Indicator Assessment

Nutrients in transitional, coastal and marine waters

Indicator Assessment
Prod-ID: IND-14-en
  Also known as: CSI 021
Published 05 Jul 2011 Last modified 11 May 2021
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  • Decreasing nutrient concentrations were found in the North Sea and in the Baltic Sea. In the Mediterranean and Black Sea, the lack of temporally and spatially comprehensive time series does not allow an overall assessment.
  • In 2008, the highest concentrations of oxidized nitrogen were found in the Gulf of Riga, and in Lithuanian, Swedish, German, Belgian, and Scottish coastal waters. Between 1985 and 2008, 12% of all the stations in the European seas reported to the EEA showed decreasing trends of oxidized nitrogen concentrations. These trends were more evident in the open Baltic Sea and in the Dutch and German coastal waters in the North Sea.
  • In 2008, the highest orthophosphate concentrations were found at Finnish coastal stations in the Gulf of Finland, the Gulf of Riga, German, Belgian, French, and Scottish coastal waters. Between 1985 and 2008, 15% of all the stations in the European seas reported to the EEA showed a decrease in orthophosphate concentrations, mainly because of improved waste water treatment. This decrease was most evident in Norwegian, Lithuanian, Danish, Belgian and Dutch coastal water stations, and in the open waters of the North and Baltic Seas.

Winter oxidized nitrogen (NO2 + NO3) concentrations in European seas in 2008

Note: The map shows the winter oxidized nitrogen concentrations in the European coastal and open waters in 2008. The low category refers to values within the lowest 20th percentile and the high category refers to values within the upper 20th percentile of concentrations in a regional sea

Data source:

Change in winter orthophosphate concentrations in coastal and open waters of the North East Atlantic, Baltic, Mediterranean and North Seas

Note: The figure shows change in winter orthophosphate concentrations in coastal and open waters of the North East Atlantic, Baltic, Mediterranean and North Seas, 1985-2008

Data source:

Winter orthophosphate concentrations in European seas in 2008

Note: The map shows the winter orthophosphate concentrations in the European coastal and open waters in 2008. The low category refers to values within the lowest 20th percentile and the high category refers to values within the upper 20th percentile of concentrations in a regional sea

Data source:

Baltic Sea

In 2008, the highest oxidized nitrogen concentrations (> 210 µg/l) were measured in the Gulf of Riga and in Lithuanian and Latvian coastal waters (Figure 1). Low concentrations (< 46 µg/l) were commonly observed in the southern parts of the Baltic Sea. High oxidised nitrogen concentrations in the Bothnian Bay were not reflected in chlorophyll values (see the Core Set Indicator 23), because primary production is limited by naturally low phosphate concentrations (< 5 µg/l, Figure 2). Orthophosphate concentrations were highest (> 25 µg/l) at Finnish coastal stations in the Gulf of Finland and in the Gulf of Riga (Figure 2). These high concentrations are strongly influenced by internal phosphorous loading, i.e. phosphorous released from bottom sediments during anoxic conditions.

In the Baltic Sea between 1985 and 2008 oxidised nitrogen concentrations decreased in 21% of the monitoring stations, and increased in 4% of the stations (Figure 3). Decreasing trends were detected especially in the open waters of the Baltic Proper and in addition in the Danish, Finnish, German, and Swedish coastal waters. This can be explained by decreased loading. Orthophosphate concentrations decreased in 13% of the stations and increased in 9% of the stations. Increasing orthophosphate trends were mainly detected in Finnish coastal waters, whereas decreasing trends in the open Baltic Sea, and Danish, German and Lithuanian coastal waters.

North Sea

In the North Sea, the highest winter oxidized nitrogen concentrations (> 350 µg/l) and orthophosphate concentrations (>100 µg/l) in 2008 were observed at French and Belgian coastal waters (Figures 1 & 2).

Long term time series indicate that oxidized nitrogen is decreasing at 8% of the stations and remain unchanged at 92% of the stations (Figure 2). Orthophosphate concentrations were decreasing at 28% of the North Sea stations. This can be linked especially to the Belgian and Dutch estuarine and coastal waters: At 57% of the Belgian and 75% of the Dutch monitoring stations concentrations were decreasing. This positive development can be attributed to improved waste water treatment where most North Sea countries have achieved reduction of phosphorus loading by 50% in the period 1985-2005 (OSPAR 2008). Also the open sea orthophosphate concentrations in the North Sea showed a decreasing trend at 19% of the stations.

NE Atlantic

Comprehensive assessment of winter nutrient concentrations in the NE Atlantic is not possible, because data for the year 2008 only includes observations from the French, Scottish and Northern Irish coastal waters (Figures 1 & 2). The highest oxidised nitrogen concentrations (> 100 µg/l) and orthophosphate concentrations (> 22 µg/l) were detected in the Belgian and German coastal waters.

In the NE Atlantic the time series of oxidised nitrogen concentration showed no remarkable changes. Time series of orthophosphate concentration revealed increasing trends at six Irish monitoring stations. These stations were located in the coastal waters near Dublin. Orthophosphate concentrations were decreasing at three Irish monitoring stations.


Mediterranean Sea

Nutrient concentrations in the open Mediterranean Sea are extremely low (UNEP 2007) and eutrophication is seen only in some coastal waters. Data for the year 2008 consisted only of Croatian and Cyprian coastal observations (Figures 1 & 2). In 2008 the highest winter oxidized nitrogen concentrations (> 1000 µg/l) in the Mediterranean were observed along the coasts of Malta. The highest orthophosphate concentrations (> 30 µg/l) were found along the western Italian coast. On the North East coast of Italy the high concentrations can be attributed to inputs from the Po River.

There were only few notable changes in nutrient concentrations: At two of the Croatian monitoring stations oxidised nitrogen concentrations were decreasing, whereas orthophosphate concentration was decreasing at one station (Figures 3 & 4). Those stations did not show any statistically significant trend.


Black Sea

No data of winter nutrient concentrations in 2008 has been submitted to the EEA

Supporting information

Indicator definition

The indicator shows 1) annual winter concentrations (micromol/l); 2) classification of concentration levels (i.e. low, moderate, high) and 3) trends in winter oxidised nitrogen (nitrate + nitrite) and phosphate concentration (micromol/l)in the regional seas of Europe. 

Levels and trends of winter concentrations of dissolved inorganic nutrients are used for this indicator, as it is assumed that winter concentrations are not significantly reduced due to uptake by primary producers.

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 used regional and subregional seas of Europe are in line with the geographical regions and sub-regions specified in the Marine Strategy Framework Directive (MSFD).  Other European Seas (Icelandic Sea, The Norwegian Sea, the Barents Sea and the White Sea) are not covered in this indicator due to current lack of data. 


Concentrations in micromol/l


Policy context and targets

Context description

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

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 the year 2020 at the latest, through the adoption of national marine strategies based on 11 qualitative descriptors.

Additional measures 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) 1993.


The most pertinent EU 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 regional seas, and between types of coastal water bodies, 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 and nutrient ratios for nitrogen, phosphorus and silica, where appropriate), are the relevant criteria and indicators 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 expected to be defined by 2013. 

Other relevant regional 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, the maximum allowable annual nutrient pollution inputs into the Baltic Sea should be 21,000 tonnes of phosphorus and about 600,000 tonnes of nitrogen. Sub-basin and country-wise nutrient reduction targets are also set.
  • OSPAR Eutrophication Strategy: combat eutrophication in the OSPAR maritime area in order to achieve and maintain, by 2010, 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 to 1985 
  • MAP/Mediterranean Sea: 50 % reduction in nutrient discharges from industrial sources

Related policy documents



Methodology for indicator calculation

The data used in this indicator is part of the WISE - State of the Environment (SoE) data, available in Waterbase - TCM (Transitional, Coastal and Marine) waters. Waterbase is the generic name given to EEA´s database on status, quality and quantity of Europe´s water resources. Waterbase – TCM waters contains data collected both from EEA member countries (i.e. belonging to the EIONET) and from the Regional Seas Conventions through the WISE-SoE TCM data collection process (WISE-SoE was formerly known as Eionet-Water and Eurowaternet). The resulting WISE SoE TCM dataset is therefore made of sub-samples of national data assembled for the purpose of providing comparable indicators of state and impact of transitional, coastal and marine waters () on a Europe-wide scale.

Levels and trends of winter concentrations of dissolved inorganic nutrients are used for this indicator, as it is assumed that winter concentrations are not significantly reduced due to uptake by primary producers.

Annual winter concentrations of Nitrogen and Phosphate, and classification of concentration levels

The primary aggregation consists of:

  1. Identifying stations and assigning them to countries and sea regions (in line with the geographical regions specified in the MSFD) 
  2. Creating statistical estimates for each combination of station and year and deriving the average annual winter concentration of N and P
  3. Classifying nutrient concentration levels for each station (i.e. according to low, moderate, and high boundaries)

1. Identifying stations and assigning them to countries and sea regions

All geographical positions defined in the data are assigned to a sea region by coordinates. The used regional and subregional seas of Europe are in line with the geographical regions and sub-regions specified in the Marine Strategy Framework Directive (MSFD) (see below). Other European Seas (Icelandic Sea, The Norwegian Sea, the Barents Sea and the White Sea) are not covered by this indicator due to current lack of data. Also, because of the limited amount of data, only the following (sub)regions are distinguished in the maps: Baltic Sea, Celtic Seas, Greater North Sea, Bay of Biscay and Iberian coast, Mediterranean Sea, Black Sea.

Regional SeaSubregional Sea
Baltic Sea None

North East Atlantic Ocean

Greater North Sea

Celtic Seas

Bay of Biscay and the Iberian coast

Macaronesian region

Mediterranean Sea

Western Mediterranean Sea

Adriatic Sea

Ionian Sea and Central Mediterranean

Aegan - Levantine Sea

Black Sea none

The stations are then further classified as coastal or off-shore (>20 km from coast) by checking them against the coastal contour. Off-shore stations – open seas -  are distinguished per sub-regional sea, whereas coastal stations are further attributed to country. These classifications are done in ArcView. Smaller regions within the regional and sub-regional seas described above are used in the aggregation process of different determinants. 

EIONET stations

WISE SoE TCM data reported directly from countries are assigned to station identifiers (i.e. EIONET stations) that are listed with coordinates. For these data, which are mostly along the coast of the reporting country, stations are kept as defined.

Regional Seas Conventions data 

For the data reported through the Regional Sea Conventions (and assembled by ICES), there are no consistent station identifiers available in the reported data, only geographical positions (latitude/longitude). 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 station on exact position may therefore fragment time series too much.

Furthermore, duplicates between Eionet and RSC data may occur for coastal stations. A visual inspection of coastal data (< 20 km from shoreline) is therefore needed to eliminate these duplicates. For the open waters (>20 km from shoreline) coordinates are rounded to 2 decimals, and this is used to create stations (i.e. for the purpose of establishing time series) with station names derived from rounded coordinates. As coordinates for the stations are used averages over visits to the station, rather than the rounded coordinates. This ensures that in cases where most observations are in a tight cluster within the rounding area, a position within the cluster is used. The open water observations are not assigned to countries, but listed as belonging to 'Open waters' in the Country column, without reference to country.

For the coastal ICES stations, there may be overlap with Eionet stations, and for the stations close to the coast, rounding coordinates to 2. decimal may be too much (about 500 m to 1 km). However, in this update, the rounding is done also for coastal stations, but the grouping of observations to rounded coordinates is done only within observations from each country separately, and the originator country is listed. Note that these stations are not necessarily close to the coast of the originator country.

2. Annual concentration of N and P per station

The statistical aggregation for calculating annual concentrations for Nitrogen (i.e. Nitrates, Nitrites and Total Oxidised Nitrogen) and Phosphate (i.e. Orthophosphates) is done in two- or three-stage query sequences, which include:

  • Selecting season (month) and depth
  • If needed, building a cross-table with determinants in columns, and water samples in rows, and deriving composite determinants from that. 
  • Aggregating over depth for each combination of station and date.
  • Aggregating over dates within each combination of station and year. 

The basic data consists of two tables:

Measurement values table
WaterbaseID (Country and Station)
Date (Year, Month and Day)
Determinant, with Determinant codes "Nitrate", "Nitrite", "Total oxidised nitrogen" and "Orthophosphate".

Stations table
Unique identifier: data provider, Country and Station ID
Sea region (Atlantic, North Sea, Baltic, Mediterranean and the Black Sea)

The two tables are combined in a query which joins data to stations, linked by WaterbaseID, and including Country Code and Sea Region (used in Selection Criteria below). This query (or a table made from it) is used in the Aggregate queries.

Description of specific aggregation query sequencesNitrogen and Phosphate
Step 1

Crosstable query, with determinands "Nitrate", "Nitrite", "Total oxidised nitrogen" and "Orthophosphate" as columns, and row heading Sea Region, WaterbaseID, Year, Month, and SampleDepth.
Include data for:
SampleDepth <=10 m and 
Month = 1,2,3 (Jan. - Mar.) for stations east of longitude 15 degrees (Bornholm) in the Baltic Sea
Month = 1,2 (Jan.- Feb.) for all other stations.

Step 2

For each combination of WaterbaseID*Year*Month*Day, calculate [Total Oxidised Nitrogen]: Calculate best possible estimate of nitrate including nitrite:Oxidised Nitrogen is equal to Total Oxidised Nitrogen if Total Oxidised Nitrogen is measured, else calculate Oxidised Nitrogen equal to sum of Nitrate and Nitrite.

Aggregate arithmetic mean of Oxidised Nitrogen and Orthophosphate over depths.

Step 3

For each combination of WaterbaseID *Year,

calculate the arithmetic mean over the depth averages from Step 2.
Export result to Aggregate database as table 't_Base_Metadata_N_and_P'

3. Classification of N and P concentration levels, for each station

For each (sub)regional sea, the observed concentrations are classified as Low, Moderate or High. Concentrations are classified as Low when they are lower than the 20-percentile value of concentrations within a (sub)region. Concentrations are classified as High when they are higher than the 80-percentile value of concentrations within a (sub)region. The classification boundaries therefore change between regional and/or sub-regional seas.

Trend analysis of Nitrogen and Phosphate concentrations

Consistent time series are used as the basis for assessment of changes over time. The trend analyses are based on time series from 1985 onwards.  Selected stations must have at least data in the last four years of the current assessment (2007 or later), and 5 or more years in the overall assessment period (since 1985). For nitrogen nutrients nitrate+nitrite is used, but gaps may be populated with nitrate alone to complete the time series.. Trend detection for each time series was done with the Mann-Kendall Statistics using a two-sided test with a significance level of 5% (Sokal & Rohlf, 1995).

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 nutrient 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 p-value < 0.05 are reported as significantly increasing or decreasing. The test analyzes only the direction and significance of the change, not the size of the change.

Methodology for gap filling

For oxidized nitrogen, the sum of nitrate and nitrite is used. However, if nitrite values are not available, gaps may be populated by assuming that oxidized nitrogen is equivalent to the prevalent nitrate fraction, in order to complete the time series.

Methodology references



Methodology uncertainty

The Mann-Kendall test for the detection of trends is a robust and accepted approach. However, due to the multiple trend analyses, approximately 5% of the tests conducted will turn out significant if in fact there is no trend. Also, the accuracy at the regional level is largely influenced by the number of stations for which data is available.

There are also a number of uncertainties related to temporal and spatial use 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). 

Moreover, two types of geographical aggregation are performed in the current methodology, based on the Country Code and Sea Region. In both cases, differences in physical, chemical and biological characteristics between sampling stations are not taken into account. Measured nutrient concentrations should be related to natural background values that reflect spatial/geographical differences. Furthermore, data collected over the different years is obtained from different laboratories, possibly following different methodologies,,and  it is being combined in the same trend analysis. This might influence the results as well.

Data sets uncertainty

Data for this assessment is still scarce considering the large spatial and temporal variations inherent to the European transitional, coastal and marine waters. Long stretches of European coastal waters are not covered in the analysis due to lack of data or sufficiently long and recent time series.

Trend analyses are only consistent for the North Sea and the Baltic Sea, for which data is updated yearly within the OSPAR and HELCOM conventions, as well as for some stations in Croatian coastal waters.

Rationale uncertainty

Due to variations in freshwater discharges and 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
Frequency of updates
Updates are scheduled once per year
EEA Contact Info


Geographic coverage

Temporal coverage