The key controls on allochthonous DOC quality, quantity, and catchment export in response to climate change are still not fully understood. More detailed knowledge of these processes is required so that changes in DOC and its interactions with nutrients and trace metals can be better predicted based on changes caused by changing climate. More studies are needed concerning the effects of trace metals on DOC, the effects of changing DOC quality and quantity on trace metals and nutrients, and how runoff and temperature-related changes in DOC export affect metal and nutrient export to rivers and lakes.
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
Photochemical transformation of dissolved organic matter (DOM) has been studied for more than two decades. Usually, laboratory or “in-situ” experiments are used to determine photodegradation variables. A common problem with these experiments is that the photodegradation experiments are done at higher than ambient temperature. Five laboratory experiments were done to determine the effect of temperature on photochemical degradation of DOM. Experimental results showed strong dependence of photodegradation on temperature. Mathematical modeling of processes revealed that two different pathways engaged in photochemical transformation of DOM to dissolved inorganic carbon (DIC) strongly depend on temperature. Direct oxidation of DOM to DIC dominated at low temperatures while conversion of DOM to intermediate particulate organic carbon (POC) prior to oxidation to DIC dominated at high temperatures. It is necessary to consider this strong dependence when the results of laboratory experiments are interpreted in regard to natural processes. Photodegradation experiments done at higher than ambient temperature will necessitate correction of rate constants.
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