Figure 1. Vertical distribution of oxygen concentrations in the Gotland Depression
In the deep basins of the Baltic Sea, such as the East Gotland Basin, long periods of oxygen deficiency occur in the water layers beneath the halocline due to the nature of water exchange and eutrophication (Figure 1). The water layer below the halocline, which is oxygen deficient, is not classified as “near bottom”. Therefore, an “Oxygen Deficiency Indicator” was developed and agreed upon by HELCOM Member States. The oxygen deficit is calculated for a deep basin (separated from another basin by seabed elevation) as a volume-specific quantity.
HELCOM Member States have agreed that the Allowable Oxygen Deficiency (AOD) for the East Gotland Basin is 8.66 ml L-1. HELCOM assessments, such as HELCOM HOLAS II, calculate the ratio of the oxygen deficit(eutrophication ratio – ER) for the period of interest to this threshold concentration. The HELCOM HOLAS II assessment for the period 2011-2015 gives an ER value of 1.24 for the East Gotland basin, i.e. below-GES. No significant changes have been observed compared to the previous period, i.e. the state of the environment remains unsatisfactory. The long-term trend of oxygen deficiency in the central Baltic Sea shows that a qualitative jump from good to poor conditions occurred between 1950 and 1960 (HELCOM HOLAS II). In the following years, oxygen deficiency has remained at a poor level.
In the other catchments, the national indicator, bottom oxygen concentration, was used to assess the environmental status. Data on bottom oxygen concentrations in the Baltic Sea coast are available from 1984. The first observation was carried out in the first half of 2008 (Figure 2), but it should be noted that observations have not been regular and in some years (e.g. during 1984-1986 only one station was monitored). In the period 1984-2016. In the period from 2007 to 2010, the oxygen concentration at the bottom ranged from 4 ml/l to almost 7 ml/l. The average bottom oxygen concentration over the whole period was 5.8 ml/l, which is higher than in the four regions considered in the Gulf. The maximum oxygen concentration (6.7 ml/l) was observed in 2006 and the minimum (4.2 ml/l) in 1984. There is no clear decreasing or increasing trend in near-bottom oxygen concentrations along the Baltic Sea coast.
Figure 2. Summer mean values (ml/l) of bottom water layer oxygen concentration in the Gulf of Riga and the Baltic Sea. A – coastal waters of the Baltic Sea, B – western coast of the Gulf of Riga, C – open waters of the Gulf of Riga, D – eastern coast of the Gulf of Riga, E – transitional waters
Data on bottom oxygen concentrations in the western coast of the Gulf of Riga are available from 1988 to 2006. The number of observations was very low in 1996-1998 (Figure 2), but it should be noted that observations have not been regular and in some years (e.g. 1996-1998) observations were made at only one station. In the period 1988-2016 groundwater oxygen concentrations ranged from 4-7 ml/l. The average near-bottom oxygen concentration over the whole period was 5.5 ml/l, the maximum oxygen concentration (6.6 ml/l) was observed in 1988. The minimum level (4.2 ml/l) was observed in 2011. Overall, oxygen concentrations at the bottom decreased slightly until 2000. After 2000 the oxygen concentrations observed in the Baltic Sea show no further downward trend.
Data on bottom oxygen concentrations in the central part of the Gulf of Riga are available since 1971 and shows the best spatial and temporal distribution compared to the other regions considered. In the period 1971-2016 the summer seasonal near-bottom oxygen concentrations were mostly in the range of 2-7 ml/l. The average benthic oxygen concentration over the whole period was 4.4 ml/l, the maximum oxygen concentration (6.5 ml/l) was observed in 1995 and the minimum (2.2 ml/l) in 2014. It should be noted that the central part of the Gulf of Riga has significantly lower mean oxygen concentrations (including the minimum value) than the other regions considered, which is due to the fact that the central part of the Gulf is much deeper than the other regions considered. The bottom oxygen concentration from the beginning of the considered period to 1984 is generally lower than it is between 1985-2002, when the oxygen concentration often exceeded 5 ml/l and was rarely lower than 4 ml/l. After 2002 oxygen concentrations returned to lower levels, and only exceeded oxygen concentration of 5ml/l in 2012. Years with oxygen concentrations below 3 ml/l are also more common – 2006, 2009, 2014.
Data on bottom oxygen concentrations in the eastern Gulf of Riga are available since 1998 (Figure 2), although one measurement was also carried out in 1976. As on the west coast of the Gulf, it should be noted that observations have not been regular and in some years (e.g. 1996, 2008) observations have been made at only one station. In the period 1988-2016 groundwater oxygen concentration was around 3-7 ml/l. The average benthic oxygen concentration over the whole period was 5.3 ml/l, the maximum oxygen concentration (6.5 ml/l) was observed in 1998. The minimum level (3.1 ml/l) – in 2009. On the eastern coast of the Gulf of Riga, a significantdecrease in bottom oxygen concentrations can be observed, i.e. in the period 1988-1993. The oxygen concentration was around 6 ml/l in the period 1999-2009, but from 1999 onwards the oxygen concentration was around 5- 6 ml/l. In the first three years, oxygen concentrations were mostly in the 5-6 ml/l range or even lower, i.e. in three years the bottom oxygen concentration was below 4 ml/l, which was not the case on the west coast of the Gulf, where all oxygen values were above 4 ml/l.
Data on bottom oxygen concentrations in the transitional waters of the Gulf of Riga are available since 1971, although it should be noted that the observations were made at one station (101A). In the period 1971-2016 near-bottom oxygen concentrations were mostly in the range of 3-7 ml/l. The average bottom oxygen concentration over the whole period was 5.2 ml/l, the maximum oxygen concentration (6.2 ml/l), similar to the west coast of the Gulf, was observed in 1988. The minimum level (2.9 ml/l) was observed in 2007. An analysis of long-term observations shows that near-bottom oxygen concentrations up to about year 2000 are mostly in the 5-6 ml/l range, but after 2000 oxygen concentrations have been gradually decreasing, with more values in the 4-5 ml/l range and values below 4 ml/l were observed similarly to the eastern Gulf coast.
Table 1. Assessment of the state of the environment in 5 different regions of the Baltic Sea and the Gulf of Riga 2007-2011 and 2012-2016. The number indicates the average value found over the period.
|Water object||Threshold||Period||Trends||Confidentiality of the assessment|
|2007 – 2011||2012 – 2016|
|Coastal waters of the Baltic Sea||5||5.54||5.93||↗|
|West coast of the Gulf of Riga||5||4.97||5.43||↗|
|Open waters of the Gulf of Riga||5||4.02||3.63||↘|
|Eastern coast of the Gulf of Riga||5||4.74||5.08||↗|
Looking at the oxygen concentration results from the previous (2007-2011) and current (2012-2016) assessment periods in the Gulf of Riga and the Baltic Sea, it can be seen that there is little or no change in oxygen concentrations in the water bodies examined (Table 1). Overall, the observed concentrations are only slightly above or below the threshold values for good environmental status. Therefore, the status (good or bad) found in the assessment (Table 1) should be treated with caution. The exception is the central part of the Gulf of Riga, where the observed concentrations are significantly below the threshold value for good environmental status.