Rapport nr 76: Undersökning av vattenkvalitet (år 1989) och bottenfauna (år 1990) vid Ålands forell i Järsö, Lemland
SUOMALAINEN, S (1990)
Åbo Akademi, Husö biologiska station
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Husö biological station has carried out investigations of the brakish water environment at the fish farm Ålands Forell on Åland (N. Baltic Sea) since its beginning year 1981. Water quality and the organic content of sediment (ignition loss) was investigated in June-August 1989. A temporary temperature stratification and anaerobic conditions below the thermocline was observed at the deeper stations near the fish farm during the summer as previously. The oxygen saturation in the surface water was high in the bay in the vicinity of the fish farm. High pH-values (8.4-8.8 in June-July) and high chlorophyll-a -values (5-23 µg/I) indicate a high primary production in the bay. The phytoplankton composition was dominated totally by the dinoflagellate Heterocapsa triquetra. The high total phophorus contents (24-74 µg/l) in the water (1-5 meter) indicate eutrophic conditions in the bay whereas the phosphorus contents (20 µg/I) in the more open sea in connection to the bay indicate mesotrophic conditions. The total nitrogen contents in surface water were also high (290-640 µg/I). Considerably high contents of total phosphorus and total nitrogen in the bottom water (P 13-20 times, N 4 -6 times higher) occured during anaerobic conditions. The high contents of organic matter (20-41 %) in sediments at the deeper stations indicate deposition of the matter in the deeper parts of the bay. Sludge suction has been carried out at the fish farm. The organic content of the sediment showed a decreasing trend at the station nearby the fish farm compared with the 2 previously years. The softbottom fauna was periodically absent at the deeper stations in the bay. A relatively rich bottom fauna community (11-16 species/groups) with Macoma baltica as the dominating species occured at the shallower stations. The biomasses were increasing at these stations. The investigated parameters indicate eutrofic conditions in the brackish-water bay in the vinicity of the fish farm.