Tilstandsvurdering av 3 kroksjøer i Vannområde Leira-Nitelva 2019

The water district Leira-Nitelva includes several oxbow lakes, of which three are situated at the Leira floodplain in Skedsmo and Fet municipalities. The purpose of this project has been to assess the ecological status of the 3 oxbow lakes Stilla, Ringstilla and Tomtestilla, to suggest potential measures to improve the ecological status, and to discuss potential restoration measures. The flow of water between the river and the oxbow lakes is low. The oxbow lakes are turbid and eutrophic (Stilla) or hypereutrophic (Tomtestilla and Ringstilla). The water chemistry indicated poor status in Stilla and bad in Ringstilla and Tomtestilla. All three lakes were surrounded by helophyte vegetation, which was particularly luxurious in Tomtestilla and Ringstilla. The vegetation coverages showed only minimal increase from 1990 to today. Stilla had the highest diversity of aquatic macrophytes, and several submerged species were recorded. The diversities in Tomtestilla and Ringstilla were low, and the vegetation mainly consisted of floating leaved and free-floating species. Based on the TIc-index developed for lakes, the ecological status was assessed as poor in Stilla and bad in Ringstilla and Tomtestilla. Also micro-crustaceans had highest diversity in Stilla,. No micro-crustacean indices are available, but the species compositions in the three lakes reflect the increasing degree of eutrophication from Stilla via Ringstilla to Tomtestilla. We used eDNA to determine the potential existence of the amphibia Triturus cristatus, Lissotriton vulgaris, Rana arvalis, Rana temporaria and Bufo bufo. All samples were negative. However, certain detection of these species with eDNA can only be done in the mating season in May or early June. The overall ecological status shows poor status in Stilla, and bad status in Ringstilla and Tomtestilla. The main reason for the low status in the oxbow lakes is most probably nutrient input from the surroundings, combined with low exchange of water with the main river. Hence, reduced nutrient inputs from agriculture and urban areas are important measures to improve the status. Focussing on the surveyed species, we have discussed restauration measures and indicated important factors for maintaining the biological values in these waterbodies. Before initiating new actions, we suggest thoroughly discussing the main goals of the present management regime, and to consider a revision of these.