AN EXPERIMENTAL AND PALEOECOLOGICAL STUDY OF ALGAL RESPONSES TO LAKE ACIDIFICATION AND LIMING IN 3 CENTRAL SWEDISH LAKES

Contemporary phytoplankton and palaeolimnological studies were made of the algal response to acidification and liming in three lakes in Hals ingland, central Sweden (Njupfatet, Sjosjon, Djuptjarn). Surveys and e xperimental studies of the phytoplankton response to liming were under taken at Njupfatet, together with an experiment designed to determine the possible role of lake sediments as an inoculum for any new species arriving in the water column. Liming had little quantitative or quali tative effect on the phytoplankton diversity at Njupfatet, but did res ult in the loss of the dominant contributor to algal biomass, Merismop edia tenuissima. None of the four new species recorded in the lake fol lowing liming was hatched from the sediment inoculum experiment. A fre eze core from Njupfatet was dated by Pb-210 and carbonaceous fly-ash p article profiles were also determined for Njupfatet and Sjosjon, which provided an approximate chronology for the latter lake. Diatom analys es were made of the three lakes and pH and dissolved organic carbon in ferred using weighted averaging methodology. Njupfatet had no plankton ic diatoms over the last 200-300 years, and was dominated by benthic d iatoms, but the diatom-inferred pH suggests that the lake was acidifie d to pH < 5 prior to liming in 1989. Both Djuptjarn and Sjosjon were d ominated by planktonic diatoms, and were acidifying from (c. 1970) pri or to liming, Sjosjon from pH 6 to similar to 5 and Djuptjarn from 65 to similar to 5.5. These pH inferences suggest that at least some lake s in this region are susceptible to atmospheric acidification. The res ponse of the diatoms to liming is discussed, in particular the rapid e xpansion at Njupfatet, of Cyclotella glomerata a diatom which was also present in Sjosjon for a period prior to acidification, and at other lakes in south-west Sweden. Possible reasons for the expansion of this small centric diatom are discussed.