Inferring recent changes in the ecological state of 21 Danish candidate reference lakes (EU Water Framework Directive) using palaeolimnology

Abstract

Summary The European Water Framework Directive (WFD) requires that all natural European waterbodiesshould 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. 2. We describe the changes in ecological state in 21 Danish lakes using  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. 3. 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. 4. 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. 5. 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. 6. 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.

Disentangling natural and anthropogenic drivers of changes in a shallow lake using palaeolimnology and historical archives

“…Ayres et al, 2008) but most are multi-proxy (e.g. Bjerring et al, 2008;Davidson et al, 2013). Here, we present a complete multi-proxy sequence of lake changes from pristine conditions which existed here relatively recently comparing with other European lakes.…”

“…Due to the short history of human impact in the catchment the lake started to deviate from 'reference conditions' in the 19 th century, which is relatively late compared to other parts of Europe (e.g. Bjerring et al, 2008).…”

“…Leira et al, 2006;Bennion & Battarbee, 2007;Bjerring et al, 2008;Bennion et al, 2011a, b). Many palaeolimnological studies have used diatoms to infer the amount of nutrients (DI-TP) and changes in taxonomic composition (Leira et al, 2006;Bennion et al, 2011a, b), although there are some problems with applying diatom transfer function for shallow lakes (Bennion et al 2011a, b).…”

“…Many palaeolimnological studies have used diatoms to infer the amount of nutrients (DI-TP) and changes in taxonomic composition (Leira et al, 2006;Bennion et al, 2011a, b), although there are some problems with applying diatom transfer function for shallow lakes (Bennion et al 2011a, b). They use inter-site comparisons (Leira et al, 2006;Bjerring et al, 2008) or detailed investigations of a single site (e.g. Davidson et al, 2005).…”

“…According to historical archives and published datasets, Lake Rotcze had previously only sparse macrophyte cover but lost its ''pre-disturbance'' condition relatively recently (Kowalewski, 2013) when changes in the catchment resulted in the establishment of macrophytes in the lake. Therefore, tracking macrophyte succession in this lake offers a unique opportunity to study the influence of both natural and anthropogenic activity starting from an undisturbed condition, which can be considered to represent 'reference conditions' as required by the WFD (Bennion & Battarbee, 2007;Bjerring et al, 2008).…”