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Benthic diatom communities in streams and lakes of acid sensitive areas in Central and North Europe (Poland and Finland) experiencing different degrees of anthropogenic stress, were studied. The aim of these studies was to examine the diatom community structure and water chemistry according to water body type (streams, peat bogs, small pools and lakes), altitude (high mountain, mountain, upland and lowland waters) and to different geographical regions. Diatoms were also used to indicate differences in water quality and diatom methods to infer pH were tested. Optimum as well as tolerance values of different taxa for pH, alkalinity and TOC were calculated and ecological notes about acid water taxa were recorded. Diatoms were also used to evaluate and to indicate restoration and recovery of acidiﬁed aquatic ecosystems. Biological recovery of waters in relation to chemical recovery was assessed using periphytic and surface sediment diatoms and the results were campared to those from earlier projects (in mid-1980s and 2001-2002). Finally, the suitability of diatom-based methods for monitoring human impacts and for assessment of water quality was discussed.
ln general, water chemistry in most of the regions investigated was categorized as poorly buffered and acidic, with alkalinities <0.5 mval l-1. However, the regions varied widely in geology, climate, soil topography, land use and anthropogenic inﬂuences, and there were signiﬁcant differences in water chemistry. The Finnish sites were typically small, rocky drainage basin lakes as well as lakes of various sizes within a peaty catchment areas. They represented different water types from weakly to strongly acidic, clear lakes as well as acidic humic lakes with different dystrophy levels. They were oligotrophic, weakly buffered and ion poor lakes. The acidic and usually clear water streams and lakes in the Polish study areas were characterized by higher nutrient concentrations and signiﬁcantly higher conductivities when compared with the Finnish areas. Both number of taxa and evenness varied considerably between the study areas, however, both of these regions generally had low species number when compared with non-acid ecosystems. Several dominant taxa in Finnish soft-acidic lakes were absent in Polish clear acidic waters of the same pH, which were characterized by signiﬁcantly higher conductivity. However, while some diatom communities from different subareas had unique charateristics, many dominant taxa were common, widespread and typical for European acidic waters.
According to the results of the CCA, acidity (pH and alkalinity) and conductivity were the most important factors in determining diatom composition and distribution. The humic substances content, reﬂected by COD, was a somewhat less important factor than acidity and ionic composition. The results, based on periphytic and surface sediment diatoms from different lake types, showed that pH, TOC and alkalinity could be successfully reconstructed from benthic diatom communities. Diatom-inferred pH equations were tested in two Polish lakes and the method was used to study the biological recovery of Finnish acidic lakes after chemical recovery (from decreased deposited sulphur). Correlation between diatom-inferred pH values and measured pH values was strong (r2 = 0.602). Diatom communities clearly indicate an increase in the pH of some Finnish headwater lakes (0.5—0.7 pH units). In Central Finland, however, changes were not clearly demonstrated probably because the natural organic acidity of those lakes was relatively high.