Inferring recent changes in the ecological state of 21 Danish candidate reference lakes (EU Water Framework Directive) using palaeolimnology.
The European Water Framework Directive (WFD) requires that all natural European waterbodies should be assigned to one of five ecological categories defining the degree to which present-day conditions deviate from those uninfluenced or only negligibly impacted by anthropogenic activities (the reference condition). By 2015, all relevant waterbodies must have obtained 'good' ecological quality. We describe the changes in ecological state in 21 Danish lakes using AD 1850 as a benchmark for reference conditions. Sediment samples representing 1850, 1900, 1950 and 2000 were analysed for diatom and cladoceran subfossils. Ecological status since 1850 was evaluated using correspondence analysis and dissimilarity measures to assess assemblage changes, and existing transfer functions were applied to infer changes in total phosphorous concentrations from diatoms (DI-TP) and submerged macrophyte coverage (SUB-COV) and benthi-planktivorous fish catch per unit effort (BP-CPUE) from cladoceran subfossils. Eighteen lakes underwent significant changes, most markedly during the past 50-100 years, in either or both diatom and cladoceran community structure. Low floristic and faunal alteration was found only in three lakes; these were, however, already nutrient-rich in 1850. In 1850, most lakes were already characterized by high DI-TP (median of 17 lakes=86 µg TP L-1), high inferred BP-CPUE and low inferred SUB-COV, and these eutrophic conditions still prevail. In addition, the accumulation rate of sediment and cladoceran subfossils and the pelagic dominance of diatoms and cladocerans have increased. When applying the thresholds proposed by a recent WFD classification for Danish lakes to the DI-TP values, only one lake could be described as having a 'good' ecological state with a concurrent low community change since 1850, limited to the cladoceran community, however. This suggests that this lake alone may serve as a potential reference site. Synthesis and applications. Our study, demonstrating the potential of a palaeolimnological approach to assess deviations from reference conditions, suggests that Danish reference lakes may be difficult to find, most probably due to the country's long history of cultural impact. Lake managers consequently face great challenges in their endeavour to ensure 'good' ecological state by 2015. Therefore, further restrictions on land-use and nutrient loading in lake catchments are needed as is the initiation of restoration activities to improve the ecological state of the lakes.