Dissolved inorganic phosphorus
Figure 1. Long-term average winter concentrations of dissolved inorganic phosphorus (DIP) in the Gulf of Riga and the Baltic Sea. A – Baltic coastal waters, 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 waters of the Baltic Sea (data source – SMHI).
Observational results of dissolved inorganic phosphorus concentrations in the open and transitional waters of the Gulf of Riga during the winter season are intermittently available from 1974 onwards. In the coastal waters of the Gulf of Riga, however, observations are available from 1990, in the open waters of the Baltic Sea from 1964 and in the coastal waters from 1984. Similarly to nitrogen, the reconstructed phosphorus concentrations for the Gulf of Riga and the Baltic Sea show a significant increase in concentrations starting in the 1950s. In contrast to nitrogen, phosphorus concentrations in both the Gulf of Riga and the Baltic Sea have continued to increase after 1989 (Figure 1). At the same time, it should be noted that long-term increases and decreases in concentrations in coastal water bodies cannot be assessed because observations are patchy. Phosphorus concentrations, like those of nitrogen, also show inter-annual variability, but not as pronounced as for nitrogen. This is probably because phosphorus has a much longer residence time in sea basins than nitrogen, e.g. 38 years in the Gulf of Riga (5.4 years for nitrogen). Consequently, nitrogen in the Gulf of Riga responds much more rapidly than phosphorus to changes in river discharge.
The winter season DIP concentrations during the assessment period as well as the previous period are summarised in Table 1. Unfortunately, the frequency of surveys during the reporting period varied from twice per period to none (Figure 1). For the open waters of the Gulf of Riga it was possible to model the missing values, but not for the other basins. The level of confidentiality of the assessment is therefore low. In general, only the Baltic Sea coast shows a decrease (improvement) in concentrations, while the Gulf of Riga basins show a deterioration. At the same time, it should be noted that in no basin, including the Baltic Sea coast, do the concentrations of DIP comply with the threshold value for good environmental status.
Given the very limited amount of information available for the evaluation, the level of confidentiality of the evaluation is assessed as low.
Table 1. Winter seasonal dissolved inorganic phosphorus (DIP) (µmol L-1) threshold values, assessment and previous period mean values, trends, and assessment confidentiality assessment.
| Water object | Threshold | Period | Trends | Confidentiality of the assessment | |
| 2007 – 2011 | 2012 – 2016 | ||||
| Open waters of the Baltic Sea | 0,291 | 0,65 | 0,66 | <-> | Low |
| Coastal waters of the Baltic Sea | 0,62 | 0,83 | 0,643 | ↘ | Low |
| West coast of the Gulf of Riga | 0,72 | 0,9 | 1,143 | ↗ | Low |
| Open waters of the Gulf of Riga | 0,411 | 0,9 | 1,03 | ↗ | Low |
| Eastern coast of the Gulf of Riga | 0,72 | 1,04 | – | – | Low |
| Transitional waters | 0,92 | 1,11 | 1,26 | ↗ | Low |
1HOD39-2012
2Directive2000/60/EC
3Based on one year of data