Palaeolimnological assessment of the reference conditions and ecological status of lakes in Estonia - implications for the European Union Water Framework Directive

The European Union Water Framework Directive (WFD) requires an assessment of reference conditions for lakes, i.e. the conditions expected with only minimal human impact on water bodies. Limnological monitoring records seldom go back more than a few decades and so rarely document the onset of human impact on lakes. Methods of palaeolimnological approaches especially fitted for the purposes of the WFD are described and two case studies, on lakes Rõuge Tõugjärv and Pappjärv, are presented. The palaeolimnological study of Rõuge Tõugjärv demonstrated that a commonly held belief that man-made eutrophication of Estonian lakes is a relatively modern matter of concern and is related to post-industrial population growth and intensification of agriculture is a misconception. The lakes, particularly those in rich soil areas, have been mediated by human impact over millennial time-scales. In many European countries it has been agreed that AD 1850 approximately represents the reference conditions for lakes. Our observations in Rõuge Tõugjärv showed that during that period anthropogenic disturbance on the lake was the greatest. Lake Pappjärv is an example of recent human influence on the aquatic ecosystem that has undergone severe degradation due to infiltration into the ground of a variety of substances from the local bitumen plant, mineral fertilizer storage tanks, and road service sand and salt mixing-grounds that have been accumulating in the lake since the 1950s

Sedimentary record of heavy metals in Lake Rõuge Liinjärv, southern Estonia

Anthropogenic impact on Lake Liinjärv (Rõuge, southern Estonia) was studied back to the mid-19th century on the basis of heavy metals (Pb, Cu, Zn, Mn, and Hg) and geochemical parameters of a short sediment core dated by 210Pb isotopes. The development of the lake and its sediment composition are heavily influenced by the inflow of saturated calcareous waters that cause precipitation of calcium carbonates. The concentrations of most of the metals started to increase at the end of the 1970s. This is most clearly observable for Zn, Cu, and Pb. At the same time the distribution pattern of Mn seems to be controlled mainly by the redox conditions in the hypolimneon. The main sources of pollutants in Lake Liinjärv, due to its large catchment area, are the influence of agricultural activity and atmospheric input. Organic matter is the main factor affecting heavy metal (Pb, Hg, Cu, and Zn) distribution in lake sediments

Palaeolimnological assessment of environmental change over the last two centuries in oligotrophic Lake Nohipalu Valgjärv, southern Estonia

The main objectives of this study were to reconstruct the environmental conditions for a small oligotrophic lake during the last two centuries, to determine if the environment of the lake was anthropogenically mediated, and to assess the pre-impact reference conditions with palaeolimnological techniques. A short sediment core from Lake Nohipalu Valgjärv was analysed in detail for diatom assemblages as well as for loss-on-ignition measurements. Accurate chronology of the sediment core was established and evaluated by different independent approaches – 210Pb, 137Cs, and 241Am dating, and the distribution of spheroidal fly-ash particles in sediments. Quantitative inference models based on sedimentary diatoms were applied to reconstruct changes in past lake water pH. Before the mid-19th century, Nohipalu Valgjärv was an oligotrophic lake with clear water continuously transparent down to the bottom and with rich benthic diatom flora. Since the early second half of the 19th century, presumably as a result of forest logging around the lake, water transparency decreased and benthic diatom productivity diminished, and the lake did not recover any more to natural baseline conditions. Due to peat mining activities in the Meenikunno bog, the quality of lake water has changed during the last two decades. The lowered lake level, deteriorated light climate, and decreased pH are the most important environmental variables that have influenced the lake ecosystem

A radical shift from soft-water to hard-water lake: palaeolimnological evidence from Lake Kooraste Kõverjärv, southern Estonia

The Water Framework Directive (WFD) of the European Union requires the quality of all European water bodies to be examined, and aims to achieve good status by 2015. This study was initiated to assess whether a potential reference lake for identifying lake-type specific reference conditions meets the WFD requirements, of being minimally impacted by human activity during the last centuries. The sediments of Lake Kooraste Kõverjärv were analysed for diatom assemblages and sediment composition; past changes in the lake-water pH and total phosphorus were reconstructed, using quantitative models on sedimentary diatoms. The chronology of sediments was established, using spheroidal fly-ash particles stratigraphy. Palaeolimnological investigations, supported by information from historical maps, revealed that man-made changes around the lake have severely influenced its ecological conditions. The lake, which had been oligotrophic with soft and clear water before the mid-17th century, has been trans­formed into a hard-water lake by modifications to the inflow and outflow. The lake water quality has also been altered by the infiltration of nutrients from a nearby hypertrophic lake that was used for flax retting since the 19th century. Although the ecological status of the lake has remained good despite all these changes, it is still questionable whether to nominate it as a reference lake for stratified hard-water lake types

Palaeoecological evidence of agricultural activity and human impact on the environment at the ancient settlement centre of Keava, Estonia

Peat record from a very small basin, the former mire of Verevainu, in the nearest vicinity of ancient Keava settlements (8th-11th centuries) and hillforts (5th–13th centuries), was investigated by means of palaeoecological approach, namely by pollen, charcoal, and loss-on-ignition analyses and radiocarbon dating. The study aimed at ascertaining the appearance of prehistoric man in the area and reconstructing the local vegetation history and human impact on the environment around the inhabitation centre. The first sporadic cereal pollen grains appeared in the sediments in the Late Bronze Age at ca 1500 BC. Both peat ignition residue values as indicators of topsoil erosion and pollen evidence suggest forest clearance, opening of landscape, and cultivation of cereals from the Pre-Roman Iron Age. Approximately at AD 350–500 the rate of human impact upon environment increased notably

Late glacial multiproxy evidence of vegetation development and environmental change at Solova, southeastern Estonia

Reinvestigation of the late glacial Solova (Remmeski) basin, based on plant macro­fossil and diatom record, AMS 14C chronology and sediment composition (loss-on-ignition and magnetic susceptibility data), provided information on vegetation history and palaeoenvironmental and palaeoclimatic changes since the time of the deglaciation of the area around 14 000 cal yr BP. The chronology of the sequence is based on seven AMS dates on terrestrial macrofossils, providing evidence of rapid sedimentation in between 14 000 and 13 500 cal yr BP. Loss-on-ignition data show a clear short-lived warming episode centred to 13 800 cal yr BP, tentatively correlated with the GI-1c warming of the event stratigraphy of the Last Termination in the North Atlantic region, which suggests that at least parts of the Haanja Heights were ice-free by 14 000 cal yr BP. Macrofossil evidence indicates Betula nana–Dryas octopetala-dominated open tundra communities with Saxifraga on dry ground, and Carex sp. and Juncus on wet ground at that time. The first evidence of the postglacial presence of tree birch (Betula pendula) in Estonia is dated back to 13 500 cal yr BP. However, conifer remains were not found in the late glacial sediment sequence of Solova Bog. The late-Allerød (GI-1a) organic deposits, which are quite typical of other parts of Estonia and indicate general warming, are missing at Solova, most probably due to a hiatus in sedimentation in this very small and shallow upland basin