We examined a sediment record from Lake Hiidenvesi in southern Finland
using paleolimnological methods to trace its limnoecological history. In
our record, beginning from the 1940s, chironomid (Diptera) assemblages
shifted from typical boreal taxa towards mesotrophic community
assemblages at ~ 1960–1980 CE being finally replaced by eutrophic taxa
from the 1990s onward. The diatom (Bacillariophyceae) assemblages
reflected relatively nutrient rich conditions throughout the record
showing a further increase in eutrophic taxa from the 1970s onward. A
chironomid-based reconstruction of late-winter hypolimnetic dissolved
oxygen (DO) conditions suggested anoxic conditions already in the 1950s,
probably reflecting increased inlake production due to allochthonous
nutrient inputs and related increase in biological oxygen consumption.
However, the reconstruction also indicated large variability in
long-term oxygen conditions that appear typical for the basin. With
regard to nutrient status, chironomid- and diatom-based reconstructions
of total phosphorus (TP) showed a similar trend throughout the record,
although, chironomids indicated a more straightforward eutrophication
process in the benthic habitat and seemed to reflect the intensified
human activities in the catchment more strongly than diatoms. The DO and
TP reconstructions were mostly similar in trends compared to the
measured data available since the 1970s/1980s. However, the increase in
TP during the most recent years in both reconstructions was not visible
in the monitored data. The results of our multiproxy study emphasize the
significance of including both epilimnetic and hypolimnetic systems in
water quality assessments and provide important long-term
limnoecological information that will be useful in the future when
setting targets for restoration.