Rapport 78: Undersökning av Boda-, Ivarskärs- och Svartsmarafjärden samt sjön Vargsundet på NW Åland sommaren 1990: hydrografi och vattenkvalitet
ÖSTMAN, T (1990)
ÖSTMAN, T
1990
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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:951-649-856-6
https://urn.fi/URN:ISBN:951-649-856-6
Tiivistelmä
Hydrography and water quality were investigated in an archipelago area (Boda-, lvarskärs- and Svartsmarafjärden) in NW Åland in the summer of 1990. Temperature, conductivity, pH, Secchi depth, spatial distribution of nutrients, phytoplankton productivity and chlorophyll-a were investigated. Due to windy weather conditions in the summer, temperature stratification, temporary oxygen depletion and a weak halocline appeared only at deep and sheltered localities. The nutrient concentrations (total phosphorus and total nitrogen) in surface water were relatively high in the area, indicating eutrophic conditions. High nutrient concentrations in bottom waters occurred at the stratified sampling sites. The primary production measurements, chlorophyll-a analyses and the Secchi depth values indicate a change since 1979 when a similar investigation was made.
The hydrography and water quality were also investigated in Lake Vargsundet, the deepest archipelago lake (maximum depth over 30 m) of Åland, in the summer of 1990. The lake was thermally stratified and salt water penetrating from the recently dredged outflow to Bodafjärden caused a halocline at 15 m. Oxygen conditions were poor in the summer (total oxygen depletion below 8 m in August). A bloom of the potentially toxic cyanobacterium Oscillatoria agardhii occurred above the thermocline and caused high chlorophyll-a concentrations in July. The total phosphorus and total nitrogen concentrations were high in the epilimnion indicating eutrophic conditions. Very high nutrient concentrations occurred in the bottom water, indicating a strong internal loading. According to this study it is of great importance to avoid further dredging of the outflow. Enlargement of the canal would cause deteriorate consequences to the lake and the outside archipelago area.
The hydrography and water quality were also investigated in Lake Vargsundet, the deepest archipelago lake (maximum depth over 30 m) of Åland, in the summer of 1990. The lake was thermally stratified and salt water penetrating from the recently dredged outflow to Bodafjärden caused a halocline at 15 m. Oxygen conditions were poor in the summer (total oxygen depletion below 8 m in August). A bloom of the potentially toxic cyanobacterium Oscillatoria agardhii occurred above the thermocline and caused high chlorophyll-a concentrations in July. The total phosphorus and total nitrogen concentrations were high in the epilimnion indicating eutrophic conditions. Very high nutrient concentrations occurred in the bottom water, indicating a strong internal loading. According to this study it is of great importance to avoid further dredging of the outflow. Enlargement of the canal would cause deteriorate consequences to the lake and the outside archipelago area.