Hindcasting baseline values for water colour and total phosphorus concentration in lakes using sedimentary diatoms — implications for lake typology in Finland

Background and present total phosphorus (TP) concentrations and water colour have been assessed with palaeolimnological methods in 27 lakes in eastern Finland. Sediment samples from core bottoms (sediment depth 29–185 cm) and core tops (sediment depth 0–1 cm) were analysed for diatom assemblages, and TP and water colour were estimated by weighted averaging methods. Changes in DI-TP and water colour between the core bottom and top samples were used for identification of potential reference lakes, as defined by the EU Water Framework Directive. In addition, implications for lake typology in Finland are discussed. Our results suggest that naturally eutrophic lakes may be more common in Finland than previously thought. Water colour, as well as TP, seems to have been subject to considerable changes in many lakes. According to our results, oligotrophic clearwater lakes appear to be little changed from their pre-disturbance conditions, but most of the presently highly coloured lakes have developed towards increasing water colour.