A conceptual model for eutrophication in the Baltic Sea

The word ‘eutrophication’ has its roots in Greek: ‘eu’ meaning ‘well’ and ‘trope’ meaning ‘nourished’. The modern use of the word is related to nutrient enrichment as a consequence of nutrient inputs. The problems of eutrophication are generally related to changes in the structure and functioning of the marine ecosystem.

A primary effect is an increase in the pro­duc­tion of planktonic algae, often manifested as algal blooms. This increased production of organic matter often has secondary and drastic negative consequen­ces, e.g. reduced water transparency, increased sedi­men­tation, increased mineralisation and oxygen con­sumption leading to oxygen depletion in bottom waters. Benthic communities such as meadows of submerged aquatic vegetation are directly affected by reduced water trans­parency. Benthic invertebrate communities and fish are affected by oxygen depletion, the ultimate effects being kills of animals. A conceptual model of eutrophication is given in the figure below.

The limited water exchange with the North Sea, stratification of the water column, and the long resi­dence time of water are the main reasons for the sensi­tivity of the Baltic Sea to eutrophication. High nutrient loads in com­bination with long residence times of water means that nutrients discharged to the sea will remain for a long time. In addition, the vertical strati­fication of the water mas­ses increases the vulnerability of the Baltic Sea to eutrophication. The most impor­tant effect of strati­fi­ca­tion in terms of eutro­phication is that it hinders or prevents ventilation and oxy­gena­tion of the bottom waters and sediments by verti­cal mixing of water, a situation that often leads to oxygen depletion. Hypoxia and anoxia have an effect on nutri­ent transformations, such as nitrification and denitri­fication processes, as well as the capacity of the sedi­ments to bind phos­phorus. In the absence of oxy­gen, sediments re­lease sig­ni­ficant quan­tities of phosphorus to the overlying water.

Large parts of the Baltic Sea are in a state of so-called repressed recovery, sometimes referred to as a vicious cycle, be­cause of interconnected processes involving nitro­gen, phosphorus and oxygen. Wide­spread hypoxia facilitates the release of of phosphorus from sediments and fuels blooms of nitro­gen fixing blue-green algae that tend to counter­act reduc­tions in external loads of nitro­gen and phosphorus.

Further reading

HELCOM 2009a. Eutrophication in the Baltic Sea - An integrated thematic assessment of the effects of nutrient enrichment in the Baltic Sea region. Executive Summary. Helsinki Commission. Baltic Sea Environment Proceeding No. 115A

HELCOM 2009b. Eutrophication in the Baltic Sea – An integrated thematic assessment of the effects of nutrient enrichment in the Baltic Sea region (2009). Helsinki Commission. Baltic Sea Environment Proceeding No. 115B

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