Increased photoreactivity of DOC by acidification: Implications for the carbon cycle in humic lakes

Effects of ultraviolet (UV)-B radiation and acidification on pelagic carbon flux in a humic lake (dissolved organic carbon [DOC] ;15 mg C L21) were studied in a mesocosm experiment during the summer of 2000. Triplicate tanks (107 liters volume, 1 m high) were exposed to natural solar radiation, a daily extra dose of UV-B radiation, or kept dark. One set of tanks was submitted to a decrease in pH (from 5.7 to 4.7), and one set was kept at the natural pH level. During 70 d, water samples were taken regularly from the mesocosms for measurements of DOC, absorbance, dissolved inorganic carbon (DIC), and pH. Additionally, we regularly incubated samples to measure photooxidation rates, primary production, and community respiration. We found an increase in the photooxidation rates in the acidified mesocosms relative to ambient pH. The greater abiotic production of DIC (i.e., photooxidation) in acidified conditions could explain ;27% of the decline in DOC in the same conditions. Laboratory experiments were done to test the effects of pH on the dissolved organic matter (DOM) photoreactivity. At lower pH values, we found both higher abiotic DIC production and specific absorbance fading, relative to neutral pH values in water from a humic lake. In a separate experiment, samples were exposed to prolonged irradiation under laboratory conditions, resulting in complete loss of absorptivity in the wavelengths between 290 and 400 nm. Decreases in DOC in the long-term exposure caused by photochemical mineralization were ;45 and 55% of the initial pool for natural pH and acidified samples, respectively, at the end of the experiment. An increase in the dissolved organic matter photoreactivity by acidification could be an important mechanism to explain the increased water transparency and in-lake DOC removal in acid lakes found in several previous studies. Reductions in stratospheric ozone have resulted in greater amounts of ultraviolet (UV)-B (280–315 nm) radiation reaching the Earth’s surface. This effect is expected to increase and persist for several decades, even if present agreements on substitutions of freons and other ozone-destroying chemicals are enforced effectively. UV-B radiation has a direct negative influence on aquatic organisms through DNA damage. On the other hand, UV-B radiation might have indirect, positive effects through food chain alterations (Bothwell et al. 1994) and through photoproduction of bacterial substrates (Mopper et al. 1991; Lindell et al. 1995; Bertilsson and Tranvik 1998, 2000), as well as inorganic nitrogen and phosphorus (Bushaw et al. 1996; Yiyong 1996), from dissolved organic matter. Although changes in UV-B radiation alone might have environmental consequences in lakes, more severe effects could be expected from interactions between climate change, acidification, and ultraviolet radiation, because drier conditions and acidification could aggravate effects from UV radiation (Schindler et al. 1996; Yan et al. 1996). A decline in dissolved organic carbon (DOC) concentration accompanied by acidification has been observed in several natural systems 1 Corresponding author ([email protected]).