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

Fluxes of major ions and nutrients were measured in the watershed-lake ecosystem of a strongly acidified lake, Plešné jezero (Plešné Lake), in the Czech Republic in hydrological years from 2001 through 2005. The lake is situated in a Norway spruce forest and has a steep watershed between elevations of 1090 and 1378 m. The average water input and output from the ecosystem was 1372 mm and 1157 mm (37 L km−2 s−1), respectively, and the water residence time averaged 306 days. Despite ecosystem recovery from acidification occurring since the late 1980s, the Plešné watershed was an average net source of 25 mmol SO42− m−2 yr−1. Nitrogen saturation of the watershed caused low retention of the deposited inorganic N (< 44% on average) before 2004. Then, the watershed became a net source of 28–32 mmol m−2 yr−1 of inorganic N in the form of NO3− due to climatic effects (a dry summer in 2003 and a cold winter in 2004) and forest dieback caused by a bark beetle attack in 2004. Nitrogen transformations and SO42− release were the dominant terrestrial sources of H+ (72 and 49 mmol m−2 yr−1, respectively) and the watershed was a net source of 24 mmol H+ m−2 yr−1. Ionic composition of surface inlets showed seasonal variations, with the most pronounced changes in NO3−, ionic Al (Ali), and DOC concentrations, while the composition of subsurface inlets was more stable. The in-lake biogeochemical processes reduced on average 59% of the incoming H+ (251 mmol H+ m−2 yr−1 on a lake-area basis). NO3− assimilation and denitrification, photochemical and microbial decomposition of allochthonous organic acids, and SO42− reduction in the sediments were the most important aquatic H+ consuming processes (358, 121, and 59 mmol H+ m−2 yr−1, respectively), while hydrolysis of Ali was the dominant in-lake H+ generating process (233 mmol H+ m−2 yr−1). Photochemical liberation from organic complexes was an additional in-lake source of Ali. The net in-lake retention or removal of total phosphorus, total nitrogen, and silica were on average 50%, 27%, and 23%, respectively. The lake was a net source of NH4+ due to a cease in nitrification (pH < 5) and from NH4+ production by dissimilation exceeding its removal by assimilation.