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Chlorophyll in transitional, coastal and marine waters (CSI 023) - Assessment published Jul 2011

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Generic metadata


Water Water (Primary topic)

Coasts and seas Coasts and seas

soer2010 | csi | baseline | oceans | water | chlorophyll-a | thematic assessments | marine and coastal | ecosystems | coastal ecosystems
DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CSI 023
Temporal coverage:
Geographic coverage:
Albania, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Cyprus, Denmark, Estonia, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Latvia, Lithuania, Malta, Montenegro, Netherlands, Norway, Poland, Portugal, Romania, Russia, Serbia, Slovenia, Spain, Sweden, Turkey, United Kingdom

Key policy question: Is eutrophication in European surface waters decreasing?

Key messages

  • In 2008, the highest summer chlorophyll-a concentrations were observed in coastal areas and estuaries where nutrient concentrations are high, namely in the Gulf of Riga, the Gulf of Finland and along the coast of France and Belgium.
  • Although nutrient concentrations in some European sea areas decreased from 1985 to 2008 (see Core Set Indicator 21), these changes were not clearly reflected in chlorophyll-a concentrations: of the 546 stations reported to the EEA the majority of the stations (89%) indicated no statistically significant change.
  • Changes were detected mainly in Finnish, Dutch, Norwegian, Swedish and Italian coastal waters. At the Finnish and Swedish monitoring stations chlorophyll-a concentrations showed both decreasing and increasing trends, whereas in Italy, Netherlands and Norway concentrations were mainly decreasing.
  • An analysis of changes based on satellite imagery show significantly increasing trends of ocean colour (equivalent to chl-a)along the Mediterranean coast, whereas trends are significantly decreasing in large parts of the central Mediterranean and Black Seas. It also shows significantly increasing trends in the Baltic Sea, but here the analysis is less certain.


Chlorophyll-a concentrations in European seas, 2008

Note: Chlorophyll-a concentrations in European seas in 2008 based on observations

Data source:
Downloads and more info

Key assessment

Baltic Sea

In 2008 the highest measured summer chlorophyll-a concentrations (> 6 µg/l) were found in the Gulf of Riga and in the Gulf of Finland. The Gulf of Riga is influenced by nutrient fluxes from the River Daugava, and with low dilution due to restricted water exchange with the Baltic Sea. In the Gulf of Finland, high concentrations are linked to the circulation that moves nutrient rich waters originating from the River Neva and St. Petersburg along the Finnish coast. Low concentrations (<2 µg/l) are predominantly observed in the Bothnian Bay, and in the Kattegatt.

Most of the stations (86%) did not show any change in chlorophyll concentration in the period 1985-2008. Positive changes were, however, observed: chlorophyll concentrations decreased in the Finnish coastal areas of the Bothnian Bay (at one third of the Finnish monitoring stations). In total statistically significant decreasing trends were evident at 6% of the Baltic Sea stations.  In contrast, increases were observed in the Gulf of Finland and at Swedish stations in the northern Baltic Proper. The latter result is in agreement with a recent study (Fleming-Lehtinen et al. 2008), which shows that chlorophyll concentrations increased in the Northern Baltic Proper in the period 1979 to 2006. In total, statistically significant increasing trends were evident at 8% of the Baltic Sea stations.

In recent years the Baltic Sea has suffered from frequent and extensive summer blooms of cyanobacteria, which colour large areas of the sea surface bright green or brownish yellow and are a nuisance to swimmers because they are slightly toxic. Cyanobacterial blooms are intermittent, occurring during calm meteorological conditions  and when phosphorous is available from occasional upwelling of deep water (Vahtera et al. 2007). Such conditions were predominant during 2008, where large areas of the Baltic Sea were covered by cyanobacterial blooms in July.

In general eutrophication is of major concern in the Baltic Sea and there are many initiatives, including the Baltic Sea Action Plan, that work towards reducing eutrophication.

 North Sea

Only limited amount of chlorophyll data of the North Sea was submitted to the EEA by the member states for the year 2008. The data coverage was best in coastal waters of France, Belgium, and in the Skagerrak. The highest summer chlorophyll-a concentrations (> 5 µg/l) were observed along the coast of France and Belgium. These areas have suffered from frequent blooms of Phaeosystis, which causes harm to recreational use of coastal waters by producing foam.

The majority of stations (90%) did not show any statistically significant change in chlorophyll concentration between 1985 and 2008. In a recent eutrophication report OSPAR (2008) stated that further efforts are still needed to reduce nutrient inputs otherwise the aim of the OSPAR Eutrophication Strategy - a healthy marine environment where eutrophication does not occur - will not be achieved. 

NE Atlantic

The highest concentrations were found in the French estuaries (> 10 µg/l). In the north eastern Scottish coastal waters chlorophyll concentrations varied from low (< 1.5 µg/l) to high (> 6 µg/l) depending on the location of the monitoring stations. OSPAR (2008) has defined some small coastal embayments and estuaries within the Celtic Seas and the Bay of Biscay and Iberian Coast as problem areas with regard to eutrophication. Harmful algal blooms of various species are observed occasionally in the NE Atlantic, but there is no evidence that the blooms would become more frequent in recent decades (Smayda 2006, Gowen et al. 2008). No significant changes in chlorophyll concentrations were observed.

Mediterranean Sea

The highest summer chlorophyll-a concentrations (> 5 µg/l) were in the French coastal water. No offshore data of the Mediterranean chlorophyll concentrations has been submitted to the EEA for the year 2008. The open water of the Mediterranean Sea is, however, poor in nutrients and thus summer chlorophyll-a concentrations are also low. The most eutrophic waters in Mediterranean are along the northern coastline, but eutrophication problem has been increasing gradually over the last decades also in the southern shore of the sea (UNEP 2007). Harmful algal blooms have been observed commonly in the northern coastal areas. These blooms have also consisted of dinoflagellates (e.g. Dinophysis and Alexandrium) potentially causing different types of shellfish poisoning (Koukaras and Nikolaidis 2004, Bravo et al. 2008).

Only France, Italy and Croatia have submitted long enough time series to estimate trends. Croatian observations start from the year 1998 and no changes could be detected. In Italian coastal waters summer chlorophyll-a concentrations were decreasing at 6%, increasing at 2% of the stations, and no statistically significant trend can be detected at the remaining 92% of stations. Decreasing trends were found in many different areas of Italian coast, but they were concentrated especially in the Gulf of Venice and the estuary of the Po River.

The ocean colour trend analysis has been used to support the analysis based on in-situ observations. In general significantly increasing trends are found in coastal waters of the Mediterranean Sea, whereas significantly decreasing trends are observed in the open parts of the Mediterranean Sea.

Black Sea

Of the Black Sea countries only Romania submitted chlorophyll-a data to EEA in 2008. The highest values (over 30 µg/l) were measured in the River Danube delta. Satellite observations show that ocean colour intensity is significantly decreasing in a large part of the central Black Sea.




 Bravo I, Vila M, Masó M, Figueroa RI, Ramilo I . 2008. Alexandrium catenella and Alexandrium minutum blooms in the Mediterranean Sea: Toward the identification of ecological niches. Harmful algae. 7:515-522.

 Fleming-Lehtinen, V., Laamanen, M., Kuosa, H., Haahti, H., & Olsonen, R. 2008. Long-term development of inorganic nutrients and chlorophyll a in the open northern Baltic Sea. Ambio 37(2): 86-92.

 Gowen, RJ; Tett, P; Kennington, K; Mills, DK; Shammon, TM; Stewart, BM; Greenwood, N; Flanagan, C; Devlin, M; Wither, A. 2008.  The Irish Sea: Is it eutrophic? Estua-rine, Coastal and Shelf Science 76(2): 239-254.

Koukaras, K. and Nikolaidis, G. 2004. Dinophysis blooms in Greek coastal waters (Thermaikos Gulf, NW Aegean Sea). J. Plankton Research 26(4): 445-457.

McQuatters-Gollop, A., Dionysios E. Raitsos, Martin Edwards, Yaswant Pradhan, Laurence D. Mee, Samantha J. Lavender, and Martin J. Attrill, M.J.  2007. Long-term chlo-rophyll dataset reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient levels Limnol. Oceanogr., 52(2), 2007, 635–648.

 OSPAR 2008. Second OSPAR Integrated Report on the Eutrophication Status of the OSPAR Maritime Area. OSPAR Commission 2008.

 Smayda, T. 2006. Harmful algal bloom communities in Scottish coastal waters: Relationship to fish farming and regional comparisons – a review. Paper 2006/3.

 UNEP 2007. Approaches to the assessment of eutrophication in Mediterranean coastal waters (First draft). Mediterranean Action Plan. Review Meeting of MED POL Monitoring Activities and the use of indicators Athens, 12-14 December 2007. UNEP(DEPI)/MED WG.321/Inf.6 3 December 2007

 Vahtera, E; Conley, DJ; Gustafsson, BG; Kuosa, H; Pitkaenen, H; Savchuk, OP; Tamminen, T; Viitasalo, M; Voss, M; Wasmund, N; Wulff, F. Internal Ecosystem Feedbacks Enhance Nitrogen-fixing Cyanobacteria Blooms and Complicate Management in the Baltic Sea. Ambio Vol. 36, no. 2, pp. 186-194.


Data sources

More information about this indicator

See this indicator specification for more details.

Contacts and ownership

EEA Contact Info

Constança De Carvalho Belchior


EEA Management Plan

2010 1.5.2 (note: EEA internal system)


Frequency of updates

Updates are scheduled once per year in July-September (Q3)
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