Fire-Induced pH Rise in a Naturally Acid Hill-Top Lake, Southern Finland: A Palaeoecological Survey

The effects of catchment fire on lake Pieni Majaslampi are examined by means of geochemical, charcoal, pollen, and diatom analyses of surface sediments. Particular emphasis is paid to pH responses in this naturally acid, weakly buffered, small catchment lake. An increase of c. 0.9 units in diatom-inferred pH correlates with dendro-chronological evidence of catchment fire around 1890. This pH rise may have beer caused by increased turbulent mixing of the water column, as well as increased inputs of clastic erosional material and atmospheric ash, rich in base cations, after the fire. Inferred pH remained high until the 1930s, after which there is a slow recovery of the system at a rate approximately 0.2 pH units per decade. The inferred pH stabilizes to a level slightly higher than the prefire value by the 1960s. High concentrations of S and Fe and elevated Fe/Mn ratios in the upper sediment layer reflect increased in-situ alkalinity generation following fire. The processes controlling the acidity dynamics in small-catchment lakes may be more complex than has hitherto been thought.