KAZALCI OKOLJA

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Key message
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A decrease of oxygen concentration in bottom water has been recorded between late summer and autumn in less than 3% of all measured values. At depths less than 20 m, hypoxia has not been recorded.


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.


Charts

Figure MR03-1:
Show data
11.1.1990 13.2.1990 20.3.1990 11.4.1990 8.5.1990 23.5.1990 12.6.1990 9.7.1990 2.8.1990 4.9.1990
ml/l 4.8 6 6 5.5 5.4 5.4 4.4 4.1 2.7 1.4
ml/l 2 2 2 2 2 2 2 2 2 2
16.10.1990 12.11.1990 9.1.1991 20.2.1991 13.3.1991 15.4.1991 21.5.1991 17.6.1991 11.7.1991 5.8.1991
ml/l 3.8 5 3.9 6 6.3 5.8 4.9 3.8 3.4 3.4
ml/l 2 2 2 2 2 2 2 2 2 2
4.9.1991 4.10.1991 5.11.1991 16.12.1991 14.1.1992 12.2.1992 4.3.1992 6.4.1992 7.5.1992 27.5.1992
ml/l 2 2.5 5.1 5.9 6.2 6.3 6.7 5.7 6.1 5.2
ml/l 2 2 2 2 2 2 2 2 2 2
15.6.1992 13.7.1992 20.8.1992 15.9.1992 20.10.1992 24.11.1992 21.12.1992 19.1.1993 9.2.1993 16.3.1993
ml/l 4.3 3.7 3.7 3.5 4.1 4.5 5.2 5.6 5.4 6
ml/l 2 2 2 2 2 2 2 2 2 2
20.4.1993 11.5.1993 8.6.1993 13.7.1993 3.8.1993 7.9.1993 12.10.1993 9.11.1993 7.12.1993 17.1.1994
ml/l 5.5 6.2 6.1 4.8 4.3 4.8 4 3.3 5.6 5.6
ml/l 2 2 2 2 2 2 2 2 2 2
22.2.1994 16.3.1994 22.4.1994 16.5.1994 5.7.1994 26.7.1994 22.8.1994 28.9.1994 25.10.1994 16.11.1994
ml/l 6.4 6.4 5.2 4.7 5.1 5.5 4.2 1.3 3.9 2.9
ml/l 2 2 2 2 2 2 2 2 2 2
13.12.1994 12.1.1995 21.2.1995 21.3.1995 4.5.1995 22.5.1995 26.6.1995 17.7.1995 9.8.1995 11.9.1995
ml/l 4.9 5.8 6.1 5.2 5.9 4.8 4.5 4.4 3.3 3.5
ml/l 2 2 2 2 2 2 2 2 2 2
12.10.1995 12.10.1995 8.11.1995 8.11.1995 8.12.1995 11.1.1996 20.2.1996 13.3.1996 10.4.1996 14.5.1996
ml/l 0.3 0.3 5.2 5.2 6.1 4.9 6.5 7.1 7.2 5.3
ml/l 2 2 2 2 2 2 2 2 2 2
11.6.1996 23.7.1996 5.9.1996 24.9.1996 22.10.1996 21.11.1996 11.12.1996 14.1.1997 13.2.1997 12.3.1997
ml/l 5 5 4.3 3 3 4.9 5.2 5.8 5.7 6
ml/l 2 2 2 2 2 2 2 2 2 2
23.4.1997 19.5.1997 23.6.1997 7.7.1997 12.8.1997 17.9.1997 13.10.1997 18.11.1997 9.12.1997 28.1.1998
ml/l 6.2 4.6 4.4 3.8 2.2 3.8 2 4.1 5 6.3
ml/l 2 2 2 2 2 2 2 2 2 2
17.2.1998 31.3.1998 21.4.1998 12.5.1998 15.6.1998 13.7.1998 18.8.1998 15.9.1998 13.10.1998 10.11.1998
ml/l 6.5 6.1 5.5 4.3 4.6 5.4 4.5 3.3 2 4.8
ml/l 2 2 2 2 2 2 2 2 2 2
15.12.1998 14.1.1999 15.2.1999 16.3.1999 20.4.1999 18.5.1999 14.6.1999 13.7.1999 17.8.1999 13.9.1999
ml/l 5.4 5.5 6.6 6.3 5.4 4.2 4.3 3.6 2.4 2.8
ml/l 2 2 2 2 2 2 2 2 2 2
11.10.1999 17.11.1999 8.12.1999 13.1.2000 13.1.2000 9.2.2000 9.2.2000 14.3.2000 14.3.2000 11.4.2000
ml/l 4.2 5.4 5.9 6.1 6.5 7 5.9 5.8 7.2 6.4
ml/l 2 2 2 2 2 2 2 2 2 2
11.4.2000 16.5.2000 16.5.2000 13.6.2000 13.6.2000 13.7.2000 13.7.2000 9.8.2000 9.8.2000 12.9.2000
ml/l 5.3 4.6 4.2 5.1 3.8 3.9 3.1 3.1 1.5 2.6
ml/l 2 2 2 2 2 2 2 2 2 2
12.9.2000 10.10.2000 10.10.2000 15.11.2000 15.11.2000 12.12.2000 12.12.2000 17.1.2001 17.1.2001 13.2.2001
ml/l 3.4 4.4 4.5 4.7 3.5 2.7 4 6.7 6 4
ml/l 2 2 2 2 2 2 2 2 2 2
13.2.2001 12.3.2001 12.3.2001 18.4.2001 18.4.2001 15.5.2001 15.5.2001 13.6.2001 13.6.2001 17.7.2001
ml/l 6.9 7.4 5.1 3.7 6.2 5.6 6.8 3 5.3 4.3
ml/l 2 2 2 2 2 2 2 2 2 2
17.7.2001 7.8.2001 7.8.2001 11.9.2001 11.9.2001 16.10.2001 12.11.2001 12.11.2001 12.2.2002 12.2.2002
ml/l 3 4.3 4.5 3.8 3.4 0.7 5.2 5.3 5.8 5.8
ml/l 2 2 2 2 2 2 2 2 2 2
12.3.2002 12.3.2002 17.4.2002 17.4.2002 14.5.2002 14.5.2002 11.6.2002 11.6.2002 9.7.2002 9.7.2002
ml/l 5.6 5.2 4.4 4.8 6.2 5.2 4.9 3.8 4.7 3.4
ml/l 2 2 2 2 2 2 2 2 2 2
5.8.2002 5.8.2002 17.9.2002 17.9.2002 15.10.2002 15.10.2002 12.11.2002 29.1.2003 20.2.2003 19.3.2003
ml/l 4.2 2.1 4.8 4 4.6 5.9 6.4 4.9 6.2 5.9
ml/l 2 2 2 2 2 2 2 2 2 2
16.4.2003 19.5.2003 10.6.2003 31.7.2003 20.8.2003 17.9.2003 20.10.2003 18.11.2003 15.12.2003 20.1.2004
ml/l 6.5 6 5.6 5.5 3 3.4 5.3 5.4 5.9 6.1
ml/l 2 2 2 2 2 2 2 2 2 2
18.2.2004 17.3.2004 14.4.2004 11.5.2004 15.6.2004 14.7.2004 10.8.2004 14.9.2004 14.10.2004 17.11.2004
ml/l 6.2 6.5 6.3 5.9 5.1 3.8 3.4 4.5 4.9 5.5
ml/l 2 2 2 2 2 2 2 2 2 2
15.12.2004 18.1.2005 23.2.2005 23.3.2005 14.4.2005 12.5.2005 15.6.2005 20.7.2005 17.8.2005 14.9.2005
ml/l 5.7 5.9 6.7 6.9 6.1 6.3 5.7 5.4 3.4 3
ml/l 2 2 2 2 2 2 2 2 2 2
14.10.2005 15.11.2005 16.12.2005 18.1.2006 14.2.2006 16.3.2006 21.4.2006 16.5.2006 14.6.2006 19.7.2006
ml/l 3.9 2.4 5.5 6.1 6.3 4.9 5.2 5.7 5.4 4.9
ml/l 2 2 2 2 2 2 2 2 2 2
17.8.2006 21.9.2006 18.10.2006 15.11.2006 13.12.2006 22.1.2007 15.2.2007 14.3.2007 18.4.2007 24.5.2007
ml/l 4 3.2 5 5.4 5.6 5.5 5.5 6.1 5.7 4.9
ml/l 2 2 2 2 2 2 2 2 2 2
13.6.2007 12.7.2007 23.8.2007 20.9.2007 17.10.2007 13.11.2007 11.12.2007
ml/l 4.9 4.4 2.5 4.6 5 5.4 5.9
ml/l 2 2 2 2 2 2 2
Figure MR03-2:
Show data
30.5.1989 13.6.1989 5.7.1989 25.7.1989 24.8.1989 6.9.1989 27.9.1989 18.10.1989 7.12.1989 11.1.1990
ml/l 4.7 4.8 3.7 3.4 2.6 3.6 1.8 4.8 5.8 5.7
13.2.1990 20.3.1990 11.4.1990 8.5.1990 23.5.1990 12.6.1990 9.7.1990 2.8.1990 30.8.1990 16.10.1990
ml/l 4.5 6 5.4 5.1 5.2 4.6 4.8 3.8 1.7 3.3
12.11.1990 9.1.1991 20.2.1991 13.3.1991 15.4.1991 21.5.1991 18.6.1991 11.7.1991 5.8.1991 4.9.1991
ml/l 3.8 4.9 6.3 5.8 5 5.8 4.2 3.1 3.2 3.7
4.10.1991 5.11.1991 16.12.1991 14.1.1992 12.2.1992 4.3.1992 6.4.1992 7.5.1992 27.5.1992 15.6.1992
ml/l 4.4 5.4 5.3 5.3 6.2 4.6 6.2 5.1 5.1 4.5
13.7.1992 20.8.1992 15.9.1992 20.10.1992 17.11.1992 24.11.1992 21.12.1992 19.1.1993 9.2.1993 16.3.1993
ml/l 4.5 3.8 2.6 4.8 4.8 4.5 5.5 5.4 6.3 6.2
20.4.1993 11.5.1993 9.6.1993 13.7.1993 3.8.1993 7.9.1993 12.10.1993 9.11.1993 22.2.1994 16.3.1994
ml/l 6.2 6.6 6.2 5.3 5.1 4.8 5.1 5.2 6.4 6.2
22.4.1994 16.5.1994 5.7.1994 26.7.1994 22.8.1994 28.9.1994 25.10.1994 16.11.1994 13.12.1994 12.1.1995
ml/l 5.5 5.6 5.1 4.8 4.7 3.3 3.9 3.6 4.2 6.3
21.2.1995 21.3.1995 4.5.1995 22.5.1995 26.6.1995 17.7.1995 9.8.1995 11.9.1995 12.10.1995 8.11.1995
ml/l 6.4 6.3 5.7 5.1 4.2 5 4.7 4.1 3.7 5.3
8.12.1995 11.1.1996 20.2.1996 13.3.1996 10.4.1996 14.5.1996 11.6.1996 23.7.1996 5.9.1996 24.9.1996
ml/l 5.9 4.8 6.5 6.6 7 5.8 5.9 4.6 4.5 4.1
22.10.1996 21.11.1996 11.12.1996 14.1.1997 13.2.1997 12.3.1997 23.4.1997 19.5.1997 23.6.1997 7.7.1997
ml/l 4.7 4.8 5.2 5.6 5.8 5.9 6.2 5 5.4 4.6
12.8.1997 17.9.1997 13.10.1997 18.11.1997 9.12.1997 28.1.1998 17.2.1998 31.3.1998 21.4.1998 12.5.1998
ml/l 5 3.8 3 4.6 5.2 6.3 6.5 6.3 5.6 4.9
15.6.1998 13.7.1998 18.8.1998 15.9.1998 13.10.1998 10.11.1998 15.12.1998 14.1.1999 15.2.1999 16.3.1999
ml/l 5.6 5.2 4.3 2.9 4.3 4.4 5.7 5.5 6.5 6
20.4.1999 18.5.1999 14.6.1999 13.7.1999 17.8.1999 13.9.1999 11.10.1999 8.12.1999 13.1.2000 9.2.2000
ml/l 5.6 4.2 4.3 3.6 2.8 3.3 4.3 5.9 6.6 7
14.3.2000 11.4.2000 16.5.2000 13.6.2000 13.7.2000 9.8.2000 12.9.2000 10.10.2000 15.11.2000 12.12.2000
ml/l 6.9 6.6 5.2 4.3 4.3 4.2 3.4 4.4 4.5 3.7
17.1.2001 13.2.2001 12.3.2001 18.4.2001 15.5.2001 13.6.2001 17.7.2001 7.8.2001 11.9.2001 16.10.2001
ml/l 6.2 6.9 7.4 6.7 5.3 5.6 4.4 3.9 2.7 2.9
12.11.2001 11.12.2001 12.3.2002 17.4.2002 14.5.2002 11.6.2002 9.7.2002 5.8.2002 17.9.2002 15.10.2002
ml/l 5.3 6.2 6.2 3 5.3 5.6 5.1 3.8 4.9 6.4
12.11.2002
ml/l 6.4
Figure MR03-3:
Sources: 

Goals

  • Protection and preservation of the marine environment;
  • Maintaining the quality of water in order to enable thriving and growth of marine bivalves and gastropods.
  • Reduction of the quantity of nutrients in coastal waters.

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.

 

 



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