Phytoplankton in Swedish oligotrophic lakes, affected by acidification, metals and liming

Abstract: Anthropogenic airborne acidification has seriously affected thousands of lakes in Sweden, an alarming fact which was first noticed in the early 1970s, when pH had decreased to values below 5, especially in areas poor in limestone such as the Swedish west coast. This development was critical to phytoplankton, the diversity of which being markedly reduced, and where the groups Cyanophyceae, Bacillariophyceae and Conjugatophyceae were totally eliminated. A study of 500 lakes showed that this change was largely a result of low pH, since rather few species are acidophilic or acidotolerant. Secondary effects of acidification, such as phosphorus depletion and aluminium toxicity, however, were just as important, giving a combined stress upon the whole phytoplankton community, resulting in reduced species diversity. A large-scale remedial operation started in the middle of the 1970s, when the pH in many acidified lakes was increased to circumneutral values by addition of lime, in order to restore the original biotic communities. In several west coast lakes studied during 1973-1992, where the pH-values increased from 4.5-5.5 to 6.8-7.6, the treatments decreased the Al-levels and increased the concentrations of humic compounds and phosphorus in parallel with the decreased water transparency, compared with levels before liming. After a colonization period of 2-5 years, the species richness was fairly similar to that in unacidified lakes, and the species composition achieved indicated increased production. Results from long-term investigations in several limed lakes, and single analyses from other limed lakes, altogether making up a total of 63 limed lakes studied, were compared with the phytoplankton composition in 500 unlimed lakes. This showed that some species, common in unacidified lakes, were still missing several years after liming. Moreover, the original composition was not fully restored, results that could partly be explained by temporarily extreme pH-values and remaining ultraoligotrophic conditions. Regarding the consequences to the lake ecosystem, the absence of a few quantitatively unimportant species, however, does not seem to be a critical shortage.The toxicity of metals is one of the main stress factors depressing phytoplankton in lakes. The increased concentrations of inorganic and labile aluminium, caused toxic conditions that especially affected diatoms, chlorophyceans and desmids, and which were a main factor determining species composition in acidic lakes. Al-toxicity may occur temporarily also in neutral waters. Thus, laboratory tests showed that Monoraphidium griffithii was just as sensitive at pH 7 as at pH 4.8. In unacidified waters, increased concentrations of copper and zinc were recorded in connection with leakage from old mine tailings. During investigations in 41 and 32 lakes in 1992-1993 it was found that concentrations > 3-5 mg Cu l -1 and > 30 mg Zn l -1could be critical to some phytoplankton species. In the most contaminated lakes, with 1100-1500 mg Zn l -1 or 60-65 µg Cu l -1, the number of taxa was decreased by ca 50 %, but several species were frequent, indicating especially high tolerance. The lake waters were biologically characterized by algal assays. In the inorganic medium, the growth of Selenastrum capricornutum was strongly depressed at 1-2 mg Cu l -1, to be compared with 3-6 mg l -1 in lake waters with uncoloured substances, and 13-22 mg l -1 in waters with humic compounds. On the other hand, there was a positive relation between the number of diatom taxa and the manganese concentration.