Photochemical, chemical, and biological transformations of dissolved organic carbon and its effect on alkalinity production in acidified lakesJi

Abstract

We evaluated the significance of photochemical and biological degradation of allochthonous dissolved organic carbon (DOC) on in‐lake H+ budgets by laboratory experiments and with a mass budget study for major ions in three atmospherically acidified forest lakes in the Bohemian Forest. In the experiments, photodegradation of DOC from a lake tributary resulted in (1) a liberation of organically bound Al and Fe, which consumed an equivalent amount of H+, (2) a minor decrease in concentrations of organic acid anions (A−) despite a major decrease in DOC concentrations, and (3) the production of biologically available DOC. Biological degradation of the photochemically transformed DOC resulted in a lesser decrease in DOC concentrations than during photodegradation (28—45% of the total decline) but in a pronounced decrease in A− concentrations (64—85% of the total decline), leading to a significant pH increase. Hydrolysis of photoliberated metals under increasing pH partly reduced net H+ consumption within the whole process. Watersheds of the lakes studied exported more SO42−, NO3−, and H+ than they received by throughfall, and the lakes were the dominant acidity‐consuming parts of the whole ecosystems, neutralizing 50—58% of H+ input. In‐lake photochemical, biological, and chemical changes in A− fluxes consumed 56—190 meq m−2 yr−1 of H+ and were the third major internal alkalinity‐producing mechanism after the biochemical reduction of NO3− and SO42− (333—396 and 143—214 meq m−2 yr−1, respectively). In contrast, the hydrolysis of inorganic Al was the dominant in‐lake H+‐producing process (144—340 meq m−2 yr−1). The in‐lake A− removal was positively related to the DOC loading. Consequently, changes in DOC and A− fluxes should not be omitted in alkalinity budgets in lakes with low or no bicarbonate concentration and elevated DOC input.