Academic journal article Estonian Journal of Ecology

Anthropogenically Induced Changes in the Sedimentation Processes in the Littoral Zone of Lake Verevi, South Estonia/Inimtegevusest Tulenevad Muutused Settimisprotsessides Verevi Jarve Litoraalses Osas

Academic journal article Estonian Journal of Ecology

Anthropogenically Induced Changes in the Sedimentation Processes in the Littoral Zone of Lake Verevi, South Estonia/Inimtegevusest Tulenevad Muutused Settimisprotsessides Verevi Jarve Litoraalses Osas

Article excerpt

INTRODUCTION

The palaeolimnological approach is widely used for reconstructing past environmental conditions (Last & Smol, 2001). In order to understand how the palaeoinformation in the sedimentary records reflects past changes in the lake environment, it is necessary to study how historically recorded changes in the lake environment are represented in the topmost sediments. By combining observational and palaeolimnological data sets it is possible to identify short-term processes at decadal scale mainly associated with pollution pressures from human activity (Punning et al., 2004; Davidson et al., 2005). The information obtained from the littoral zone is especially important for the reconstruction of changes in the use of the lake or the surrounding land (Gaillard & Digerfeldt, 1991; Hannon & Gaillard, 1997).

The main consequence of human activity during the last century is eutrophication of lakes due to excess input of nutrients (P, N) originating from agricultural fertilizers and urban wastewaters. Although eutrophication is mainly related to the human activity during the last century, there are also records from the 12th century revealing similar changes (Ekdahl et al., 2004). Usually eutrophication processes are represented in the sedimentary records as an increase in the mass accumulation rates (MAR) of organic matter, lowering of carbon/nitrogen (C/N) ratio, and also as precipitation of calcium carbonates (Ekdahl et al., 2004; Meyers, 2006; Vreca & Muri, 2006; Xue et al., 2007).

Depending on the lake morphometry, catchment topography, and in-lake processes, these interactions are not always straightforward. To distinguish the origin of organic matter in the sediments, C/N ratios have often been used because it is known that algae have lower C/N ratios than vascular plants (Ho & Meyers, 1994; Meyers, 1994). Therefore we would expect to have lower C/N values in the sediments during times of higher eutrophication due to algal blooms, but an increase in the input of allochthonous organic matter from the catchment and higher aquatic plants could mask this. Besides, the accumulation of organic matter does not depend only on its production but also on its decomposition and preservation. Primary production and decomposition of organic matter are in turn directly related to the precipitation of carbonates, as the removal of C[O.sub.2] from the water column during photosynthesis affects the water pH and may enhance precipitation of carbonates, and on the other hand, decomposition of organic matter may lead to anoxic conditions in the sediment surface causing carbonates to dissolve (Dean, 1999). In order to separate the cause from the effect in these kind of studies, a multiproxy aproach is needed, and the changes in the lake environment have to be of large scale and historically well recorded.

This paper presents high-resolution lithological and geochemical records of four undisturbed and well-dated sediment cores taken from the littoral zone of a small Lake Verevi in Estonia. The main aim is to reconstruct the changes in the sedimentation environment by using the observational data of the lake hydrochemistry and hydrobiology, including macrophyte mapping. Lake Verevi was selected because it has a relatively flat catchment area and abundant littoral vegetation restricting input of allochthonous material; besides, good monitoring data describing some major changes in the lake environment are available for the last 100 years. The objectives of this research were to study how the historically recorded changes in the lake trophic status and vegetation are recorded in the sediments, how they affected sedimentation processes, and how well they are temporally correlated.

STUDY SITE

Lake Verevi is situated in South Estonia, within the borders of the town of Elva (Fig. 1a). It is a small lake (12.6 ha) with a maximum length of 950 m and width of 320 m. Its average depth is 3. …

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