Indicator Assessment

Chlorophyll in transitional, coastal and marine waters

Indicator Assessment
Prod-ID: IND-18-en
  Also known as: CSI 023 , MAR 006
Published 11 Apr 2019 Last modified 18 Nov 2021
17 min read
This page was archived on 18 Nov 2021 with reason: No more updates will be done
  • The trends in chlorophyll concentrations show improvements in the eutrophication status in some of Europe’s seas, due to the successful implementation of nutrient management strategies.
  • The highest chlorophyll concentrations are generally observed in transitional and coastal waters of the marine (sub)regions, in response to elevated nutrient concentrations in those waters.
  • Decreasing chlorophyll concentrations were observed in the southwestern Baltic Sea and along the continental coast of the Greater North Sea (including Kattegat), showing the effects of measures to reduce nutrient inputs (OSPAR 2017, HELCOM 2018).
  • For the other marine (sub)regions, only a few time series were available. In general, those time series did not show significant trends.

Trends in summer chlorophyll concentrations in European Seas, 1990-2017

Note: The map shows trends per station in chlorophyll concentrations per station in the upper 10 m of the water column, as observed during summer in the years 1990-2017. Red: significant increase; Green: significant decrease; Grey: no significant trend. Small symbols: ≤10 years of data; Large symbols: >10 years of data.

Data source:

Mean chlorophyll-a (Chla) concentrations in European seas, 2013-2017

Note: The map shows chlorophyll concentrations in the upper 10 m of the water column, observed in the summers of the years 2013-2017.

Data source:
  • Data on chlorophyll concentrations between 2013 and 2017 are available for the Baltic Sea, the Greater North Sea and for parts of the Celtic Seas, the Bay of Biscay, the Mediterranean Sea and the Black Sea.
  • A relatively large number of time series covering chlorophyll concentrations in the period 1990-2017 was available for the Baltic Sea, the North Sea and the Celtic Seas.

Baltic Sea

Eutrophication is still a large-scale problem in the Baltic Sea, a fact acknowledged by most, if not all, of the bordering countries (EEA, 2019)

The highest measured summer chlorophyll-a concentrations in the 2013-2017 period were found in coastal and transitional waters along the German coast and in the Gulf of Gdansk. Low concentrations were predominantly observed in the open waters of the Baltic Sea (Figure 3).

Most of the stations (86 %) did not show a significant change in chlorophyll concentration in the period 1990-2017. Overall, statistically significant decreasing trends were evident in 9 % of the Baltic Sea stations (Figure 2), which were in the southwestern part of the Baltic Sea. Chlorophyll concentrations increased at 5 % of the stations, mainly in coastal waters of the Bothnian Bay and the Bothnian Sea, and at some stations in the Baltic Proper and the Gulf of Finland (Figure 1).

Greater North Sea

Eutrophication is a problem in parts of the Northeast Atlantic. River discharges are the main sources of elevated nutrient levels caused by human activities (EEA, 2019).

In the North Sea, the highest chlorophyll concentrations were found in coastal and transitional waters along the continental coast from Belgium to Denmark.

Decreasing trends were found in transitional, coastal and offshore waters of the Kattegat and at some stations along the continental North Sea coast.

Atlantic waters: Celtic Seas, Bay of Biscay and the Iberian coast

In the Celtic Seas, only data on chlorophyll concentrations were available for transitional and coastal waters of Ireland. The concentrations generally show a decreasing gradient from inshore to offshore. In 2 % of the cases, the time series showed an increasing trend, while in all other cases there was no significant trend.

In the Bay of Biscay and Iberian coast, oxygen concentrations along the French coast were low in general (<10 μg/l). There were few time series available, none of which showed a significant trend.

Mediterranean Sea

Mediterranean Sea is probably the regional seas with fewest eutrophication problem areas. This is partly related to the fact that the offshore parts of the Mediterranenan Sea are characterized by very low nutrient concentrations (EEA, 2019).

Data for the western Mediterranean Sea mainly cover offshore waters where concentrations are low. Data for the Adriatic Sea and the Ionian Sea show very low concentrations (<1 μg/l).

There were few time series available. Only 1 out of 12 available series showed a significant increasing trend.

Black Sea

More reductions in nutrient inputs are required to restore the Black Sea to being unaffected by eutrophication (EEA, 2019).

In the Black Sea, the highest chlorophyll concentrations were found at the northwest shelf near the mouth of the river Danube.

Only three time series were available. None of those showed a significant trend.


  μg/l <5 5-10 10-20 20-40 >40 Total no of observations
Baltic Sea 1293 343 76 65 83 1860
NE Atlantic 1424 245 75 40 9 1793
  μg/l <0.5 0.5-1.0 1.0-2.0 2.0-4.0 >4.0 Total no of observations
Mediterranean Sea 1971 45 14 2 0 2032
  μg/l <4 4-8 8-16 16-32 >32 Total no of observations
Black Sea 182 31 11 5 2 231


You may find the mean concentration for the European seas in the following link:

Map of summer chlorophyll-a concentrations observed in 2013-2017.

Supporting information

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:




Baltic Sea


North-East Atlantic Ocean

Greater North Sea

Celtic Seas

Bay of Biscay and Iberian coast

Macaronesian biogeographic region

Mediterranean Sea

Western Mediterranean Sea

Adriatic Sea

Ionian Sea and Central Mediterranean

Aegean-Levantine Sea

Black Sea




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.



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



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.



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



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.

Data sources

Other info

DPSIR: Pressure
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CSI 023
  • MAR 006
Frequency of updates
Updates are scheduled every 2 years
EEA Contact Info
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