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

Abstract

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

Stone Age Pottery Chronology in the Northeast European Forest Zone: New AMS and EA-IRMS Results on Foodcrusts

“…All EA-IRMS data from the dated foodcrusts suggest a high freshwater aquatic component, but there is no correlation between foodcrust 14 C ages and EA-IRMS results that might be used to estimate FREs. Highly variable FREs were recorded in studies of modern freshwater fish in Ireland and Germany (Keaveney and Reimer 2012; Philippsen 2013a) and in recent lake sediment upstream of Kääpa (Alliksaar and Heinsalu 2012). As foodcrusts may represent single cooking episodes, such variability might account for the scatter of foodcrust 14 C ages at Kääpa, but we cannot assume that the dated sherds are contemporaneous with each other or the horse tooth.…”

Radiocarbon Dates and Stable Isotope Data from the Early Bronze Age Burials in Riigiküla I and Kivisaare Settlement Sites, Estonia

“…It appears unlikely that freshwater reservoir effects were negligible, however, given the calcareous bedrock and drift geology in Estonia, and there are indications from modern and paleolimnological studies that in some hardwater lakes the reservoir effects may be large (e.g. Olsson and Kaup 2001;Poska and Saarse 2002;Veski et al 2005;Alliksaar and Heinsalu 2012). Poska and Saarse (2002) found differences of ~500 yr between bulk gyttja and terrestrial plant macrofossil 14 C ages.…”

“…Veski et al (2005) estimated an offset of 600-700 yr in the 14 C age of bulk organic matter from Medieval varve-dated sediment. Alliksaar and Heinsalu (2012) found that bulk organic matter from lake sediment independently dated to the last century gave 14 C ages of up to 2000 yr. As the bulk organic matter must include some terrestrial material, the FRE in fully aquatic organisms in these cases must be even greater. The calibrated dates shown in Figure 2 must therefore be regarded only as maximum ages for these burials.…”

Determining reference conditions of hemiboreal lakes in Latvia, NE Europe: a palaeolimnological approach

“…Human activities that vary temporally and spatially leave even more pronounced footprints in lakes (Bradshaw et al, 2005;Battarbee and Bennion, 2011;Weckström et al, 2015;Marzecová et al, 2017). Through eutrophication caused by agricultural activity, soil leaching, over nutrient in-washing of sewage, and manure and fertilizer, biotic processes are changing in the lakes and also affect terrestrial ecosystems (Bellinger and Sigee, 2010;Alliksaar and Heinsalu, 2012;Soininen et al, 2015;Steffen et al, 2015;Mikomägi et al, 2016). It is very likely that, due to the increasing stress from urbanisation and agricultural activity, freshwater ecosystems will continue to face changes in biodiversity and productivity (Schiefer et al, 2013;Douglas and James, 2015;Liu et al, 2016).…”