Definition

There is no single and globally accepted definition of marine eutrophication. The word “eutrophication” has its roots in Greek where “eu” means “well” and “trope” means “nourishment”.

Nixon (1995) defines marine eutrophication as “an increase in the supply of organic matter”. The supply is not restricted to pelagic primary production, but also includes bacterial production, primary production of submerged aquatic vegetation, inputs of organic matter from land via rivers and point sources as well as the net advection from adjacent waters. The advantage of this definition is that it is short, simple and does not confuse causes and effects. The limitations of the definition are 2 fold. It does not take structural or qualitative changes due to nutrient enrichment into account, and it is difficult to make fully operational since the majority of existing marine monitoring programmes seldom include all the variables needed to estimate the total supply of organic matter to a given body of water.

Gray (1992) focuses on the direct effects of nutrient enrichment on productivity, the secondary effects where the produced organic material is not consumed by grazers, and the extreme and ultimate effects, which includes the growth of macroalgae, oxygen depletion and mortality of species. Richardson & Jørgensen (1996) focus both on the process, the associated effects of nutrient enrichment and natural versus cultural caused eutrophication. Prudently, Richardson & Jørgensen point out that when we speak of eutrophication it is cultural eutrophication or that, which is caused by anthropogenic activities, which is of interest.

The definition of eutrophication by OSPAR is: “Eutrophication means the enrichment of water by nutrients causing an accelerated growth of algae and higher form of plant life to produce an undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned, and therefore refers to the undesirable effects resulting from anthropogenic enrichment by nutrients” (OSPAR 1998). A number of EU Directives also defines eutrophication. In the Urban Wastewater Treatment Directive eutrophication means: “The enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned” (EU 1991). The Nitrates Directives definition is almost identical, except that it is restricted to eutrophication from agriculture (EU 1991).

The differences between the various definitions leave the definition open for interpretation. However, this is not critical, as long as there is a common understanding of the effects and agreement upon the acceptable levels of deviations from a healthy marine environment.

Eutrophication should be seen both as a process and as a continuum, since the background values may vary from area to area due to natural causes. For example, the productivity in the open Baltic Sea is relatively low compared to the southern and eastern parts of the North Sea. Therefore, when speaking of eutrophication, both the initial process and direct effects (sensu Nixon) and the derived primary and secondary effects should be taken into account, cf. Box 1:

Box 1. Definition of eutrophication

Eutrophication is the enhanced inputs of nutrients and organic matter leading to changes in primary production, biological structure and turnover and resulting in a higher trophic state. The causative factors are: elevated inputs of nutrients from land, atmosphere or adjacent seas, elevated winter DIN- and DIP concentrations, and increased winter N/P-ratios compared to the Redfield Ratio. In the case of marine waters, the primary or direct effects include: increased primary production, elevated levels of biomass and chlorophyll a concentrations, shift in species composition of phytoplankton, and shift from long lived macroalgae to short lived nuisance species. The secondary or indirect effects include increased or lowered oxygen concentrations, and changes in species composition and biomass of zoobenthos. Low oxygen concentrations in the bottom water (oxygen depletion, hypoxia) can further affect the fish, benthic invertebrates and plants. Total oxygen depletion (anoxia) can result in the release of hydrogen sulphide from the sediment, causing extensive death of organisms associated with the sea floor. As only a few species can survive these extreme conditions, and as it takes time for plants and animals to recolonise damaged areas, eutrophication can result in impoverished biological communities and impaired conditions.

Danish Environmental Protection Agency & National Environmental Research Institute • updated: