Hypoxia is increasing in coastal oceans. This is because eutrophication has increased oxygen consumption, while less oxygen is replenished to the bottom under stronger stratification. Quantifying these biogeochemical and physical drivers is important for management and predicting future trends. By using observations from the Pearl River Estuary (PRE) region (10-70 m deep) and similar coastal systems, we introduce a simple analysis to quantify both the biogeochemical and physical drivers of hypoxia. Our results show that in the PRE region, sediment respires >60% of organic matter produced in the water column, leading to high sediment oxygen uptake (average 41.1±16.3 mmol m^-2 d^-1) and shallow oxygen penetrations (2-7 mm). The sediment's effect on the bottom oxygen loss becomes stronger with the reducing thickness of the bottom boundary layer. We then construct a generic mass-balance model to quantify oxygen loss, determine timescales of hypoxia formation, and explain within- and cross-system variabilities.
 

sediment oxygen uptake, stratification, bottom boundary layer, hypoxia, coastal oceans