The acidification of the Herrenwieser See, Black Forest, Germany, before and during industrialisation

Past trends in the biology and chemistry of the Herrenwieser See in the base-poor Bunter Sandstone Black Forest were reconstructed from a sediment core. Diatom inferred pH indicated pre-industrial acidification periods with subsequent recovery. For the most recent acidification, multiple regression indicated a pH decline by 0.78 pH units to a minimum of pH 3.91 in the early 1980s; weighted averaging also indicated a pH minimum in the 1980s, but the lowest value was pH 4.58 and the overall decline only 0.30 units. The most important indicator species were Asterionella ralfsii for moderate acidification and Tabellaria quadriseptata for extreme acidification. Community diversity H′ and floristic composition, as shown by cluster analysis, were sensitive to pH changes in the lake. Chlorophyll pigments offered another indicator for pH trends since periods of acidification were accompanied by higher concentrations of chlorophyll derivatives. Acidic inputs into the lake were indirectly detected via analysis of persistent pollutants, metals and polycyclic aromatic hydrocarbons. Characteristic chemical changes indicated a metal burden from atmospheric deposition, and the release of base cations and aluminium from weathering. PAH concentrations rose simultaneously with the pH decline in the 19th century. After the input maximum in the 1960s the flux rates for PAHs decreased and are now < 20% of their peak value. The simultaneous trends in biological indicators and persistent pollutants demonstrate that recent acidification was caused by atmospheric acid deposition. So far there has been no significant reverse trend despite signs of recent changes indicated by the diatoms. However, pre-industrial acidification and recovery were also related to acid deposition, and indicate that reversal is possible in this lake.