Viia Lepane

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.

Characterization of sediment pore-water dissolved organic matter of lakes by high-performance size exclusion chromatography

Abstract.This paper demonstrates the applicability of high-performance size exclusion chromatography (HPSEC) to characterize lake-sediment pore-water dissolved organic matter (DOM). Two shallow nonstratified Estonian lakes – Harku and Karujärv – were investigated. The HPSEC results for pore waters were compared with those obtained by spectroscopic methods such as fluorescence spectroscopy and UV-absorption, and with other analyses carried out on sediments (sediment composition, C/N and H/C ratios). The average molecular weight values for UV absorbing DOM were close to pore waters of the studied lakes, not exceeding 5000 for weight- and 3000 for average-number, respectively. Low MW size fractions (<1000) dominated in Lake Harku pore waters, while the opposite was recorded in Lake Karujärv pore waters. Besides the traditional paleolimnological methods, the method used (HPSEC) seems to be promising in detailed sediment pore water characterization.

High-performance liquid chromatography (HPLC)-size exclusion chromatography (SEC) for qualitative detection of humic substances and dissolved organic matter in mineral soils and peats in Lithuania

High-performance liquid chromatography (HPLC) with size exclusion (SEC) separation function was used to isolate and examine the molecular mass (MM) distributions and polydispersity of humic substances (HSs) and dissolved organic matter (DOM) from mineral soils and peats. The aim was to improve their detailed characterisation and to inform of their soil carbon (C) sequestration and environmental quality. This is the first study conducted in Lithuania in which HSs and DOM, separated from two soil types, have been used to characterise soil at the molecular level. The HPLC-SEC, as a separation method, was coupled with diode-array detection (DAD), thus enabling the separation of molecular fractions. Results showed that HPLC-SEC can be used to determine the MM of HSs in soil, provided that the relation between retention time and MM is known and a suitable method for fitting the HS peak is available. The UV-spectra analysis showed that DOM has a larger MM (Mw = 2439–3436 Da), which contains more aliphatic C. The HS fraction has a smaller MM (Mw = 2776 Da), with aromatic structures that reflect a higher aromaticity. Separated fractions had characteristic MMs of humic acid (HA) and fulvic acid (FA) and DOM. The HSs separated from peat samples were characterised by higher aromaticity, humification and stability. The HSs extracted from mineral soil samples showed a higher degradability level. The results also show the MM distribution and polydispersity of HS and DOM fractions (Mw/Mn = 1.009–1.252) are relatively homogenous in both soil types. Findings confirm that chromatographic and spectrometric parameters can be used for characterisation of both HSs and DOM, and for detecting changes in organic matter quality. Moreover, they can also be used for a further understanding the C-cycle and could be applied for enhancing soil C-sequestration and informing environmental quality management.

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.

A 10,000 year record of sediment pore-water dissolved organic matter characteristics from Lake Peipsi as revealed by HPSEC

In this study, high-performance size-exclusion chromatography (HPSEC) combined with diode-array detection (DAD) was applied for the investigation of dissolved organic matter (DOM) from sediment pore-water from Lake Peipsi, Estonia. The age of the 4 m long sediment core was estimated using the 14C dating method and suggested sediment accumulation for the past 10,000 years. Using the HPSEC approach it was possible to evaluate temporal changes in the content of DOM and its molecular weight characteristics during the Holocene period. The content of dissolved organic carbon (DOC) calculated from spectroscopic data was in good correlation with the total peak areas computed from HPSEC chromatograms. The results revealed that the content of detected DOC in the older samples 10,300 to 2,600 years before present (BP) was twice as high as in the younger ones (from 2,400 years BP to present). Thus, HPSEC analysis with spectroscopic methods might provide useful information about temporal changes in DOM content and could be used in palaeolimnological research.

Dissolved organic matter, nutrients, and bacteria in Antarctic soil core from Schirmacher Oasis

PurposeThis study focuses on the application of HPLC in dissolved organic matter (DOM) research in Antarctic environment together with nutrients and heterotrophic bacteria (HB) analyses. The specific aims were to investigate changes in DOM components characteristics and in nutrients in soil core from ground active layer and upper permafrost, to relate obtained data to active heterotrophic bacteria records after applying statistical data treatment methods, and to explore the potential impact of environment.Materials and methodsA single Antarctic 1.9-m deep soil core drilled at a site without human impact from Schirmacher Oasis, located 70° 46′ 02″ S and 11° 45′ 11″ E, was explored. The chromophoric DOM (CDOM) was characterized by soil water analysis using multi-wavelength HPLC. Total organic carbon and total nitrogen were determined by elemental analysis, the total phosphorus by inductively coupled plasma spectrometry. The vertical changes in those nutrients and their ratios were investigated. The microbiological analysis was accomplished through the determination of psychrotrophic and psychrophilic aerobic HB numbers by colony-forming units counting method, and by epifluorescence microscopy examination. Cluster analysis using the Ward method and principal component analysis was performed on the chromatographic and microbiology data to reveal similar layers in studied soil core.Results and discussionIn active soil layer, the CDOM was missing thus indicating rather active decomposition of organic material or organic debris by the local microbial community. In deep permafrost layers, the quantity of CDOM preserved in soil water increased. The content of total organic carbon in soil was low, between 0.05 and 0.2%, and decreased down the core. The vertical changes in nutrients (total N and P), the ratios C/N and C/P, followed total organic carbon profile suggesting similar sources. Microbiological analyses showed decreasing vertical concentrations of active HB. Statistical data treatment methods enabled clustering of soil core into three zones according to depth.ConclusionsThe obtained results contribute to better understanding of organic carbon-related processes in an almost un-polluted Antarctic environment. The CDOM, macronutrients, C/N, C/P, and HB profile characteristics of the Antarctic soil core clearly demonstrate the effect of environment (active or permafrost soil layers). The study demonstrated that combining HPLC with multi-wavelength detection and microbial analyses with statistical data treatment is potentially a promising tool of investigating changes in Antarctic soil DOM and in soil waters generally.