Figure 1. Summer mean values of water transparency 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
Summer water transparency (Secchi depth) observations in the open waters of the Gulf of Riga are available from 1963, in transitional waters from 1983, in coastal waters of the Gulf of Riga from 1991 and in coastal waters of the Baltic Sea from 1999. In the open waters of the Gulf of Riga, water transparency has decreased by about 1 m between the beginning of observations and the early 1990s (Figure 1). Since the 1990s, water transparency levels have remained virtually constant, although there are relatively large inter-annual fluctuations in water transparency levels. A similar situation was observed for the Central Baltic Sea Region (HELCOM HOLAS II). In coastal and transitional waters (observations started later), however, there is no long-term decrease in water transparency, although there are brief periods of higher or lower water transparency.
In all assessed catchments, water transparency was below the criteria for good environmental status (Table 1). Overall, the differences between the values used to assess the environmental status of the period under assessment and the previous period are insignificant.
Table 1. Water transparency (Secchi depth, m) thresholds, assessment and previous period averages, trends, and assessment confidentiality.
|Water object||Threshold||Period||Changes||Confidentiality of the assessment|
|2007 – 2011||2012 – 2016|
|Open waters of the Baltic Sea||7,61||–||–||–|
|Coastal waters of the Baltic Sea||4,52||3,6||2,95||↘||Low|
|West coast of the Gulf of Riga||42||3,29||3,38||↗||Low|
|Open waters of the Gulf of Riga||5,01||3,65||3,32||↘||Low|
|Eastern coast of the Gulf of Riga||42||2,98||2,74||↘||Low|
3Based onone year of data