Impact of acidification on diatoms and chemistry of Dutch moorland pools

Old (ca 1920) and recent (1978) diatom assemblages from sixteen pristine moorland pools are compared by analysis of pH-spectra, diversity (Hill's index), (dis)similarity (number of species in common, Dyer dissimilarity) and principal component analysis. The pH-spectra of clear water pools indicate that the formerly wide range of pH (4–6) is very narrow now (3.7–4.6). No significant change of pH (ca 4.5) is observed in brown water moorland pools. Diversity significantly declines in clear water pools and has a tendency to rise in brown water pools. The number of species in common does not change. The Dyer dissimilarity significantly decreases in clear water lakes, no change is found in brown water lakes. The first principal component (PC 1) explains 61% of total variance between samples and is nearly completely determined byEunotia exigua. PC 1 has a strong positive correlation with both the absolute and relative sulfate concentration and other factors related to acidification (Ca²⁺, Al³⁺, Mg²⁺, electrical conductivity). PC1 has a negative correlation with factors characterizing humic acid waters (K MnO₄-consumption, Fe, (Na⁺ + K⁺)/ (Ca²⁺+Mg²⁺) etc.). Old samples have low scores on PC 1. Recent samples from clear waters have high scores on PC 1. The original variation, caused by regional factors, is replaced by a sulfate controled variation. The factors which are responsible for the recent differences in sulfate concentrations between pools are discussed (sulfate reduction, accumulation by dry deposition in adjacent pine forests and drought). Some observations contribute to the opinion that acidification may be considered as eutrophication and not as oligotrophication.