Changes of Humic Substance Constituents in Großer Arbersee during Acidification

Abstract

Sediment humic substances from an acidification chronology (210Pb‐ and 137Cs‐dating) of Großer Arbersee, a small cirque lake in the Bavarian Forest, Germany, have been studied by means of 1H NMR spectroscopy. The acidification of the lake and, particularly, its catchment caused a more or less continuous decrease in diatom‐inferred pH from 5.4 to 4.75. Combined with diagenetic processes in the sediment, acidification reduced the bulk concentration of humic material in the water column and altered the composition of humic material, too: The share of humic acids (IHSS protocol) increased from 12 to 17% of the total humic carbon. Statistically, the reduced pH values account for 60%. While the constituents changed in the fulvic acid fraction, they remained more or less unchanged in the humic acid fraction. Within the fulvic acid fraction, percentages of carbohydrates increase, whereas those of aliphatic as well as aromatic compounds decrease. These changes may be explained (a) by reductions in enzymatic activity, particularly of cellulases and proteases, in the raw humic material in the catchment, (b) protonation of acidic functional groups (carboxylic acids, phenolic compounds) which leads to an immobilization and retention of these compounds in the catchment, or/and (c) by increased chemical oxidation or precipitation of the aromatic portions, leaving the more transparent aliphatic chains. As a consequence of reduced chromophoric DOC content, light penetration, particularly of UV irradiation, has increased and the biogeochemical detoxification mechanism will be reduced. The residual fauna and flora in acidified Großer Arbersee will be exposed to another severe physical and chemical stress, because acidification is generally accompanied by a variety of organic micro‐contaminants.