The evaluation of total nitrogen in the open-sea areas is made as the average of total nitrogen concentrations in the upper (0-10 m) water layer throughout the year (annual). Out of the 13 sub-basins assessed, two were found to be in good status (below threshold value) during their assessment period 2011-2016: The Kattegat and Great Belt. The remaining assessed sub-basins were above the threshold values, with a number of sub-basins being near to the threshold (Results figures 1 and Results table 1). Some assessment open sea units could not be assessed as threshold values still need to be agreed upon at the HELCOM-wide level.
As some Contracting Parties favoured the use of seasonal (i.e. summer) instead of annual mean values when developing the indicator, a comparison was made between both methods. In most of the open sea basins, the annual and summer concentrations were at a relatively equal levels when measured annually or for summer months only (difference 0-1.5 µmol l-1). Annual concentrations were clearly higher than summer values in the Gulf of Riga (difference 6.2 µmol l-1), and slightly higher in the Great Belt and the Quark (difference 2-3 µmol l-1). As a consequence of this study, the annual average was used for the indicator in open-sea areas.
In general concentrations during the current assessment period for open sea assessment units remained relatively stable, though there were some fluctuations and an apparent decrease observed in the Gulf of Riga (Results figure 2).
Results figure 1. Detailed eutrophication assessment with the Eutrophication ratio (ER) of total nitrogen being split into 5 classes to show a more differentiated picture than the 2-class division used in the key message figures. ER is calculated as the ratio of the average concentration during assessment period and the threshold value (Fleming-Lehtinen et al. 2015). Please note that for some sub-basins threshold values are still under discussion and that coastal areas shown are from national reporting and can reflect annual or summer data (see Results table 2 fro details).
Some coastal areas in the south-western Baltic are highly eutrophied. In the remaining coastal areas, seasonal instead of annual averages were used for assessment. Based on mean summer concentrations (June-August / June-September), some areas along the coast of Sweden, Finland, Estonia, and Poland were classified as achieving good status (Results figure 1 and Results table 2).
Results figure 2. Average annual surface total nitrogen concentrations (black line; average for 2011-2016). The red dashed line represents the threshold values. For Kiel Bay, Bay of Mecklenburg, Arkona and Bornholm Basin threshold values are still under discussion. Therefore, no dashed red line is shown in these cases, and these basins occur as "not assessed" in the maps above.
Results table 1. Threshold values, present concentration (as average 2011-2016), eutrophication ratio (ER) and status of total nitrogen in the open-sea basins. ER is a quantitative value for the level of eutrophication, calculated as the ratio between the present concentration and the threshold value – when ER >1, threshold value has not been reached.
*NOTE: Danish measurements presented in this report are underestimated. This might affect content and conclusions in this report in regard to the status assessment and assessment of nutrient inputs to Danish waters.
Coastal area assessment results are based mostly on summer and annual average total nitrogen values, and in some cases (in Sweden) also winter averages are used (Results table 2). In Estonian coastal waters, the threshold value has been achieved in 7 waterbodies out of 16 and in Finland in 4 out of 14, based on summer estimates. In the coastal areas of Germany, the threshold value has been achieved in one waterbody out of 45, based on annual values. In Lithuania none of the 6 coastal areas achieved the threshold value. In Poland 4 coastal areas were assessed using annual values and in 3 waterbodies the threshold value was achieved. For the remaining 15 areas, summer averages were used, with 1 of them achieving the threshold value. In Sweden both summer and winter averages were used in assessing coastal areas. Concerning summer values, the threshold value was achieved in 10 areas out of 24, and using winter values in 9 areas out of 22. Concerning the percentage of total assessed area to achieve the threshold value (i.e. good status) per country (if the threshold value was achieved in some basins), over half of the area is in good status in Poland, Sweden (summer values) and Finland; 56%, 52% and 51%, respectively. According to winter values in Sweden, 38% of the assessed area is in good status. Estonia has 32% of the area in good status and Germany <1%.
Results table 2. Results for national coastal total nitrogen indicators by coastal WFD water type/water body. The table includes information on the assessment unit (CODE, defined in the HELCOM Monitoring and Assessment Strategy Annex 4), assessment period (start year and end year), average concentration during assessment period, threshold values, units, and Eutrophication Ratio (ER). The ER is colored red or green to denote if the status evaluation has been failed or achieved, respectively. Data and status evaluations were reported by Contracting Parties for different seasons, indicated by * = annual, ** = winter, and no symbol = summer. - indicates only status provided and not raw result value.
Temporal trends provide additional information on the total nutrients in the Baltic Sea that supports the interpretation of the indicator results. Data for this extended time period indicate that decreasing concentrations are found in several open sea assessment units, though an increasing trend is recorded in the Bornholm Basin (Results figure 4). It should be noted that the temporal trends do not affect the indicator result, which is a status assessment where a concentration is compared to a threshold value.
Results figure 4. Temporal development of total nitrogen (TN) concentrations in the open-sea assessment units in 1970-2016. Dashed lines show the five-year moving averages and error bars the standard deviation. Green lines denote the indicator threshold. Significance of trends was assessed with Mann-Kendall non-parametric tests for period from 1990-2016. Significant (p<0.05) improving trends are indicated with blue and deteriorating trends with orange data points.
The confidence in the presented total nitrogen status evaluation for the open sea areas (Results figure 5) is high in most of the assessed sub-basins. The data confidence was moderate in the Quark and low in the Åland Sea. It should be noted that the confidence is only based on data availability, not the threshold confidence since the latter was not available for the indicator calculation.
Results figure 5. Indicator data confidence, determined combining information on data availability for the indicator when using observations from all months of the year. Low indicator confidence calls for increase in monitoring.
The indicator confidence was estimated only for the indicator data (ES-Score) due to absence of ET-Score, which describes the uncertainty of the threshold value setting procedure. The ES-Score is based on the number as well as spatial and temporal coverage of the observations for the assessment period 2011-2016. To estimate the overall indicator confidence, the ET-score should be defined and ET- and ES-Scores combined. See Andersen et al. 2010 and Fleming-Lehtinen et al. 2015 for further details.
As the indicator period and method of calculation varies between open-sea and coastal areas, and thus the threshold- or assessment concentrations are not directly comparable between the open-sea and coast, nor between all coastal assessment units where nationally binding threshold values may have been set, only the confidence for the open-sea areas are shown in Results figure 5.