In open sea areas, good status (Secchi depth above defined threshold value, which reflects good conditions) for water clarity was achieved in the Kattegat and The Sound. For all open sea areas, the eutrophication ratio (ER) was below 1.5, with the highest value (1.38) being observed in Northern Baltic Proper (Results figure 1 and Results table1). In general the average water clarity has remained relatively constant during the assessment period (Results figure 2).
Results figure 1. Status of the water clarity indicator, presented as eutrophication ratio (ER). ER shows the present water clarity condition in relation to the threshold value, increasing along with increasing eutrophication. The threshold value for ER ≤ 1.00, with values below this threshold achieving good status.
Results figure 2. Summer (June-September) Secchi depth (black line; average for years 2011-2016) and threshold values (red broken line).
Results table 1. Threshold values, present concentration (as average 2011-2016), eutrophication ratio (ER) and status of Secchi depth in the open-sea basins. ER is a quantitative value for the level of eutrophication, calculated as the ratio between the threshold value and the present concentration – when ER > 1, good status has not been reached.
In coastal waters, good status was only achieved in some water bodies along the coast of Sweden and Finland, especially in coastal assessment units located in Bothnian Bay, The Quark and Bothnian Sea, and in certain coastal assessment units in Poland, Latvia and Estonia. For certain coastal areas, measured water clarity was far from reaching threshold values (ER > 2.0, Results figure 1 and Results table 2).
Results table 2. Results for national coastal Secchi depth (water clarity) 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 coloured red or green to denote if the status evaluation has been failed or achieved, respectively. Data are generally reported for the summer season, though some marked with * are annual. - indicates only status provided and not raw result value.
The long-term series for water clarity show a steadily deteriorating situation over several decades, most profoundly in the north-eastern sub-basins (Fleming-Lehtinen & Laamanen 2012). In more recent years, however, the decrease in water clarity has levelled off across most of the Baltic Sea (Results figure 3). Over the period from 1990-2016, water clarity has decreased significantly in four of the 17 sub-basins (Arkona Basin, Bornholm Basin, Western Gotland Basin and Northern Baltic Proper). In the Kattegat and the Great Belt, the water clarity has increased (Results figure 3).
Results figure 3. Temporal development of water clarity (measured as Secchi depth in summer) in open sea assessment units from 1970s to 2016. Dashed lines show the five-year moving averages and error bars the standard deviations. Green lines indicate the indicator threshold values. Significance of the trends was assessed with Mann-Kendall tests for the 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 indicator status evaluation is based on the spatial and temporal coverage of data as well as the accuracy of the target-setting protocol. This was not adequate in all sub-basins an dresulted in lowered confidence in the assessment. The Quark, Åland Sea, Gulf of Finland and Gulf of Riga assessments were determined to be of low confidence. High confidence was found in the Southwestern assessment units (Kattegat, Great Belt, The Sound, Kiel Bay, Bay of Mecklenburg, Arkona Basin, Bornholm Basin and the Eastern Gotland Basin). In the remaining open-sea basins, the indicator confidence was moderate.
Results figure 4. Indicator confidence, determined by combining information on data availability and the accuracy of the target-setting protocol. Low indicator confidence calls for increase in monitoring.
The indicator confidence was estimated through confidence scoring of the target (ET-Score) and the indicator data (ES-Score). The ET-Score was rated based on the uncertainty of the target 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- and ES-Scores were combined. See Andersen et al. (2010) and Fleming-Lehtinen et al. (2015) for further details.