This indicator shows relative frequency of the occurrence of low concentrations (< 2 ml/l) of dissolved oxygen in the bottom water layer during the summer period between May and November. The mean values of dissolved oxygen concentration at the depth of over 10 metres are shown. Averages for measuring stations in the period 1989–2005 are shown in figures MR3-1 and MR3-2. Only the stations where hypoxia in the bottom water layer was recorded are shown, i.e. the stations in the middle (00CZ) and south-western (000G) section of the Gulf of Trieste (Figure MR3-3).

The main factors causing hypoxia in the bottom water layer are an increased quantity of organic particles and deposition. Vertical mixing of water masses, on the other hand, improves oxygen supply, thus reducing hypoxia in the bottom water layer.

The Northern Adriatic Sea is one of many coastal water areas where various stages of hypoxia in the bottom water layer can be observed in the late summer or autumn months. Among these areas is the Gulf of Trieste, where chemical characteristics of the water column and movement of water masses change considerably depending on seasonal conditions. During the winter, the water column is relatively uniform, while in spring, layers begin to form due to the warming of the surface layer and the influx of fresh river water. The layers are most distinct during the summer months. Seasonal variation of temperature layers is strongly affected by solar radiation. Heavy storms can also mix the layers in summer. Autumn cooling and winds cause the water column to become uniform again.

The first thorough and systematic research of oxygen content in the bottom water layer was conducted in the 1990s. It coincides with the establishment of the “Northern Adriatic Observatory” and the monitoring of the central part of the Gulf of Trieste. Prior to that, only data from the shallower, narrow coastal stripe was available.

The Gulf of Trieste is a semi-closed, shallow bay in which a seasonal decrease of oxygen in the bottom water layer has been observed in late summer and autumn. Partial or complete hypoxia in the Gulf of Trieste was observed in 1974, 1983, 1987 and 1990. The extent of complete hypoxia at the sea bottom was the largest in 1983, when approximately one third of the Gulf was affected and such conditions persisted for two weeks. The consequence of hypoxia was a massive die-off of benthic organisms. In other cases, smaller areas in the central part of the Gulf were affected. Later, measurements that regularly included the central part of the Gulf revealed that oxygen levels approach the critical minimum between August and October of almost every year. On the other hand, oxygen content in the south-eastern part of the Gulf is usually higher.

Frequency of the occurrence of low oxygen concentrations in the bottom water layer in the coastal and marine environment in the Gulf of Trieste does not indicate a recognisable trend. In the period 1989–2005, hypoxia occurred occasionally in the central part of the Gulf of Trieste: in the late summer or autumn of 1989, 1990, 1994, 1995, 2000 and 2001 (figures MR3-1 and MR3-2). In the south-eastern part of the Gulf, only two events of hypoxia were recorded (in 1989 and 1990) in the period 1989–2005. At all sampling sites, the occurrence of hypoxia represents less than 3% of all measured values. Hypoxia has not been recorded at depths greater than 20 metres.