Rapport nr 99: Uppföljning av situationen i Vargsundet sommaren 1999 samt en miljökonsekvensbedömning av den planerade slussen
NUMMELIN, C (2000)
NUMMELIN, C
Åbo Akademi, Husö biologiska station
2000
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https://urn.fi/URN:ISBN:952-12-0589-X
https://urn.fi/URN:ISBN:952-12-0589-X
Tiivistelmä
Lake Vargsundet is a eutrophicated lake with problems as large external and internal loading and, it is thus under high risk oj phytoplankton blooms. In 1997 there was a bloom of the haptophyte Prymnesium parvum which killed large amounts of fish. There is an inflow of brackish water from the sea to the northern basin. This inflow forms a layer of denser water on the bottom of the lake. In this layer, there is no oxygen and the nutrient concentrations are much higher than in the surface layer. To stop the inflow of salt water there are plans of building a lock which would allow fish and boats to pass through but would prevent the inflows of saline water. If the inflow of saline water stops, the meromictic conditions in the norther part will disappear over time and the nutrients from the deeper layers will become available in the euphotic zone, which can result in heavy phytoplankton blooms.
In summer 1999 the hydrography, phytoplankton composition, zoobenthos, standing crop of crayfish and fish were studied. Results from this study showed that the situation in the lake was satisfactory, there was no phytoplankton bloom and a high amount of crayfish and fish were recorded. Though the nutrient levels especially in the deeper layers of the northern basin were high.
It is essential that the inflow of saline brackish water is stopped but also other actions to minimise the loading of nutrients would be requested. A large part of the external loading comes from agriculture, cattle and settlement, and these sources should be controlled and minimised. The internal loading can be treated with different methods, for instance airing of the deeper water layers and sediment, dredging the polluted sediment, dilution and removal of the deeper eutrophicated water layers, biomanipulation or/and removal of vegetation.
In summer 1999 the hydrography, phytoplankton composition, zoobenthos, standing crop of crayfish and fish were studied. Results from this study showed that the situation in the lake was satisfactory, there was no phytoplankton bloom and a high amount of crayfish and fish were recorded. Though the nutrient levels especially in the deeper layers of the northern basin were high.
It is essential that the inflow of saline brackish water is stopped but also other actions to minimise the loading of nutrients would be requested. A large part of the external loading comes from agriculture, cattle and settlement, and these sources should be controlled and minimised. The internal loading can be treated with different methods, for instance airing of the deeper water layers and sediment, dredging the polluted sediment, dilution and removal of the deeper eutrophicated water layers, biomanipulation or/and removal of vegetation.