Dissolved inorganic nitrogen
Figure 1. Long-term average winter concentrations of dissolved inorganic nitrogen (DIN) 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).
Winter observations of dissolved inorganic nitrogen in the open and transitional waters of the Gulf of Riga are intermittently available since1974. In the coastal waters of the Gulf of Riga, however, observations are available since 1990, in the open waters of the Baltic Sea since 1971 and in coastal waters since 1984. The reconstructed nitrogen concentrations for the Gulf of Riga and the Baltic Sea show a marked increase starting in the 1950s and peaking in 1989 in the Gulf of Riga and in 1991 in the Baltic Sea (Figure 1). Decreasing concentrations are observed in the following years. 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. All water basins show a high interannual variability in the calculated mean values, which is most pronounced in transitional waters.
The winter season DIN 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. In addition, only two stations represent the open part of the Baltic Sea in this assessment. The level of confidentiality of the assessment is therefore low. In general, all the basins considered, except the open Baltic Sea, show a decrease (improvement) in concentrations, but only one basin could be below the threshold value. However, it should be noted that in this case the assessment is based on the results of only one year (2016) and therefore has a low level of confidentiality. In addition, it should be noted that all the basins considered, except the open waters of the Baltic Sea, are relatively shallow (low water volume) and therefore subject to high interannual variability due to different river flow regimes. This is particularly true for transitional waters, where the most rapid improvement is observed. Therefore, the use of individual years of the reporting period in the assessment may give a misleading impression of significant negative or positive changes compared to the previous period.
Table 1. Winter seasonal dissolved inorganic nitrogen (DIN) (µmol L-1) limit values, mean values of the assessment and the previous period, trends, and assessment confidentiality assessment.
|Water object||Threshold||Period||Trends||Confidentiality of the assessment|
|2007 – 2011||2012 – 2016|
|Open waters of the Baltic Sea||2,51||3,32||3,34||<->||Low|
|Coastal waters of the Baltic Sea||82||19,08||12,083||↘||Low|
|West coast of the Gulf of Riga||112||11,26||9,013||↘||Low|
|Open waters of the Gulf of Riga||5,21||13,82||12,25||↘||Low|
|Eastern coast of the Gulf of Riga||112||26,43||–||–||Low|
3Based onone year of data