Indicator Assessment

Nutrients in transitional, coastal and marine waters

Indicator Assessment
Prod-ID: IND-14-en
  Also known as: CSI 021 , MAR 005
Published 09 Apr 2019 Last modified 18 Nov 2021
22 min read
This page was archived on 18 Nov 2021 with reason: No more updates will be done
  • Examples of successful implementation of nutrient management strategies can be found in, for example, the Baltic Sea and the North Sea regions, where decreasing trends are observed.
  • The highest nitrogen and phosphorus concentrations are generally observed in transitional and coastal waters of the marine (sub)regions, which reflects the influence of direct and diffuse inputs of nutrients in the upstream catchments.
  • In the southwestern Baltic Sea, decreasing nitrogen and phosphorus concentrations were observed between 1990 and 2017. These trends illustrate the effects of reductions in nutrient inputs. Phosphorus concentrations in this period increased in other parts of the Baltic Sea, because of phosphorus release from sediment under anoxic conditions (HELCOM 2018).
  • In the Greater North Sea, nitrogen and phosphorus concentrations decreased between 1990 and 2017 at a large number of stations in transitional and coastal waters, with the exception of total phosphorous concentrations in parts of the Kattegat. Again, these trends reflect the effect of reductions in nutrient inputs (OSPAR 2017).
  • In the Celtic Seas, some offshore stations showed decreasing nutrient concentrations between 1990 and 2017.
  • In the Black Sea, time series of phosphorus concentrations showed significant decreases in the northwest shelf area, while nitrogen concentrations showed a more variable pattern.


Trends in annual mean total nitrogen concentration in the European seas

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Trends in winter mean orthophosphate concentrations in European seas

Note: The map shows the trends per station in orthophosphate (PO4) concentrations in the upper 10 m of the water column, observed during winter of the years 1990-2017.

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Trends in winter mean orthophosphate concentration in European Seas

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Trends in annual mean total phosphorus concentration in European Seas

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  • Data on nutrient concentrations for the period 2013-2017 are available for the Baltic Sea, the Greater North Sea, the Celtic Seas and for a few stations in the Adriatic Sea and the Black Sea.
  • A relatively large number of time series covering nutrient concentrations during the 1990-2017 period 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)

In the Baltic Sea, the highest concentrations of nitrogen (dissolved organic nitrogen (DIN), total-N) and phosphorus (orthophosphate, total-P) were found in coastal waters in the southwestern Baltic Sea, the Gulf of Gdansk and Lithuania, and along the Finnish coast. 

With the exception of parts of the southwestern Baltic Sea, where time series covering concentrations of phosphorus (orthophosphate, total-P) showed a decrease between 1990 and 2017, significant increasing trends were observed for many stations in the Baltic Proper, the Gulf of Finland, the Bothnian Sea and the Bothnian Bay. These increasing trends were generally based on long time series (> 10 years).

Time series covering concentrations of winter mean DIN concentrations showed decreases in the southwestern Baltic Sea but also at stations in the Baltic Proper. Annual means of total-N concentrations increased, however, at some stations in the Baltic Proper, the Gulf of Finland, the Bothnian Sea and the Bothnian Bay.

Temporal trends are well-documented and recovery is underway on a Baltic-wide scale due to reductions of nutrient inputs over the past decades. However, additional reductions are required to meet the objectives of the BSAP, WFD and MSFD (EEA, 2019).

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).

The highest concentrations of nitrogen (DIN, total-N) and phosphorus (orthophosphate, total-P) were found in transitional and coastal waters along the continental coast of the Greater North Sea (including Skagerrak and Kattegat).

Nitrogen and phosphorus concentrations showed decreasing trends at a large proportion (30-50 %) of stations in the Greater North Sea, predominantly in transitional and coastal waters. Offshore stations, however, showed no significant trend. The only exception is the increase in total-P concentrations at stations in the Kattegat, which is probably related to the exchange of water with the Baltic Sea.

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

The highest concentrations of nitrogen (DIN) and phosphorus (orthophosphate) were found in transitional and coastal waters of the UK and Ireland. Time series covering DIN and orthophosphate concentrations showed significant decreases during the 1990-2017 period at a number of stations in offshore waters.

No data on concentrations in the 2013-2017 period or time series from 1990 to 2017 were available for the Bay of Biscay and the Iberian coast.

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).

For the Mediterranean Sea, only some data were available for the Adriatic Sea and the Ionian Sea. Concentrations were generally low.

Time series for nutrient concentrations were only available for a few stations along the Adriatic coast of Croatia and Montenegro, and in the Gulf of Trieste. Phosphorus concentrations did not show significant trends, with the exception of one station in the Gulf of Trieste where a decreasing trend was observed. For nitrogen, increasing trends in DIN were found in 3 out of 19 stations (16 %).

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, relatively high concentrations of nitrogen (total-N, DIN) and phosphorus (orthophosphate, total-P) were found at the northwest shelf, and along the coast of Bulgaria, Romania and Ukraine. For other parts of the Black Sea, very few data for the 2013-2017 period were available.

DIN concentrations decreased at some stations at the northwest shelf, and increased in coastal waters along the coast of Russia. In contrast, total-N concentrations increased at some stations (18 %) along the northwest shelf and decreased at other stations (18 %). Phosphorus concentrations (total-P) showed a decrease at 41 % of stations, all at the northwest shelf


You may find the mean concentrations for the European seas in the following links:

Map of winter mean dissolved inorganic nitrogen observed in 2013-2017

Map of annual mean total nitrogen concentrations observed in 2013-2017

Map of annual mean total phosphorus concentrations observed in 2013-2017

Map of winter mean orthophosphate concentrations observed in 2013-2017

Supporting information

Indicator definition

The indicator illustrates the following levels and trends in concentrations in the regional seas of Europe:
  • winter means of dissolved inorganic nitrogen (nitrate + nitrite + ammonium)
  • winter means of orthophosphate
  • annual means of total nitrogen
  • annual means of total phosphorus

The winter period is defined as follows:

  • January, February and March for stations east of longitude 15 degrees (Bornholm) in the Baltic Sea
  • January and February for all other stations.
The following marine (sub)regions 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 the Iberian coast

Macaronesian biogeographic region

Mediterranean Sea

Western Mediterranean Sea

Adriatic Sea

Ionian Sea and Central Mediterranean

Aegean-Levantine Sea

Black Sea



Concentrations are measured in micromoles per litre (µmol/l)


Policy context and targets

Context description

Measures to reduce the adverse effects of excess anthropogenic inputs of nutrients and to protect the marine environment are being taken as a result of various initiatives at global, European, regional (i.e. through Regional Sea Conventions and/or regional Ministerial Conferences) and national levels.

There are a number of EU Directives aimed at reducing the loads and impacts of nutrients, including the Nitrates Directive (91/676/EEC) aimed at the protection of waters against pollution caused by nitrates from agricultural sources; the Urban Waste Water Treatment Directive (91/271/EEC) aimed at reducing pollution from sewage treatment works and from certain industries; the Integrated Pollution Prevention and Control Directive (96/61/EEC) aimed at controlling and preventing pollution of water from industry; 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 seas by 2020 at the latest. The MSFD requires the adoption of national marine strategies based on 11 qualitative descriptors, one of which is Descriptor 5: Eutrophication.

Measures also arise from 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); the OSPAR Convention 1998; 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.


The most pertinent target with regard to concentrations of nutrients in water arises from the implementation of the Water Framework Directive, where one of the environmental objectives is to achieve good ecological status. Member States have defined water-type specific environmental standards to support the achievement of good ecological status.

As natural and background concentrations of nutrients vary between and within the subregional seas, nutrient targets or thresholds for achieving good ecological status have to be determined while taking into account local conditions.

Within the scope of the Marine Strategy Framework Directive, nutrient levels (nutrient concentrations in the water column for total and dissolved inorganic nitrogen and phosphorus) are the relevant primary criteria (D5C1) in marine waters under Descriptor 5: Human-induced eutrophication. The assessment of eutrophication in marine waters needs to combine information on nutrient levels as well as a range of ecologically relevant primary effects and secondary effects, taking into account relevant temporal scales. The nutrient targets and thresholds for achieving good environmental status are determined by Member States.

Other targets related to nutrient pollution are:

  • Baltic Sea Ministerial Declaration: 50 % reduction in nutrient discharges based on mid 1980s levels by 1995;
  • HELCOM/Baltic Sea Action Plan: for good environmental status to be achieved, maximum allowable annual nutrient inputs into the Baltic Sea have been defined for the Baltic Sea and its sub-basins;
  • OSPAR Eutrophication Strategy: combat eutrophication in the OSPAR maritime area in order to achieve and maintain, by 2020, a healthy marine environment where eutrophication does not occur;
  • OSPAR: reduce inputs of phosphorus and nitrogen into areas where these are likely to cause pollution, in the order of 50 % compared with 1985; 

      MAP/Mediterranean Sea: 50 % reduction in nutrient discharges from industrial sources.

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 assessment of the 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.

In previous assessments, the description of nitrogen was limited to only oxidised nitrogen (nitrite + nitrate). The concentration of oxidised nitrogen does not include ammonium (NH4+), which is another important inorganic nitrogen compound. This approach deviates from the common practice in most RSCs, and the WFD and MSFD where all dissolved inorganic nitrogen compounds (described as DIN = nitrite + nitrate + ammonium) are taken into consideration. In many cases, ammonium is a substantial fraction of winter average DIN and a description of oxidised nitrogen (nitrite + nitrate) alone may therefore lead to a biased assessment result. Consequently, in the assessment results published in 2015, the concentrations of DIN were included as well. In the current assessment, only the results for DIN are presented.

The use of winter means of dissolved inorganic nitrogen and phosphorus is common practice in the northern regional seas (Baltic Sea, North Sea) but may be less suitable to describe the nutrient levels in southern seas like the Mediterranean Sea and the Black Sea where the growing season is longer. To improve the assessment, the new data extraction was also used to collect data on annual means of total nitrogen and total phosphorus to complement the data on inorganic nutrients. In the current assessment, data on total nitrogen and total phosphorus are included as well to provide a better year-round picture of nutrient concentrations. 

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 and 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 from 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 the erroneous aggregation of data belonging to stations close to each other or the 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 January-March, depending on region, for dissolved inorganic nutrients; all year for total nutrients)
  • 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 colour scale.


Trend analysis

Trend analysis was carried out for each station in a region having at least data in 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. The accuracy on regional level is of course 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

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

There are also a number of uncertainties related to temporal and spatial coverage of the data. Currently, the winter period is defined as January and February for all stations except for stations east of longitude 15 degrees (Bornholm) in the Baltic Sea. However, this definition may be too broad to reflect the climatic differences across the European Sea regions. For example, for the Black Sea, it is suggested to also consider spring concentrations due to the nutrient enrichment of coastal waters as a result of increased riverine inputs (BSC, 2010), whereas annual means are deemed more suitable for the Mediterranean Sea. This issue is solved to some extent by also including annual means of total nitrogen and total phosphorus in the analysis.



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. 

Rationale uncertainty

Due to variations in freshwater discharge, the hydro-geographic variability of the coastal zone and internal cycling processes, trends in nutrient concentrations as such cannot be directly related to measures taken in nearby river basins. However, overall trends reflect the effects of measures to reduce nutrient pollution.



Data sources

Other info

DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CSI 021
  • MAR 005
Frequency of updates
Updates are scheduled every 2 years
EEA Contact Info


Geographic coverage

Temporal coverage