>> Background information

Development

Oxygen depletion of the bottom water layer is an effect of extensive eutrophication. At worst, eutrophication may result in anoxia, which is the total absence of oxygen in the seabed. Proportion of anoxic seabed areas has been high throughout the 2000s. During this time period more than 50% of the monitored seabed areas have in an average year suffered from the absence of oxygen.

Occurence of benthic fauna in the Gulf of Finland is strongly correlated with seabed oxygenation. During the anoxic periods benthic communities are locally destroyed and begin to recover again when oxygen conditions improve. The year 2008 seems to have been more favourable for the benthic fauna than any of the previous years since 2001. Still, approximately one third of the seabed monitoring stations had no fauna present in 2008.

Impact on biodiversity

Lethal level of oxygen depletion is species dependent. Some species can survive in low oxygen conditions only for a few hours, others even several weeks. Species diversity is however always decreased. In the Gulf of Finland, the so called key species, for example the amphipods Monoporeia affinis and Pontoporeia femorata, are the first ones to suffer as oxygen level loweres. If oxygen concentration drops below zero toxic hydrogen sulfide is released and all macrofauna is killed.

Some species recover from oxygen depletion or anoxia more efficiently than others. This results in dominance of these species in areas of periodic oxygen depletion, which in turn leads to changes in the benthic community composition and effects the whole ecosystem. Diversity of benthic communities is one of the most important descriptors for the state of an aquatic ecosystem.