ION MASS BUDGETS FOR SMALL FORESTED CATCHMENTS IN FINLAND

Ion mass and H+ budgets were calculated for three pristine forested ca tchments using bulk deposition, throughfall and runoff data. The catch ments have different soil and forest type characteristics, A forest ca nopy filtering factor for each catchment was estimated for base cation s, H+, Cl- and SO42- by taking into account the specific filtering abi lities of different stands based on the throughfall quality and the di stribution of forest types. Output fluxes from the catchments were cal culated from the quality and quantity of the runoff water. Deposition, weathering, ion exchange, retention and biological accumulation proce sses were taken into account to calculate catchment H+ budgets, and th e ratio between external (anthropogenic) and internal H+ sources. In g eneral, output exceeded input for Na+, K+, Ca2+, Mg2+, HCO3- (if prese nt) and A(-) (organic anions), whereas retention was observed in the c ase of H-+, NH4+, NO3- and SO42-. The range in the annual input of Hwas 22.8-26.3 meg m(-2) yr(-1), and in the annual output, 0.3-3.9 meg m(-2) yr(-1). Compared with some forested sites located in high acid d eposition areas in southern Scandinavia, Scotland and Canada, the catc hments receive rather moderate loads of acidic deposition. The consump tion of H+ was dominated by base cation exchange plus weathering react ions (41-79%), and by the retention of SO42- (17-49%). The maximum net retention of SO42- was 87% in the Hietajarvi 2 catchment, having the highest proportion of peatlands. Nitrogen transformations played a rat her minor role in the H+ budgets. The ratio between external and inter nal H+ sources (excluding net base cation uptake by forests) varied be tween 0.74 and 2.62, depending on catchment characteristics and acidic deposition loads. The impact of the acidic deposition was most eviden t for the southern Valkeakotinen catchment, where the anthropogenic ac idification has been documented also by palaeolimnological methods.