Results and Confidence

Current status of the Baltic Sea chlorophyll-a

In open sea areas, good status (concentrations of chlorophyll-a below the threshold value) has been achieved in the Kattegat. In the remaining 16 sub-basins the threshold value was failed, thus the status was not good. The open sea assessment units causing greatest concern regarding chlorophyll-a status (ER > 2.0) are the Gulf of Finland, Northern Baltic Proper, Western Gotland Basin, and Bornholm Basin. The Bothnian Sea, Åland Sea, Eastern Gotland Basin and Gransk Basin (ER values between 1.5 and 2.0), and the Bothnian Bay, The Quark, Gulf of Riga, Arkona Basin, Bay of Mecklenburg, Kiel Bay, Great Belt and The Sound (ER values between 1.0 and 1.5), fail their threshold values more narrowly and thus also receive not good status (Results figure 1 and Results Table 1). Trends during the current assessment period and the respective threshold values for open sea areas are shown in Results Figure 2, and longer term trends are discussed in detail below.


Results figure 1. Status of the Chlorophyll-a indicator, presented as eutrophication ratio (ER). ER shows the present concentration in relation to the threshold value, increasing along with increasing eutrophication. The threshold value is ER ≤ 1.00.


Results figure 2. Summer (June-September) chlorophyll-a concentration (black line, average for 2011-2016) and threshold value as agreed by HELCOM HOD 39/2012 (red broken line). Where no data was available an empty spaces is shown where the bar would be. It should be noted that the results for Bornholm Basin strongly depend on stations in the open-sea area of Pomeranian Bay, which is influenced by the Odra plume.

Results table 1. Threshold values, present concentration (as average 2011-2016), eutrophication ratio (ER) and status of Chlorophyll-a 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.

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In coastal waters, good status is found in some areas for Sweden, Denmark, Germany, Poland, Lithuania and Estonia. Certain coastal assessment units had very high (ER > 2.0) eutrophication ratios (Results figure 1 and Results Table 2).


Results table 2. Results for national coastal chlorophyll-a 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. *indicates data used are annual, all other data are for the summer season, - indicates only status provided and not raw result value.


Chlorophyll-a estimates measured on different platforms

The chlorophyll-a indicator is a multiparametric indicator and is based on combined data from in-situ measurements, FerryBox flow-through measurements (in open sea areas) and remote sensing data (Earth Observation satellite data in open sea areas – only for the year 2011). Data are combined where applicable (and agreed on by Contracting Parties) and where available to evaluate the indicator status, for example FerryBox data is only applied in some agreed open sea areas and remote sensing satellite data is only incorporated for 2011.




Results figure 3. Status of the chlorophyll-a -indicator shown based on each individual methodology: measured in-situ (top), FerryBox (middle) and remote sensing satellite (bottom – NOTE: only available for year 2011), presented as eutrophication ratio (ER). ER shows the present concentration in relation to the threshold value, increasing along with increasing eutrophication. The threshold value has been reached when ER ≤ 1.00. The overall chlorophyll-a status evaluation is based of combined annual information of the three parameters (as and where available).


Long-term trends

The long-term trends are provided as additional information and do not influence the status assessment for the current assessment period (2011-2016).

An increase in summer chlorophyll-a concentration was evident in most of the Baltic Sea sub-basins from the 1970/80s to the late 1990s/early 2000s. Only in some southwestern areas, the Kattegat and Arkona Sea, were these increases not observed. In the Bornholm Basin a decrease in summer chlorophyll-a concentration could even be observed during this period (Fleming-Lehtinen et al. 2008).

Chlorophyll-a concentration trends suggest that during the 1990-2016 period there has been little change. There are some exceptions, for example in the most southwestern parts of the Baltic Sea, where decreasing trends are observed and the Bornholm Basin where an increasing trend is seen (Results Figure 4). These decreasing trends corresponds well with decreases in nitrogen inputs and concentrations in the southwestern parts, where nitrogen is considered the most limiting nutrient for phytoplankton growth. In the central and eastern parts of the Baltic Sea, where summer chlorophyll-a concentration is mainly related to phosphorus concentrations the indicator shows no changes. A significant increasing (deteriorating) trend was detected only in the Bornholm Basin, which is attributed to influence from measurements at shallow stations in the Pomeranian Bay and outflow from the river Odra.


Results figure 4. Temporal development of chlorophyll-a (chl-a) concentrations in the open-sea assessment units from 1970s to 2016. Dashed lines show the five-year moving averages and error bars the standard deviation. Green lines denote the indicator threshold value. Significance of trends was assessed with a 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 colour.


Confidence of the indicator status evaluation

The confidence of the indicator status evaluation, based on the spatial and temporal coverage of data as well as the accuracy of the threshold value-setting protocol, was high for all assessment units, except the Kattegat, where it was moderate due to lower confidence in the applied target.


Results figure 5. Indicator confidence, determined by combining information on data availability and the accuracy of the threshold-setting protocol. Low indicator confidence calls for increase in monitoring or scientific re-evaluation of threshold values and targets.


The indicator confidence was estimated through confidence scoring of the threshold value (ET-Score) and the indicator data (ES-Score). The ET-Score was rated based on 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- and ES-Scores were combined. See Andersen et al. 2010 and Fleming-Lehtinen et al. 2015 for further details.