Eutrophication

Eutrophication means that the water has an increased volume of nutrients, as nitrogen and phosphorus for example, which has undesirable effects on water quality. Rivers and streams transport nutrients to the ocean naturally, but nutrient loads have significantly increased in recent decades as a result of human activities, as agriculture, forestry, settlements, sewage treatment and the use of livestock. Still, farmlands are considered the largest source of nutrient enrichment. The use of oil and gas also adds nutrients from the atmosphere to the oceans, and especially nitrogen.

Image 1: Eutrophication

Some species benefit from eutrophication, while some species do not. This can result in changes of the overall composition of species. Fast-growing algae, for example, benefit from eutrophication and cause murky water, which in turn causes habitat changes since it decreases light penetration through the water column. The decomposition of these algae also require a lot of oxygen which causes oxygen depletion, or hypoxia. Hypoxia results in so-called dead zones where few species are able to survive. Furthermore, eutrophiciation causes beaches to overgrow, increasing sedimentation of organic matter on the seafloor and substantial algae blooms. From a human perspective, this might have negative effects on the fishing industry, tourism and recreation.



Image 2 and 3: Algae bloom

On the Swedish coast there are no clear signs of eutrophiciation, while in the protected bays of the Kvarken archipelago, where nutrient loads to coastal waters are particularly high, show signs of eutrophiciation in some places. This is probably due to the slow circulation of the water in the archipelago on the Finnish side, as well as the differences in land use.

Image 4: Potamogeton pectinatus and Corda filum

Sources

Statens miljöförvaltnings webbtjänst (2013). Övergödning. www.miljo.fi.

Sjöling, S.(2008). Östersjöns döda bottnar. Havsutsikt, Nr 1/2008, Stockholm.

Lundberg, C. et al (2012). Undervattensmiljö i Norra Östersjön – Viktigt att tänka på vid havsnära planering. Närings-, trafik- och miljöcentralen, Helsingfors.

Sedin, A (2002). Miljööversikt – Havets tillstånd i Kvarkenområdet. Kvarken miljö, Vasa.