Living coast: Characteristics and natural assets of Björnöfjärden

Characteristics and natural assets of Björnöfjärden

The deep and elongated nature of the inlet of Björnöfjärden results in a thermocline of between six and ten metres during the summer months. As a result of this stratification, the bottom water is not mixed and oxygenated. The bays have a rich flora and fauna above the stratification. Below the thermocline, the effects of eutrophication are clear: ever higher life is eliminated, and the bottom water has extremely high levels of nitrogen and phosphorus.
The stratification can mask eutrophication during the summer as a result of nutrients being kept below the thermocline, where they cannot contribute to algal blooms. This means that water quality during the summer months can periodically be experienced as good, for example because of a good visibility depth. When the thermocline weakens, due to strong wind in the longitudinal direction of the bays, or in the autumn when the surface water temperature drops, nutrient-rich bottom water will rise and create large algal blooms.
When the algal blooms drop to the bottom, the algae are broken down and all the oxygen is consumed. The result is dead bottom zones with poor ability to bind phosphorus.

Vegetation
The vegetation on the shallow bottoms in the Björnöfjärden bay system has high status (Quarfordt et al. 2011). This applies in particular to species-rich plant communities with large coverage of sandy and soft bottoms down to a depth of around five metres. The hard-bottom communities are considerably less species-rich and are more sparse, which is probably due to high sedimentation. The whole bay system is classified as highly protected in terms of wave exposure, which means that sediments are not flushed away form the hard bottoms by the motion of waves and water. Below the thermocline (> 7 metres depth) poorer conditions prevail, with low oxygen levels and little light availability. There is high coverage of cyanobacteria and sulphur bacteria on these bottoms.

Fish
In trial net fishing and inventories of fish fry, a total of eleven and nine different species respectively were caught (Lindqvist 2011; Arvidsson 2011). In terms of numbers, perch, bleak, silver bream and roach dominated (93 per cent). Most were caught in the depth range from zero to three metres. Catches already decreased sharply in the depth range from three to six metres. Fishing pointed to good fry recruitment among the dominant species perch, bleak, white bream and roach. Recruitment of pike appears to be working fairly well. The ratio of perch to cyprinids and the proportion of fish-eating perch in Björnöfjärden indicate that the bay system is eutrophicated.

Benthic animals
In the shallower parts of Björnöfjärden (depth < 10 metres) the number of species, the number of individuals and the biomass were low (Lindqvist, 2011). Only a small number of animals were encountered in the deeper parts of the bay
(> 10 metres). The lack of benthic fauna indicates poor oxygen conditions during most of the year.