Annual hypoxia dynamics in a semi-enclosed Mediterranean gulf

Abstract

In this paper we study the hypoxia dynamics in a fjord-like gulf, the Amvrakikos Gulf, in the Mediterranean Sea and highlight the factors which control the development of hypoxia. Bimonthly physicochemical data were collected in order to describe the hypoxic conditions and a three-dimensional model was used to describe the hydrodynamic circulation and explain the variations in hypoxic conditions. According to our results, the horizontal intrusion of well oxygenated open seawater can lead to the oxygenation of the deepest water in this semi-enclosed gulf. The key factor in determining the success of this horizontal intrusion is the density difference between the gulf's deepest area and the open sea outside the gulf. When the well oxygenated open seawater is denser than the bottom water in the gulf, it inflows into the gulf near the bottom and slowly re-oxygenates the isolated deep waters through mixing and diffusion, as the water travels eastward. However, when the gulf's deep water is denser than the open sea's water, the inflow stops. This, in combination with the eutrophic conditions which characterize the Amvrakikos, causes the re-establishment of hypoxic/anoxic conditions.