Chlorophyll a
Figure 1. Summer mean values of chlorophyll a 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, F – open Baltic Sea
For the assessment of the environmental status, the results of summer season measurements of chlorophyll a from 1996 onwards are used. During this period, observations have been made 1-3 times per season, except in some years when no observations have been made in one of the sea basins.
Along the Baltic Sea coast, chlorophyll a concentrations have fluctuated over the whole observation period over a wide range, from1.15 to 6.37 mg/m3 on average (Figure 1), showing several concentration peaks followed by troughs. Overall, no consistent trend of increasing or decreasing concentrations can be identified over the period considered. In the open waters of the Baltic Sea, however, the year-to-year variation in concentrations is much smaller than in the coastal waters(1.88-3.16 mg/m3), except in 2008, when a concentration of 5.73 mg/m3 was detected. In the open waters of the Baltic Sea, there is also no overall trend in concentrations.
Similar to the Baltic Sea coast, the Gulf of Riga shows significant interannual variability in chlorophyll a concentrations (Figure 1). In contrast to the Baltic Sea, the Gulf of Riga shows a trend of increasing concentrations from the 1990s to the 2000s in practically all water bodies, followed by decreasing concentrations. At the same time, it should be noted that the eastern shore of the Gulf of Riga in 2010 had an unusually high chlorophyll a concentration(22.06 mg/m2), which was almost twice the maximum concentration observed in previous years(12.07 mg/m3, 2005).
Summer concentrations of chlorophyll a during the assessment period are reflected in Table 1 . Overall, the environmental status does not meet the criteria for good environmental status in any of the water bodies examined. Chlorophyll a values were extracted in two periods – from 2007 to 2011 and from 2012 to 2016. Changes in environmental status varied between basins, with an upward trend in the Baltic Sea coast and the open part of the Gulf of Riga. In the open Baltic Sea, the Gulf of Riga and transitional waters, summer chlorophyll a concentrations tend to decrease. It should be noted here that the confidentiality of the assessment of both the state of the environment and its changes between periods is low, as in both periods the assessment is based on data from the same seasonal month, August. This is particularly true for the eastern coast of the Gulf of Riga, where one year (2010) of extremely high concentrations resulted in the highest inter-annual variability.
Table 1. Chlorophyll a thresholds, assessment and historical averages, trends, and assessment confidentiality assessment.
| Water object | Threshold | Period | Trends | Confidentiality of the assessment | |
| 2007 – 2011 | 2012 – 2016 | ||||
| Open waters of the Baltic Sea | 1,91 | 3,54 | 2,43 | ↘ | low |
| Coastal waters of the Baltic Sea | 1,82 | 4,29 | 4,79 | ↗ | low |
| West coast of the Gulf of Riga | 2,72 | 4 ,21 | 3,55 | ↘ | low |
| Open waters of the Gulf of Riga | 2,71 | 3,96 | 4,2 | ↗ | low |
| Eastern coast of the Gulf of Riga | 2,72 | 10,43 | 4,89 | ↘ | low |
| Transitional waters | 3,02 | 5,72 | 5,26 | ↘ | low |
1HOD39-2012
2Directive2000/60/EC