Tiiu Alliksaar

History of anthropogenically mediated eutrophication of Lake Peipsi as revealed by the stratigraphy of fossil pigments and molecular size fractions of pore-water dissolved organic matter

We investigated stratigraphic changes in fossil pigments and the molecular structure of the UV-absorbing fraction of pore-water dissolved organic matter in a sedimentary record from Lake Peipsi (Estonia/Russia) temporally covering the 20th century. The aims of the study were to define the onset of eutrophication in the lake and to track its course. An attempt was also made to reconstruct lake conditions before the intensive nutrient loading began. Fossil pigment analysis indicated that the eutrophication of the lake started in the 1960s and accelerated in the 1970s. Sedimentary pigments also indicate a continuing tendency of the lake ecosystem towards eutrophy in the 1980s and 1990s. However, changes in the molecular size structure of pore-water dissolved organic matter indicated that the contribution of autochthonous matter to the organic pool of the lake ecosystem had already started to increase around the end of the 1930s. We conclude that this rise was generated by a coincidence of several anthropogenic and natural factors. The pore-water data also show that a slight relative reduction in the autochthonous organic matter took place in the 1990s. A discordance in the paleodata obtained for the beginning of the 20th century complicates clear conclusions about earlier conditions in the lake. On the one hand, the qualitative characteristics of pore-water dissolved organic matter and the low concentration of chlorophyll a indicate that the phytoplankton biomass was low in Lake Peipsi during that period. On the other hand, the concentrations of marker pigments of specific phytoplankton groups are high, comparable with the values in the recent sediments. Possible reasons for the high levels of these pigments in the early 1900s sediments, such as a shift in the preservation conditions of organic substances and their transport from the lake’s catchment, are discussed.

Water level changes in a large shallow lake as reflected by the plankton:periphyton-ratio of sedimentary diatoms

Biostratigraphic diatom analyses were carried out on a short sediment core from the large shallow-water Lake Võrtsjärv, Estonia, in order to relate the diatom composition to the instrumental water level record. We dated the sediment core by radiometric methods (210Pb, 137Cs, 241Am) and spheroidal fly-ash particle abundance chronology and evaluated the statistical significance of the relationships between the percentage of planktonic diatoms and the water level continuously monitored since 1871. Before the 1960s, the percentage of planktonic diatoms in the sediment showed quite strong positive relationship to water level. The impact of eutrophication after the 1960s presumably masked the influence of water level changes on the diatom community. In addition, statistical analysis of the upper part of the sediment core (1970—present day) together with measured limnological parameters of the lake showed that water transparency had the strongest influence on diatoms, while temperature, pH and alkalinity had lesser impacts. Our study shows that the planktonic:periphytic diatom ratio in the sediment can be used to track overall trends of the lake-level changes in Lake Võrtsjärv before the onset of cultural eutrophication; however, the results have to be interpreted carefully, taking into consideration other possible limnological factors such as water transparency, nutrients and wind.

Sediment diatom assemblages and composition of pore-water dissolved organic matter reflect recent eutrophication history of lake peipsi (Estonia/Russia)

A paleolimnological approach was used for the assessment of the recent eutrophication history and identification of possible reference conditions in the large, shallow, eutrophic Lake Peipsi. Lake Peipsi is the fourth largest lake by area, and the largest transboundary lake in Europe, being shared between Estonia and Russia. Lake Peipsi has been anthropogenically impacted over a longer time-scale than that covered by instrumental limnological monitoring. The 210Pb record and down-core distribution of fly-ash particles in the 40-cm core from the middle part of the lake suggest 130 years of sediment accumulation. Diatom assemblages indicate alkaline mesotrophic conditions and a well-illu-minated water column, sediment pore-water fluorescence index values suggest low autochthonous productivity and a stable aquatic ecosystem similar to natural reference conditions during the second half of 19th and early 20th century. Nearsynchronous stratigraphie changes including the expansion of the eutrophic planktonic diatom Stephanodiscus parvus, the appearance of new species associated with eutrophic lakes and the decrease in the relative abundance of littoral diatoms, together with changes in the fluorescence properties of sediment pore-water dissolved organic matter, imply increased nutrient availability, enlarged phytoplankton crops, reduced watercolumn transparency and the onset of humaninduced disturbances in the lake since the mid-20th century. The most conspicuous expansion of eutrophic planktonic diatoms and maximum concentration of siliceous microfossils occur simultaneously with changes in the fluorescence indexes of pore-water dissolved organic matter, indicating a pronounced increase in the contribution of autochthonous organic matter to the lake sediment. This implies that nutrient loading and anthropogenic impact was at a maximum during the 1970s and 1980s. Sedimentary diatom flora may reflect a reduction of phosphorus loading since the 1990s. However, the absolute abundance of planktonic diatoms and sediment pore-water fluorescence index values vary greatly implying that the lake ecosystem is still rather unstable.

Recent patterns of sediment accumulation in a small closed eutrophic lake revealed by the sediment records

A short-core palaeolimnological investigation was undertaken with the aim ofacquiring knowledge of sediment deposition. Analyses of the lithological composition of sediments from the whole-lake basin were performed on the small eutrophic L. Linajärv (northern Estonia) and the concentrations of mineral and organic matter were measured on 647 sub-samples from 14 sediment cores. The accumulation rate of the sediment sequences was established and C/N ratios of organic matter in some cores were recorded. Results indicate that the water depth, basin slopes and distance to the shore have the most important impact on the physical sediment properties. It was shown that variations in the mineral matter concentrations were influenced by the changes in deposition conditions in the areas with steep slopes. The study indicated that more objective information about the sedimentation mechanisms is obtained using analysis of the concentration ratio of mineral and organic matter since it reduces the implied role of diagenetic compaction.

A 700-year decadal scale record of lake response to catchment land use from annually laminated lake sediments in southern Estonia

Lakes, particularly those in fertile soils may have been anthropogenically influenced over long timescales for hundreds to thousands o f years by agricultural activities and may have become more productive and/or disturbed (BERGLUND 1991). Human impact on these landscapes, starting with the development of primitive agriculture, followed by forest clearance and more advanced land-use practices, has affected the lakes through various catchment processes. In this way human-induced nutrient enrichment has caused major problems to aquatic ecosystems. Paleolimnological investigations o f lake sediment cores can be used to document the history of land-use activities and related effects on lake water quality (FRITZ 1989). Varve chronology based on annually deposited lake sediments yields a continuous high-resolution temporal record and allows absolute dating of the sediment sequence. Furthermore, aquatic sub-fossils preserved in varved sediments provide precise data for reconstructing past trophic changes in lakes, whereas pollen evidence allows tracking of prehistoric human impact on vegetation in the lakes catchment area. The aim ofthe present study was to follow the impact of agriculture on water quality in Lake Rõuge Tõugjãrv, southem Estonia using high-resolution proxies ( diatoms, pollen, Cladocera, loss-on-ignition) from varved sediment during the past 700 years.

The Spatial Distribution of Characterised Fly-Ash Particles and Trace Metals in Lake Sediments and Catchment Mosses: Estonia

The concentrations of fly-ash particles were studied in the surface sediments of 42 lakes in Estonia. Concentrations of both spheroidal carbonaceous particles (SCPs) and inorganic ash spheres (IASs) were determined and the particles chemically characterised to different fuel types. This was accompanied by trace metal analyses from lake surface sediments and catchment mosses in order to determine the distribution of atmospheric pollutants. The obtained spatial distribution data for fly-ash particles varied to a great extent, partly due to differences in sedimentation rates but mainly due to higher deposition in industrial areas. The distribution patterns of the different particle types in lake sediments, as well as the trace metals in mosses, show that the deposition of airborne particulates is of local origin around pollution sources and that the share of long-range transported atmospheric impurities is high along the main cyclonic routes in Estonia.

A high-resolution spectroscopic study of pore-water dissolved organic matter in annually laminated lake sediments: a new tool for reconstructing eutrophication history

Spectroscopic methods have been used widely in studies of dissolved organic matter (DOM) in a Iarge variety of water bodies (ZUMSTEIN & BVFFLE 1989, KOWALCZUK et al. 2003) and marine sediments (DEFLANDRE & ÜAGNÉ 2001, S!ERRA et al. 2001). These methods allow predicting quantities as well as revealing qualitative properties of DOM. The main merits of the methods are (l) simplicity, (2) small sample volume required for analyses and (3) no need for chemical pretreatment of samples. In the present research, spectroscopic methods were employed to detect changes in characteristics of pore-water DOM (pwDOM) in lake sediments. Fluorescence an d absorption spectra were analysed to examine conformity of spectral indicators with the paleolimnological proxies. A lake with annually laminated sediments was chosen, which allowed dating sediments with high accuracy and tracking temporal variations o f paleoindicators in high-resolution scale.

Partitioning of metals between operational fractions in the sediment record from Lake Peipsi

A sequential extraction procedure was used to study the partitioning of metals (Ca, Mn, Fe, Cu, Zn and Cd) between operationally defined fractions in the interval covering the last 150 years of the sediment record from Lake Peipsi, Estonia. The results indicated decreased total and bioavailable Cu and Zn levels from the 1980s to the present, and increased Cd. The variability in Mn and Fe concentrations was possibly induced by changes in redox conditions at the bottom of the lake. The dissolved organic fraction of the sediment core was characterised by high-performance size-exclusion chromatography and spectroscopy. The dissolved organic matter pool was comprised mainly of humic substances and of a very small high molecular weight fraction. This study revealed increasing trends for general sediment characteristics (organic matter, dissolved organic carbon and absorbance ratio) since the 1960s, together with some molecular characteristics (peak areas of humic and high molecular weight fractions). Statistical cluster analysis revealed that metal concentration data in combination with some chromatographic and spectrometric parameters can be used to reveal periods with similar characteristics in Lake Peipsi sediments.

Paleolimnological assessment of eutrophication history of large transboundary Lake Peipsi, Estonia/Russia

Introduction Lake Peipsi (3555 km2, mean depth 7.1 m), shared between Estonia and Russia, is the fourth largest lake by area and the largest transboundary lake in Europe. Adequate information about the status and historical development ofL. Peipsi is needed for the protection of its ecosystem and implementation of the Water Framework Directive (DIRECTIVE 2000). The nutrient load into L. Peipsi increased most dramatically during the 1980s followed by a sharp decrease in nitrogen (N) loadings resulting from the collapse of soviet-type agriculture in the early 1990s (NõGES & NõGES 2004). Phosphorus (P) load was not significantly decreased, and the decreased N/P ratio created favourable conditions for bloom-forming Nz-fixing cyanobacteria (NõGES et al. 2004). However, the time-span of available limnological data is too short to determine when the changes in the ecosystem began. To establish the onset o f man-made eutrophication and natural background conditions, a short sediment core from L. Peipsi was studied for diatom remams.

A comparison of the palaeolimnology of Peipsi and Võrtsjärv: connected shallow lakes in north-eastern Europe for the twentieth century, especially in relation to eutrophication progression and water-level fluctuations

We applied a multi-proxy palaeolimnological approach to provide insights into the natural variability and human-mediated trends of two interconnected temperate large shallow lakes, Peipsi and Võrtsjärv, during the twentieth century. The history of the lakes was assessed on the basis of age-related changes in the sediment main constituents (water, organic matter and carbonate), sub-fossil pigments, diatom assemblages and organic matter dissolved in pore water. The temporal changes in the palaeodata indicate an increase of the in-lake biological production in both lakes from about the 1960s, suggesting enhanced nutrient inputs. In subsequent decades, the gradual increase of autochthonous organic matter becomes more obvious, indicating progressive eutrophication of the lakes. Palaeolimnological indicators from the sediment record of Lake Peipsi indicate a slight recession of the lake’s eutrophication in the 1990s but not for Lake Võrtsjärv. The results of the study also suggest that after the lakes became eutrophied, the climatically induced water-level fluctuations ceased to be the main driver determining the abundance of phytoplankton. Responses of the lakes to human-induced impacts are better recorded in the sediments of Lake Peipsi than in those of Lake Võrtsjärv, which is shallower of the two and where the wave-induced resuspension of deposits markedly smooths or erases the signals of environmental changes. The results of the investigation expand the knowledge on how large shallow lakes respond to human-mediated and natural perturbations, including those in the lake catchment areas and the capability of the lakes to store the chronology and sequence of these changes.