Eutrophication (EO 5)

Common Indicator 13: Concentration of key nutrients in water column

Geographical scale of the assessment:
Regional, Mediterranean Sea
Contributing countries:
Albania, Bosnia and Herzegovina, Croatia, Cyprus, Egypt, France, Greece, Israel, Italy, Montenegro, Morocco, Slovenia, Spain, Tunisia, Turkey
Mid-Term Strategy (MTS) Core Theme:
1-Land and Sea Based Pollution
Ecological Objective:
EO5. Human-induced eutrophication is prevented, especially adverse effects thereof, such as losses in biodiversity, ecosystem degradation, harmful algal blooms and oxygen deficiency in bottom waters
IMAP Common Indicator:
CI13. Key nutrients concentration in water column (EO5)
Indicator Assessment Factsheet Code:

GES Definition: Concentrations of nutrients in the euphotic layer are in line with prevailing physiographic, geographic and climate conditions  

GES Targets:

  • State: Reference nutrients concentrations according to the local hydrological, chemical and morphological characteristics of the un-impacted marine region 17
  • State: Decreasing trend of nutrients concentrations in water column of human impacted areas, statistically defined
  • Pressure: Reduction of BOD emissions from land based sources
  • Pressure: Reduction of nutrients emissions from land based sources



Eutrophication is a process driven by enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, leading to: increased growth, primary production and biomass of algae; changes in the balance of nutrients causing changes to the balance of organisms; and water quality degradation (IMAP, 2017). Seawaters depending on nutrient loading and phytoplankton growth are classified according to their level of eutrophication. Low nutrient/ phytoplankton levels characterize oligotrophic areas, water enriched in nutrients is characterized as mesotrophic, whereas water rich in nutrients and algal biomass is characterized as eutrophic. The Mediterranean is one of the most oligotrophic seas in the world and most of its biological productivity takes place in the euphotic zone (UNEP, 1989, UNEP/MAP, 2012).

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Assessment methods

At the moment only some of the Mediterranean countries have developed a boundary approach for the assessment of eutrophication and no general assessment criteria have been agreed for the Mediterranean area for the key nutrient concentrations in the water column. This assessment effort was based only on the presentation of the geographical variability of some key nutrients (DIN – dissolved inorganic nitrogen and TP- total phosphorous; µmol L-1).

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Results and Status, including trends

The trophic status of the Mediterranean Sea is controlled by the highly populated coastal zone and the riverine input from a draining area of 1.5 million km2 (Ludwig et al. 2009) that induce eutrophic trends in coastal areas. The blue offshore waters of the Mediterranean have been characterized as extremely oligotrophic with an increasing tendency for oligotrophy eastwards (Turley 1999). Eutrophication and oligotrophy in the Mediterranean is illustrated as chlorophyll a distribution in remote sensing imagery (Figure 1). It is observed that the Eastern Mediterranean Sea (EMS) is still the most oligotrophic area of the whole Mediterranean basin. This is due to the low nutrient content of EMS; the maximum concentrations recorded for nitrate were about 6 μmol L-1, for phosphate 0.25 μmol L-1, and for silicate 10–12 μmol L-1, with the nitrate to phosphate ratio (N/P) >20 and in deep waters about 28:1, the EMS has been characterized as the largest phosphorus-limited body of water in the global ocean.

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The available data show that in areas were assessment is possible the key nutrient concentrations are in ranges characteristic for coastal areas and in line with the main processes undergoing in the interested area. The result also confirm the validity of this indicator as support in assessing eutrophication. Coastal Water type assessment criteria for reference condition and boundaries for key nutrients in the water column have to be built and harmonised through the Mediterranean region, which will greatly help the implementation of a clear sampling strategy with a simplified approach in monitoring design and data handling for the future implementation of IMAP.

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Key messages 

  • The available data show that assessment is possible. Key nutrient concentrations are within characteristic ranges for coastal areas and in line with the main processes undergoing in concerned interested area.
  • Criteria for reference condition and boundaries for key nutrients in the water column have to be built and harmonised through the Mediterranean region.

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Coste, B., Le Corre, P., Minas, H. J. (1988). Re-evaluation of nutrient exchanges in the Strait of Gibraltar. Deep-Sea Resarch, 35, 767–775.

EEA (1999). Nutrients in European ecosystems. Environmental assessment report No 4.

Ignatiades, L., Gotsis-Skretas, O., Pagou, K., & Krasakopoulou, E. (2009). Diversification of phytoplankton community struc- ture and related parameters along a large scale longitudinal east–west transect of the Mediterranean Sea. Journal of Plankton Research, 31(4), 411–428.

IMAP (2017). Integrated Monitoring and Assessment Programme of the Mediterranean Sea and Coast and Related Assessment Criteria UNEP, Athens, 52 pp.

Krom, M. D., Emeis, K. C., and Van Cappellen, P. (2010). Why is the Mediterranean phosphorus limited? Progress in Oceanography. doi:10.1016/j.pocean.2010.03.003.

Ludwig, W., Dumont, E., Meybeck, M., and Heusser, S. (2009). River discharges of water and nutrients to the Mediterranean and Black Sea: major drivers for ecosystem changes during past and future decades? Progress in Oceanography, 80, 199–217.

Tanhua T., Hainbucher D., Schroeder K., Cardin V., lvarez M. A. and Civitarese G. (2013) The Mediterranean Sea system: a review and an introduction to the special issue. Ocean Sci., 9, 789–803.

Turley, C. M. (1999). The changing Mediterranean Sea: a sensitive ecosystem? Progress in Oceanography, 44, 387–400.

UNEP (1989). State of the Mediterranean Marine Environment. MAP Technical Series No. 28, UNEP, Athens.

UNEP/FAO/WHO (1996). Assessment of the state of eutrophication in the Mediterranean Sea. MAP Technical Report Series No. 106, UNEP, Athens, 455 pp.

UNEP/MAP, 2003. Eutrophication monitoring strategy of MED POL, UNEP(DEC)/MED WG 231/14, 30 April 2003, Athens 24 pp.

UNEP/MAP (2007). Eutrophication Monitoring Strategy for the MED POL (REVISION), UNEP(DEPI)/MED WG.321/Inf. 5, 9 November 2007, Athens.

UNEP/MAP (2012): State of the Mediterranean Marine and Coastal Environment, UNEP/MAP – Barcelona Convention, Athens, 2012.

UNEP/MAP (2016). Decision IG.22/7 - Integrated Monitoring and Assessment Programme (IMAP) of the Mediterranean Sea and Coast and Related Assessment Criteria. COP19, Athens, Greece. United Nations Environment Programme, Mediterranean Action Plan, Athens.