A quasi-persistent hypoxic water mass in an equatorial coastal sea, Jakarta Bay, Indonesia

Apart from in deep and mainly enclosed systems, the occurrence of hypoxic conditions in coastal seas is generally only a seasonal phenomenon. Despite this, we observed quasi-persistent hypoxia in an equatorial shallow open bay (Jakarta Bay, Indonesia) between 2015 and 2018. There are 3 reasons for the persistence of this hypoxic water mass, namely 1) This bay tends to be stratified due to weak tidal mixing and high river discharge; 2) Due to the lack of continuous vertical convection caused by surface cooling in an equatorial climate, here the water column tends to remain stratified; 3) The oxygen demand in the water is high. Recovery from hypoxia in Sep. 2017 followed the intrusion of offshore water induced by wind. In this study it is suggested that estuaries and ROFIs (Region of Fresh Water Influence) in equatorial regions are susceptible to hypoxia under increasing anthropogenic impacts. •Though Jakarta Bay is a shallow open bay, a hypoxic water mass was formed quasi-persistently.•The hypoxia was generated by large oxygen demand of water and persistent formation of stratification.•Persistent stratification was generated by weak tidal mixing, high river discharge and lack of continuous convection.•The recovery of hypoxia was caused by the intrusion of offshore water in the bottom layer induced by wind.