Sub-regional assessment of benthic invertebrate communities in open sea areas of the Baltic Sea

Note that this assessment is based on data covering only the open sea areas of the Baltic Sea. 

Benthic invertebrate diversity and, therefore, reference conditions differ markedly between sub-basins owing to the gradient in salinity, which constrains species distributions. A total of eight basins were evaluated and the reference conditions, measured as the average number of species, varied between 18.3 in the Arkona Basin and 2.0 in the Bothnian Bay. For the years 2003–2007, benthic invertebrate status varied considerably between sub-basins and was related to the widespread occurrence of hypoxia and anoxia in the Baltic Proper and the Gulf of Finland (see figure below). None of the sub-basins can be regarded as pristine and even the Gulf of Bothnia, where EQR values were the highest at 0.83, showed a 17% reduction from defined reference conditions. The entire Baltic Proper, from the Bornholm Basin to the northern Baltic Proper and the Gulf of Finland, was in a severely disturbed state.

The Arkona Basin is regularly flushed by saltwater inflows; the average EQR value for the assessment period was 0.71, which is above the good/moderate border. However, this value is based on limited data, and larger deviations from reference conditions probably occurs. In the Bornholm Basin conditions were severely disturbed, with an average EQR value of 0.44 (range: 0.25–0.81), representing a 56% reduction from defined reference conditions. Also in the southeastern Gotland basin and the northern and central Eastern Gotland Basin, conditions were severe, with EQR values of 0.33 and 0.12 (range: 0.06–0.19), respectively.

In the northern Baltic Proper, no fauna whatsoever was recorded for the assessment period, resulting in an EQR value of 0. This reflects the consistently bad oxygen conditions in this open sea area. Conditions in the Gulf of Finland were more variable, but also there benthic invertebrate status was bad, with an EQR of 0.24 (range: 0.13–0.34).

In the Gulf of Bothnia, where water column stratification is weak and oxygen conditions are generally good, EQR values were above the good/moderate border, i.e. an average of 0.91 in the Bothnian Sea (range: 0.69–1.0) and 0.83 in the Bothnian Bay (range: 0.71–0.94).

Please note that there is an uncertainty regarding the assessment for the Arkona Basin and the SE Gotland Basin due to limited data. Hence the results for these regions should be viewed as indicative only. All relevant data is presented in the table and the figure (below).

Figure. Reference values and the border between good and moderate (G/M) ecological status in the different sub-basins in open sea areas of the Baltic Sea depicted as EQR and the average number of species. Benthic invertebrate status is described as an average for the assessment period 2003–2007.

Table 1. (Click to enlarge)

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Last updated: 27 April 2010