Photochemical, chemical, and biological transformations of dissolved organic carbon and its effect on alkalinity production in acidified lakes
Limnol. Oceanogr., 48(1), 2003, 106-117 | DOI: 10.4319/lo.2003.48.1.0106
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.