Browsing by Subject "ahven"

The main motivator of this thesis was to discover the importance of Chaoborus in a eutrophic and dystrophic lake and evaluate suitable restoration methods to enhance the state of the study lake. The role of Chaoborus in the study lake, Lake Jouttenus, was studied with sampling of both the water column and the sediment, echo-surveys, and diet analyses of fish. The sampling was planned as comprehensive, and the sampling stations were distributed across the whole lake area to examine the density and distribution of Chaoborus. The deeper areas of the lake were emphasized more in sampling because Chaoborus tend to favor those areas. The Chaoborus density was calculated with stratified sampling, which gives a more precise mean density estimate than simple random sampling. In addition, fish were caught on four (4) study occasions to find out if they had included Chaoborus in their diets. The mean density of Chaoborus in Lake Jouttenus was a little lower than expected, only 271 individuals/m2 in areas ≥2 m depth. The highest density of larvae was found from mid-depths between 6.0-7.9 m in the sediment. Only the deepest areas (≥8 m depth) had limnetic Chaoborus and more limnetic than benthic larvae. The mean length of larvae was 8.4 mm in the sediment and 9.0 mm in the water column. The length distributions appeared to be unimodal. Echo-surveys confirmed that the larvae occurred in the water column only in the deepest area in the north of the lake where the hypolimnion had a low oxygen concentration below 4 m depth. The diet analyses showed that roach and perch had eaten Chaoborus but the number of Chaoborus was high only in the diet of roach in early July. In comparison with other studied lakes, the density of Chaoborus was the lowest in Lake Jouttenus. In addition, the mean length of benthic Chaoborus in Lake Jouttenus was lower than in the other studied humic lakes. The distribution of limnetic Chaoborus appeared as restricted to the deepest areas in the lake and elsewhere the larvae occurred only in the sediment. The slow growth of Chaoborus and their low mean density could be explained by the lack of an efficient refuge and probably also the lack of resources. The larvae were unable to reach the epilimnion and their prey safely at daytime and/or the amount and quality of food items for the larvae were poor. The darkness caused by humic substances and low oxygen concentration in the hypolimnion created a refuge for the limnetic larvae only in the deepest area of Lake Jouttenus. Roach and perch ate Chaoborus occasionally. However, mass removal of fish is not recommended as it would decrease the predation pressure by fish on Chaoborus and increase the risk of Chaoborus population growth at the deepest areas and enable their range to extend. Instead of mass removal of fish all methods that aim in reducing the humic substances in the water especially at the lake catchment area might enhance the state of the lake. The clarification of water would diminish the living conditions for Chaoborus in long-term and help with controlling the other troubling factors such as Gonyostomum semen blooms in the study lake.