We evaluated the significance of photochemical and biological degradation of allochthonous dissolved organic carbon (DOC) on in-lake H ϩ 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 ϩ , (2) a minor decrease in concentrations of organic acid anions (A Ϫ ) 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 Ϫ concentrations (64-85% of the total decline), leading to a significant pH increase. Hydrolysis of photoliberated metals under increasing pH partly reduced net H ϩ consumption within the whole process. Watersheds of the lakes studied exported more SO , NO , and H ϩ than they receivedby throughfall, and the lakes were the dominant acidity-consuming parts of the whole ecosystems, neutralizing 50-58% of H ϩ input. In-lake photochemical, biological, and chemical changes in A Ϫ fluxes consumed 56-190 meq m Ϫ2 yr Ϫ1 of H ϩ and were the third major internal alkalinity-producing mechanism after the biochemical reduction of NO and SO (333-396 and 143-214 meq m Ϫ2 yr Ϫ1
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