The humic lake acidification experiment (HUMEX): Main physico-chemical results after five years of artificial acidification

The HUMEX-project is a whole catchment manipulation experiment where the effects of the addition of H2SO4 and NH4NO3 to a humic-rich lake, Lake Skjervatjern, and its catchment were studied. The lake was physically divided into an experimental lake (Basin A) and a control lake (Basin B). Two years after the division, Basin A and its catchment were artificially acidified. Hydrological data, meteorology, precipitation, and runoff chemistry collected during a 2-y pre-acidification period and during 5 y of acidification were evaluated. Randomized intervention analysis (RIA) was used to evaluate statistical significant differences between runoff chemistry from the two basins before and after the acidification. RIA showed significantly higher concentrations of SO42- H+, NH4+, NO3-, Al-n+, Ca2+, Mg2+, total reactive Al (RAL), and labile Al (LAL) in Basin A after treatment compared with the control basin. After the treatment, the acid neutralizing capacity (ANC) in Basin A was significantly lower than in Basin B. However, the average ANC is substantially higher in the control basin after acidification compared with the two years before acidification, while unchanged in the manipulated catchment. The main reason for this is the long lasting effect of Na leakage after seasalt-episodes. No significant changes were observed regarding the amount of total organic carbon (TOC), water color, or UV-absorbency after the treatment, but the anion deficiency (A(-)) was significantly lower in the treated basin. This indicates that the organic acids are more protonated in the treated basin compared with the control basin. After a cold winter in 1993/94, an extreme NH4+ increase was observed in runoff water from Basin A. This increase was accompanied by increases in water color and UV-absorbency, but without any increase in TOC.