Assessment of ecological status in the Oslo Rivers. Macroinvertebrates and fish in the watershed of River Ljanselva and the River Alna

Summary

The ecological status, according to the Water Framework Directive, was assessed in the rivers Ljanselva and Alna in 2017. In River Ljanselva the ecological statues have gone down compared to the latest surveys in 2004 and 2012, for all but the station highest in up in the water course (LJA1). However, only two stations (LJA2 and LJA5) score nEQR values degrading them to a lower ecological status than found in previous years. The trout (Salmo trutta) density in River Ljanselva showed high productivity at LJA2 and LJA5, whereas low recruitment was found at LJA3 and LJA4, despite high densities of older age classes. Like previous years very little trout was found at LJA1 and no recruitment was registered. In River Alna the ecological states calculated from the macroinvertebrate community show no changes in ecological status and remains at “Very bad” for the four lower stations in the river (ALN3 to ALN6). ALN1 scored the highest ecological status of “Very good”, and ALN2 “Moderate” as previously found. The trout density at ALN1 and ALN2 showed good recruitment and higher densities that found in recent years. Historically trout have only rarely been registered at ALN3 and not farther downstream. In 2017 we found densities of 40.4 and 15.8 fish per 100 sqm at ALN4 and ALN6, respectively. However, recruitment at these two stations seem to be limited.