BS7 Oxygen and benthic communities

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Development

Oxygen depletion of the bottom water layer is a consequence 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 period more than 50% of the monitored seabed areas have in an average year suffered from the absence of oxygen. However, the situation has been slightly better in 2010s than in 2000s.

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. In the first half of the 2000s the benthic fauna was observed in less than one third of monitored areas yearly. Since 2008 the share of observed fauna has increased remarkably. In 2013 benthic fauna was found in 75 % of the monitored areas. Nevertheless, one fourth of the areas had no fauna present at all.

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.