RECENT PALEOLIMNOLOGY OF 3 SHALLOW DANISH LAKES

Recent eutrophication histories of three shallow lakes (mean depths < 3 m) were studied using palaeolimnological methods. Freeze-cores were dated using Pb-210, Cs-137, Cs-134 and Am-241 Resultant chronologies were problematical at two sites (Veang So and Vesterborg So) due to sediment-water interface mixing, indicated by uniform CS-137 profiles over the surface 20-30 cm. Sediments at Langeso and Vesterborg So have a high carbonate content, which together with the high mixing rate have resulted in diatom dissolution below 30 cm at Vesterborg So. Diatom stratigraphy indicates relatively small biological changes at both Vaeng So and Langeso; both lakes have been eutrophic for at least the last 150-200 years. Vaeng So is dominated by planktonic diatoms together with high percentages of benthic Fragilaria spp., and Langeso by planktonic diatoms, especially Cyclostephanos dubius and Stephanodiscus parvus. Epilimnetic phosphorus concentrations were inferred using weighted averaging; at Vaeng So the diatom-inferred TP (DI-TP) concentrations were close to observed values in the early 1980s, but failed to record the post-biomanipulation decrease to 55 mug l-1 after 1988, presumably due to the smoothing of the sediment record by resuspension and mixing. At Langeso two increases in DI-TP suggest a two-phase enrichment of the lake, initial eutrophication approximately 1880, and hypertrophy after 1950. The recent DI-TP values are lower than the annual observed values, and reasons for this are discussed. Despite problems associated with sediment mixing, the sediment records of these shallow systems can still be used for monitoring and for environmental reconstructions.