Changes of humic substance constituents in Grosser Arbersee during acidification

Sediment humic substances from an acidification chronology (Pb-210- and Cs- 137-dating) of GroBer Arbersee, a small cirque lake in the Bavarian Forest, Germany, have been studied by means of H-1 NMR spectroscopy. The acidifica tion of the lake and, particularly, its catchment caused a more or less con tinuous decrease in diatom-inferred pH from 5.4 to 4.75. Combined with diag enetic processes in the sediment, acidification reduced the bulk concentrat ion of humic material in the water column and altered the composition of hu mic material, too: (I) The share of humic acids (IHSS protocol) increased from 12 to 17 % of t he total humic carbon. Statistically, the reduced pH values account for 60 %. (II) While the constituents changed in the fulvic acid fraction, they remai ned more or less unchanged in the humic acid fraction. (III) Within the fulvic acid fraction, percentages of carbohydrates increas e, whereas those of aliphatic as well as aromatic compounds decrease. These changes may be explained (a) by reductions in enzymatic activity, particul arly of cellulases and proteases, in the raw humic material in the catchmen t, (b) protonation of acidic functional groups (carboxylic acids, phenolic compounds) which leads to an immobilization and retention of these compound s in the catchment, or/and (c) by increased chemical oxidation or precipita tion of the aromatic portions, leaving the more transparent aliphatic chain s. As a consequence of reduced chromophoric DOC content, light penetration, pa rticularly of UV irradiation, has increased and the biogeochemical detoxifi cation mechanism will be reduced. The residual fauna and flora in acidified GroBer Arbersee will be exposed to another severe physical and chemical st ress, because acidification is generally accompanied by a variety of organi c micro-contaminants.