Hydro-environmental processes governing the formation of hypoxic parcels in an inverse estuarine water body: Assessment of physical controls

Abstract

Hypoxia is often thought of as the key factor responsible for fish kill events in coastal areas but fish kill events are too complex to be governed by a single factor. The events are influenced by a combination of chemical, biological and physical processes. Hydrodynamics play a key role in understanding the formation of hypoxia in shallow waters. This study aims to identify the settings of the physical forces that lead to a large-scale depletion of dissolved oxygen in Kuwait Bay at the northwest of the Arabian Gulf. The assessment, made with a validated three-dimensional numerical model (Alosairi and Alsulaiman, 2019), revealed that the pollution from the outfalls leads to nearfield depletion of dissolved oxygen but has only a minor effect on the bay-scale dissolved oxygen. This is a result of the strong dynamics of Kuwait Bay, which mixes the pollutant rapidly before it is transported seawards. Offshore, a low dynamic region has been identified near Jahra Bay which is susceptible to occasional dissolved oxygen depletion. Assessment of the physical forces revealed that the density-driven current and, to a greater extent the wind regime, controlled the formation of a hypoxic parcel near Jahra Bay. The combination of neap tides and low mixed winds reduced mixing and enabled the longer residence times of Kuwait Bay. These are the most critical circumstances, as the average dissolved oxygen can be reduced by 50% during summer. The circumstance resulting in low dynamics near Jahra Bay were also found to be effective in explaining algal blooms.