Fossil midge larvae (Diptera : Chironomidae) as quantitative indicators of late-winter hypolimnetic oxygen in southern Finland : a calibration model, case studies and potentialities

We examined the relationship between fossil chironomid assemblages and environmental variables in 30 lakes in southern Finland with particular attention to hypolimnetic oxygen.Ordination techniques (DCA, CCAs) and Monte Carlo permutation tests were used to identify the most important parameters controlling the chironomid distributions.Based on the 1 : 2 ratios of the variables, the greatest potential for quantitative inferences was with hypolimnetic oxygen ( 1 : 2 = 0.767).Therefore, inference models for past late-winter hypolimnetic oxygen were developed using weighted-averaging partial least squares (WA-PLS) techniques.The best model used one WA-PLS component, with a cross-validated coefficient of determination (r 2 jack ) of 0.72 and a root-mean-squared error of prediction (RMSEP) of 2.351 mg l -1 .The model was tested on two short-core sediment sequences.The results indicated that quantitative estimations of hypolimnetic oxygen changes provide a useful tool for palaeolimnology and lake management in assessments of lake ecosystems, and in restoration projects in southern Finland.