Nutrients in transitional, coastal and marine waters

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

Justification for indicator selection

The water quality in transitional, coastal and marine regions could be adversely affected by anthropogenic activities, such as the application of agricultural fertilisers and manure, the discharge of wastewater and airborne emissions from shipping and combustion processes. These activities may result in elevated nutrient (nitrogen and phosphorus) concentrations leading to eutrophication and causing a chain of undesirable effects.

Usually a distinction is made between the direct and indirect effects of nutrient enrichment. The direct effects include a high chlorophyll concentration in the water column as a result of increased phytoplankton primary production, and changes in species composition and functioning (such as diatom to flagellate ratio, benthic to pelagic shifts and bloom events of nuisance/toxic algal blooms). The increase in the risk of algal blooms (e.g. cyanobacteria) may cause the death of benthic fauna, wild and caged fish or shellfish poisoning of humans. In addition to the effects on the aquatic ecosystem, discoloration of water may be a disturbance to bathers, thus impairing recreational activities.

The indirect effects of nutrient enrichment include increased abundance of perennial seaweeds and seagrasses (e.g. fucoids, eelgrass and Neptune grass), reduced water transparency related to an increase in suspended algae and oxygen depletion. Increased growth and dominance of fast-growing filamentous macroalgae in shallow sheltered areas may change the coastal ecosystem, increase the risk of local oxygen depletion, and reduce biodiversity and nurseries for fish. Increased consumption of oxygen due to increased organic matter decomposition can lead to oxygen depletion, particularly in areas with stratified water masses, changes in community structure and hypoxia with bottom fauna mortality. Even a short-time event of hypoxia will kill most invertebrates living on or within the seabed, creating 'dead zones'. Dead zones in marine ecosystems due to hypoxic conditions have doubled in size globally every decade since the 1960s.  

The main nutrients causing eutrophication are nitrogen (in the form of nitrate, nitrite or ammonium) and phosphorus (in the form of orthophosphate). Silicate is essential for diatom growth, but it is assumed that its input is not significantly influenced by human activity.

The marine regions of Europe have different sensitivities to eutrophication, determined by their physical characteristics. The Baltic Sea and the Black have a high sensitivity to eutrophication because of limited water exchange with connecting seas.

Scientific references

• Black Sea Commission 2008. State of the environment of the Black Sea (2001-2006/2007). Edited by Temel Oguz. Publications of the Commission of the Protection of the Black Sea against Pollution (BSC) 2008-3 Istanbul, Turkey, 448 pp.
• OSPAR, 2017. Eutrophication Status of the OSPAR Maritime Area. Third Integrated Report on the Eutrophication Status of the OSPAR Maritime Area. Publication no 694/2017
• HELCOM 2018. State of the Baltic Sea – Second HELCOM holistic assessment 2011-2016. Baltic Sea Environment Proceedings 155
• Cloern JE, 2001. Our evolving conceptual model of the coastal eutrophication problem Marine Ecology Progress Series, 210, pp. 223-253
• UNEP/MAP, 2017. Mediterranean Quality Status Report.
• UNEP/MAP, 2012. State of the Mediterranean Marine and Coastal Environment. UNEP/MAP – Barcelona Convention, Athens, 2012
• EEA 2019. Nutrient enrichment and eutrophication in Europe’s seas. EEA report in prep.
• EEA 2015. State of Europe’s seas. EEA report No. 2/2015
• EEA 2014. Marine messages. Our seas, our future — moving towards a new understanding. Brochure No 1/2014
• Nixon, S.W. 2009. Eutrophication and the macroscope. Hydrobiologia 629, 5–19.

Indicator definition

Units

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.

Targets

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

• 2003 Strategies of the OSPAR Commission
  2003 Strategies of the OSPAR Commission for the Protection of the Marine Environment of the North-East Atlantic. II - Eutrophication.
• Bathing Water Directive (76/160/EEC)
  Council Directive of 8 December 1975 concerning the Quality of Bathing Water (76/160/EEC)
• 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)
• Common Implementation Strategy for the Water Framework Directive (2000/60/EC)
  Policy Summary of Guidance document No. 23 on Eutrophication assessment in the context of European water policies 
• 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.  
• OSPAR MSFD Advice document on eutrophication
  GES 8/2012/12
• Sixth Environment Action Programme (decision No 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
• Strategic Action Plan for the Environmental Protection and Rehabilitation of the Black Sea
  This document represents an agreement between the six Black Sea Coastal states (Bulgaria, Georgia, Romania, the Russian Federation, Turkey and Ukraine) to act in concert to assist in the continued recovery of the Black Sea.  Black Sea Commission 2009
• 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 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

Classification

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

• 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).
• Sokal R.R. & Rohlf F.J. Biometry: the principles and practice of statistics in biological research. Sokal RR, Rohlf FJ, W. H. Freeman, New York, 1995
• 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.
• 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.
• 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.
• Mann H.B., 1945. Nonparametric tests against trend  Econometrica 13, 245-259 .

Data specifications

EEA data references

• Waterbase - Transitional, coastal and marine waters provided by European Environment Agency (EEA)

External data references

• Chemistry data - Data & products on marine water quality
• Oceanographic database

Data sources in latest figures

Uncertainties

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.

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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.