Verzuring van vennen : een tijdsverschijnsel

anglais. Changes in chemistry, particularly pH, of a number of soft-water moorland pools in The Netherlands are described. Several groups of organisms, including diatoms, and direct measurements are used to infer past pH values. Diatoms were studied from preserved samples from the beginning of the 20th century, from recent samples and from sediment cores. Old data about chemistry, macrophytes and human impact were obtained from published and unpublished documents.In isolated pools in nature reserves the pH declined to c. 4 from initial values between 4 and 6 at the beginning of this century. Small inputs of nutrients by agriculture, fisheries or swimming retards acidification. The diversity and dissimilarity of diatom assemblages decrease by acidification, particularly in clear water pools. Acidification boosts the growth of the diatom Eunotiaexigua .Sulphate concentrations were extremely high in 1977 and 1978 in two clear water pools where more than half of the bottom surface was exposed to the atmosphere during the extremely dry summer of 1976 and decreased afterwards, parallel to Juncusbulbosus . E.exigua decreased with a delay. Apparently reduced sulphur compounds, which accumulated in the bottom due to atmospheric deposition in the decades before, were oxidized in the dry summer of 1976. No such changes were seen after the dry summer of 1921. In these two pools the pH declined from 5-6 in 1920 to c. 4 in c. 1980. In a brown water pool, where only a small fraction of the bottom was exposed in 1976, changes by the drought were small. Also the decline of pH in this pool since 1920 (c. 0.5 unit) was relatively small.Palaeolimnological studies were conducted in three pools, which were supposed to be pristine. However, it appeared both from coring results and documented evidence that these pools had pH values between 4 and 5 in the early 19th century and became subsequently more alkaline by nutrient enrichment. After 1900 the pools were acidified because anthropogenic enrichment was finished and acid atmospheric deposition increased.To follow the effect of future changes of acid atmospheric deposition and understand the processes involved, chemical and biological monitoring should continue. Input of buffering substances in some moorland pools could maintain weakly acid conditions in Dutch surface waters.

néerlandais; flamand. Via uitgebreide bestudering van studies van derden gecombineerd met eigen onderzoekingen wordt de ontwikkeling van de verzuring van vennen gedurende de laatste 70 jaar weergegeven en zijn de invloeden op deze verzuring geanalyseerd