Vannkjemisk utvikling og biologisk tilstand etter kalkslutt i innsjøer i Hedmark

Abstract

Kalkingsprogrammet i Hedmark ble avsluttet i 2013. I etterkant bIe det iverksatt overvåking av vannkjemi, bunndyr, fisk og kreps i tidligere kalkede vannforekomster. Kalsiumnivået er redusert siden kalkingen ble avsluttet, men var i perioden 2015-2016 fortsatt høyere enn antatt ukalket nivå, særlig i sørlige deler av fylket. pH har blitt redusert, men ikke til nivåer som tilsvarer «moderat» eller dårligere tilstand i følge vannforskriften. Det samme gjelder syrenøytraliserende kapasitet (ANC). Konsentrasjonen av labilt aluminium (LAl) var lav i de fleste innsjøer. Bunndyrfaunaen indikerte «god» tilstand i 33 av 50 lokaliteter. De klareste indikasjonene på reforsuring ble påvist i Nord-Odal/Åsnes-området. Det gjaldt også for fiskebestandene som i tillegg viste negativ trend i øvre deler av Flagstadelva. I tre av fire undersøkte lokaliteter med edelkreps har bestandene ikke klart å bygge seg opp igjen etter å ha vært skadet av forsuring.

In 2013, all state funded liming of lakes in Hedmark was stopped. In this survey, the aim has been to document changes in water chemistry and the state of benthic macroinvertebrates, crayfish, and fish communities after termination of liming. The concentration of sulphate and nitrate was low and close to expected background level. Calsium has decreased since liming was terminated, but is still higher than the expected unlimed level, especially in southern parts of the county. This is most likely caused by long-term effects of sedimented lime, as shown in other surveys. The pH is reduced, but not to levels that correspond to "moderate" or poorer condition according to the national classification system, cf. the European Union’s Water Framework Directive. The same applies to acid-neutralizing capacity (ANC) where this was determined, and the concentrations of labile aluminum (LAl) were with few exceptions low. The state of the benthic macroinvertebrate fauna was assessed as “good” in 33 of 50 investigated lakes and “moderate” or worse in 12 lakes using “Forsuringsindeks 1”. The worst conditions were found in southern parts of the county, especially in the Nord-Odal and Åsnes area. In the upper parts of River Flagstadelva, the density of brown trout has decreased in the period 2014-2016, a trend that might be caused by changes in water chemistry after termination of liming. Crayfish (Astacus astacus) populations were examined in four localities historically affected by acidificatio. Historical data are available for three of the stocks, and they have not recovered.