Indicator definition

 The indicator illustrates the geographical distribution and trends in mean summer surface concentrations of chlorophyll-a (micrograms per litre) in the regional seas of Europe.

The summer period is defined as:

• June to September for stations north of latitude 59 degrees in the Baltic Sea (Gulf of Bothnia and Gulf of Finland)
• May to September for all other stations

The following marine regions and subregions are covered, in line with the Marine Strategy Framework Directive (sub)regions:

Units

The concentration of chlorophyll-a is expressed as micrograms per litre (µg/l) in the uppermost 10 m of the water column during summer.

Policy context and targets

Context description

There are a number of EU Directives aimed at reducing the loads and impacts of nutrients. These include the Nitrates Directive (91/676/EEC), aimed at reducing nitrate pollution from agricultural sources; the Urban Waste Water Treatment Directive (91/271/EEC), aimed at reducing pollution from sewage treatment works and certain industries; and the Integrated Pollution Prevention and Control Directive (96/61/EEC), aimed at controlling and preventing pollution of water from industry. Other directives includes the Water Framework Directive (2000/60/EC), which requires the achievement of good ecological status or good ecological potential of transitional and coastal waters across the EU by 2015 and the Marine Strategy Framework Directive (2008/56/EC), which requires the achievement or maintenance of good environmental status in European sea basins by 2020 at the latest, through the adoption of national marine strategies based on 11 qualitative descriptors, one of which is Descriptor 5: Eutrophication.

Measures also arise from a number of other international initiatives and policies including the UN Global Programme of Action for the Protection of the Marine environment against Land-based Activities; the Mediterranean Action Plan (MAP) 1975; the Helsinki Convention 1992 (HELCOM) on the Protection of the Marine Environment of the Baltic Sea Area; the OSPAR Convention 1998 for the Protection of the Marine Environment of the North-East Atlantic; and the Black Sea Environmental Programme (BSEP). Reduction of nutrient sources is included as one of the targets under Sustainable Development Goal (SDG) 14: By 2025, prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution.

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Targets

Natural and background concentrations of chlorophyll vary between and within subregional seas, depending on physical and biological factors, such as natural background nutrient loads, water residence time, mixing, light conditions and biological processes.

The most pertinent target with regard to chlorophyll concentrations arises from the Water Framework Directive. Target chlorophyll concentrations/ranges that support the biological quality elements at a good status (high-good boundary and good-moderate boundary) have been defined in the Commission Decision (2018/229) based on the results of the intercalibration exercise carried out by the geographical intercalibration groups in the Baltic Sea, the North-East Atlantic and the Mediterranean Sea. These target chlorophyll concentrations/ranges are determined locally for different water types and water categories, including coastal and transitional water bodies.

Chlorophyll concentration in the water column is considered as one of the primary criteria (D5C2) of the direct effect of nutrient enrichment in marine waters under the Marine Strategy Framework Directive’s Good Environmental Status Descriptor 5: Human-Induced Eutrophication. The assessment of eutrophication in marine waters needs to take into account the assessment of coastal and transitional waters under the Water Framework Directive in a way that ensures comparability, taking into consideration the information and knowledge gathered and approaches developed in the framework of regional sea conventions. Chlorophyll targets or thresholds for achieving good environmental status in marine waters are defined by Member States.

Related policy documents

• 1600/2002/EC
  Decision no 1600/2002/ec of the European Parliament and of the Council of 22 july 2002 laying down the sixth community environment action programme
• Bathing Water Directive (76/160/EEC)
  Council Directive of 8 December 1975 concerning the Quality of Bathing Water (76/160/EEC)
• COM (2002) 539 final
  Communication from the Commission to the Council and theEuropean Parliament:Towards a strategy to protect and conserve the marine environment.COM (2002) 539 final
• Commission Decision (EU) 2017/848 of 17 May 2017
  Commission Decision (EU) 2017/848 of 17 May 2017 laying down criteria and methodological standards on good environmental status of marine waters and specifications and standardised methods for monitoring and assessment, and repealing Decision 2010/477/EU
• Commission Staff Working Paper concerning the relationship between the initial assessment of marine waters and the criteria for good environmetal status
  (SEC(2011) 1255 final)
• Convention for the Protection of the Marine Environment and the Coastal Region of the Mediterranean
  The Convention for the Protection of the Mediterranean Sea Against Pollution (the Barcelona Convention) was adopted on 16 February 1976 by the Conference of Plenipotentiaries of the Coastal States of the Mediterranean Region for the Protection of the Mediterranean Sea, held in Barcelona.
• Convention on the Protection of the Marine Environment of the Baltic sea area (HELCOM).
  1992 (entered into force on 17 January 2000)
• Council Directive (91/271/EEC) of 21 May 1991
  Council Directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC)
• Council Directive (91/676/EEC) 12 December 1991
  Council Directive of 12 December 1991 concerning the protection of waters against pollution caused by nitrates from agricultural sources (91/676/EEC).
• Council Directive 96/61/EC (IPPC)
  Council Directive 96/61/EC of 24 September 1996 concerning Integrated Pollution Prevention and Control (IPPC). Official Journal L 257.
• Marine Strategy Framework Directive 2008/56/EC
  Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive)
• Ministerial Declaration of the Third International Conference on the Protection of the North Sea
  Ministerial Declaration of the Third International Conference on the Protection of the North Sea, The Hague, 8 March 1990.  
• Water Framework Directive (WFD) 2000/60/EC
  Water Framework Directive (WFD) 2000/60/EC: Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy.

Methodology

Methodology for indicator calculation

The two main sources of data for this indicator are ICES and EMODNet datasets.

ICES data come from OSPAR and HELCOM contracting parties, and are therefore sub-samples of national data assembled for the purpose of providing comparable indicators of state and impact of transitional, coastal and marine waters (TCM-data) on a Europe-wide scale. In addition, data supplied by EMODnet were combined with the ICES data.

Concentrations are presented in maps showing means per station and year for the most recent 5-year period (2013-2017).

Consistent time series are used as the basis for the assessment of development over time. The trend analyses are based on time series from 1990 onwards. Stations with at least 5  years or more in the period since 1990 are selected for trend calculations.

The primary aggregation consists of:

• Identifying (clusters of) stations and assigning them to marine (sub)regions;
• Creating statistical estimates for each combination of station and year.

The procedures of data extraction, data selection and aggregation, trend analysis and plotting of results are carried out in R.

Geographical classification: Sea region, coastal/offshore, station

All geographical positions defined in the data are assigned to marine (sub)region by coordinates.

Stations are defined geographically by position, given as longitude and latitude in decimal degrees, but the reported data do not contain reliable and consistent station identifiers. The reported coordinates for what is intended to be the same station may vary between visits, because the exact achieved position is recorded, not the target position. Identifying stations by strict position may fragment time series too much as the position of the same station may vary slightly over time.

In order to improve the aggregation into time series, data are aggregated into squares with sides of approximately 1.375 km for coastal stations within 20 km of the coastline and approximately 5.5 km for open water stations more than 20 km away from the coastline. The procedure however does not totally prevent erroneous aggregation of data belonging to stations close to each other or erroneous breakup of time series into fragments due to small shifts in position, but reduces the problem considerably.

Statistical aggregation per station and year

The aggregation includes:

• selecting season (months May/June-September, depending on region);
• selecting sample depth (0-10 m depth);
• select data per station and year;
• calculate the mean of the selected data per station and year.

Classification

No classification was applied. Maps are created per marine (sub)region using a continuous color scale.

Trend analysis

Trend analysis was carried out for each station in a region with data in at least  the last 6-year period (2013 or later), and 5  years or more in the period since 1990. Trend detection for each time series was done with the non-parametric Mann-Kendall trend test.

The Mann-Kendall method is a non-parametric test suggested by Mann (1945) and has been extensively used for environmental time series (Helsel and Hirsch, 2002; Hipel and McLeod, 2005). Mann-Kendall is a test for monotonic trend in a time series y(x), which in this analysis is chlorophyll concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau = S/D where S = sum {i<j} (sign(x[j]-x[i])*sign(y[j]-y[i])) and D = n(n-1)/2. S is called the score and D, the denominator, is the maximum possible value of S. The p-value of tau is computed by an algorithm given by Best and Gipps (1974). The tests reported here are two-sided (testing for both increasing and decreasing trends). Data series with a p-value < 0.05 are reported as significantly increasing or decreasing. The test analyses only the direction and significance of the change, not the size of the change.

The Mann-Kendall test is a robust and accepted approach. Because of the multiple trend analyses, approximately 5 % of the conducted tests will turn out significant (identify a trend) if there is no trend. Only data from the Baltic Sea area, the eastern Greater North Sea, Italian coastal waters and a number of Croatian and French stations in the Mediterranean allow the analysis of trends. Accuracy regional level is largely influenced by the number of stations for which data are available.

Methodology for gap filling

Gap filling in the time series is not necessary for the trend analysis that uses the Mann-Kendall test.

Methodology references

• HELCOM (2013) Approaches and methods for eutrophication target setting in the Baltic Sea region. BSEP133
• OSPAR (2013) Common Procedure for the Identification of  the Eutrophication Status of the OSPAR Maritime Area (Reference number: 2013-8).
• Carletti A, Heiskanen AS. 2009. Water Framework Directive intercalibration technical report. Part 3: Coastal and Transitional waters. JRC-IES EUR 23838 EN/3
• Ferreira JG, Andersen JH, Borja A, Bricker SB, Camp J, Cardoso da Silva M, Garcés E, Heiskanen AS, Humborg C, Ignatiades L, Lancelot C, Menesguen A, Tett P, Hoepffner N, Claussen U, 2011. Overview of eutrophication indicators to assess environmental status within the European Marine Strategy Framework Directive. Estuarine Coastal and Shelf Science 93, 117-131.
• Helsel, D.R., Hirsch, R.M., 2002. Statistical methods in water resources. Techniques of Water Resources Investigations Book 4, chapter A3. U.S. Geological Survey. 522 pp.
• Hipel K.W. & McLeod A.I., 2005. Time Series Modelling of Water Resources and Environmental Systems Electronic reprint of our book originally published in 1994.
• Mann H.B., 1945. Nonparametric tests against trend Econometrica 13, 245-259

Uncertainties

Methodology uncertainty

In the current application, two growing seasons are distinguished, one for the northern part of the Baltic Sea (June-September) and one for the southern part of the Baltic Sea, the North Sea, the North-East Atlantic, the Mediterranean Sea and the Black Sea (May-September). It is questionable whether using one growing season for all waters that range geographically from the Mediterranean and the Black Sea to the North Sea and the Baltic Sea is appropriate. Moreover, currently only surface concentrations are considered. However, in the Black Sea, not only do the chlorophyll concentrations show peaks in late winter, late spring and autumn, these peaks not only occur at the surface but also in the subsurface layer (BS SoE, 2008).

The Mann-Kendall test for the detection of trends used for statistical analysis of the data is a robust and accepted approach. Because of the multiple trend analyses, approximately 5 % of the conducted tests will turn out significant (decrease or increase) if in is no trend (type I error).

Data sets uncertainty

Data for this assessment are still scarce considering the large spatial and temporal variations inherent in European transitional, coastal and marine waters. Data to describe concentrations for individual stations have a higher availability than data needed for trend analysis, which requires time series of several years for stations. For the latter, there is good coverage for the Baltic Sea and the continental coast of the North Sea, and more limited coverage for (predominantly coastal) stations in the Celtic Sea, the Bay of Biscay and Iberian coast, and the Mediterranean Sea and Black Sea. 

For the assessment of chlorophyll-a concentrations, different analytical methods are generally used. Although these different analytical methods generally give comparable results with reasonable to good correlations between methods, simple fluorometric and photometric methods are less accurate and therefore may be a source of uncertainty.

Low sampling frequencies increase the risk of not detecting phytoplankton blooms and differences in sampling frequency between stations are an additional source of uncertainty.

Rationale uncertainty

Due to variations in freshwater run-off, light climate, hydro-geographic variability of the coastal zone and internal cycling processes, trends in chlorophyll-a concentrations cannot be directly related to measures taken to reduce eutrophication, but must be evaluated in a broader context.