Identifying reference conditions for dimictic north German lowland lakes: implications from paleoecological studies for implementing the EU-Water Framework Directive

Using published paleolimnological results from 14 dimictic calcareous lakes, this study identifies total phosphorous (TP) reference values for the European lake type CB 1. The initial increase in settlement-associated pollen occurred in the catchments between ad similar to 1000 and similar to 1820. A departure from diatom-inferred TP reference conditions occurred during periods of increased human activities during Early to Late Medieval Times (ad similar to 1110-1325; four lakes), early Modern Times (ad similar to 1575-1600; two lakes), after the 30 years' war (> ad 1650; two lakes) and during the Anthropocene (after ad similar to 1850, three lakes). Only one lake continuously has TP reference values until recent days, whilst TP reference values could not be detected in two cases. Thus, we refrain from setting a fixed point in time for defining reference conditions for lakes in the European Central Plains. This study also validates TP reference levels calculated based on common lake models for CB 1-lakes and assesses the range of TP reference levels using paleolimnological diatom studies. The highly variable diatom-inferred TP reference levels only partly support the modelled levels. Thus, we recommend using two subtypes (CB 1a and 1b), based on the watershed to volume ratio to better meet the requirements of lake type-specific reference levels.