability in DO off Eastern Luzon is analyzed with the large-scale variability offshore of source waters to determine the relative influence of biogeochemical cycling in the area.