Pressure on oxygen levels of Jakarta Bay

•High nutrient concentrations in Jakarta Bay coastal waters foster phytoplankton bloom formation.•Organic nitrogen makes up to 92% of the total nitrogen pool.•An oxygen deficiency area covering 20 km2 was identified in the nearshore waters.

Jakarta Bay is subject to inputs of large amounts of dissolved inorganic nutrients and organic matter deriving from the Jakarta Metropolitan Area and its hinterland. As a consequence, inorganic and organic nutrient concentrations in the nearshore area are considerably increased. The hydrographic pattern inside the bay fosters phytoplankton bloom formation due to pronounced vertical density gradients. High Biomass Blooms (HBB) of phytoplankton are observed recurrently, representing an additional source for oxygen consuming degradable organic matter. An oxygen deficiency area of 20 km2 can be identified in the eastern part of Jakarta Bay where oxygen levels decrease to 60% saturation, equaling a noxiously low concentration of 4.0 mg·dm− 3 O2. The absolute minimum detected was 51% O2 or 3.2 mg·dm− 3 O2. The observed oxygen concentrations fail Indonesian quality standards for seawater.