Element fluxes in watershed-lake ecosystems recovering from acidification: Čertovo Lake, the Bohemian Forest, 2001–2005

Fluxes of major ions and nutrients were measured in the watershed-lake ecosystem of a strongly acidified lake, Čertovo jezero (Čertovo Lake), in the 2001 through 2005 hydrological years. Water balance was estimated from precipitation and throughfall amounts, and measured outflow from the lake. The average water input into and outflow from the watershed-lake ecosystem was 1461 mm and 1271 mm (40 L km−2 s−1), respectively, and the water residence time in the lake averaged 662 days. The ecosystem has been recovering from acidification since the late 1980s. Still, however, Čertovo watershed was an average net source of 23 mmol m−2 yr−1 of SO42−. Nitrogen saturation of the watershed caused low retention of the deposited inorganic N (23% on average). After a dry summer in 2003 and a cold winter in 2004, the watershed became a net source of inorganic N (19 mmol m−2 yr−1). Nitrogen transformations and SO42− release were the dominant terrestrial sources of H+ (81 and 47 mmol m−2 yr−1, respectively) and the watershed was a net source of 42 mmol H+ m−2 yr−1. Ionic composition of tributaries showed seasonal variations with the most pronounced changes in NO3−, base cations, DOC, and ionic Al (Ali) concentrations. The in-lake biogeochemical processes reduced the incoming H+ by ∼50% (i.e., neutralized on average 222 mmol H+ m−2 yr−1, on a lake-area basis). Denitrification, SO42− reduction, and photochemical and microbial decomposition of allochthonous organic matter were the most important in-lake H+ consuming processes (215, 85, and 122 mmol H+ m−2 yr−1, respectively), while hydrolysis of Ali was the dominant H+ generating process (96 mmol H+ m−2 yr−1) in Čertovo Lake. Photochemical liberation from organic complexes was an additional in-lake source of Ali. The net in-lake retention or removal of nutrients (carbon, phosphorus, nitrogen, and silica) varied between 18% and 34% of their inputs.