**3. Results and discussion**

#### **3.1 Main characteristics of the sediment profiles**

The descriptions and dating values for analysed sediment cores are reported in Table 1. Both sediment cores covered roughly 150 years of sediment deposition. The characteristics of L. Peipsi sediment core solid-phase have been reported previously (Lepane, 2010b). Briefly, the organic matter content was 23 to 25% for the period 1860–1950 and increased up to 27% after the 1950s. This increase was followed by an increase in DOC values from the 1960s.

In the second case investigated, L. Rõuge Tõugjärv, the organic component of the particulate sediment matrix had low values (11-13%) between 1850 and the 1880s, followed by a distinct peak (18%) around the 1900s and a progressive rise to 27% thereafter (Alliksaar et al., 2005). The rest of the sediment consisted mainly of terrigenous mineral matter eroded into the lake from the catchment. In L. Rõuge Tõugjärv, the low organic matter values in sediment probably indicated land derived input from human-induced topsoil erosion and dilution of organic matter by addition of clastic mineral particles, while the increase in organic matter presumably reflected a lower contribution of terrestrial mineral material with a reduction in the overall rural activities. The DOC values corresponded to an increase in organic matter from 1850 to the 1890s; thereafter the decrease has been significant up to the present. The low DOC values may be due to a high amount of aliphatic organic compounds resulting from the microbial activity. Since the determination of DOC was based on the spectroscopic method, the aliphatic organic matter fraction was not determined in present study.

#### **3.2 Multi-wavelength HPLC analyses**

The sediment DOM was characterized by *pw* sample analysis using HPLC (HPSEC) with DAD. Absorbance spectroscopy with a single detection wavelength has been verified as a suitable detection method after HPLC separation (Lepane, 2010a). The UV absorbance at 250–280 nm has been widely used to provide an estimation of aromatic compounds (Filella, 2010). In the present study, multi-wavelength HPLC analysis has been carried out to detect changes in *pw*DOM composition and molecular mass profiles down the cores. The absorbance spectra were examined from 200 to 400 nm. Figures 2 and 3 display the HPLC chromatograms of *pw* from studied sediment cores and from different layers, respectively. The UV detector response range from 205 to 400 nm has been plotted against the retention times to obtain the multi-wavelength contour plots. Plot colours provide the visual representation of the relative absorbance intensity. The multi-wavelength plots allowed the most suitable detection wavelength for separated DOM components to be selected. Simultaneously, the chromatograms were registered at 280 nm. The chromatogram patterns suggested that the *pw*DOM molecules included two fractions in both lakes. The first peak, with shorter retention times of 6–7 min and of 8–9 min, corresponded to the

HPLC Fingerprints of Porewater Organic Compounds as Markers for Environmental Conditions 317

(a)

(b)

(c)

Fig. 2. Multi-wavelength HPLC chromatograms of porewater samples from Lake Peipsi

sediment core at different depths: (a) 4 cm, (b) 37 cm, (c) 41 cm.

fraction with larger molecules and was operationally named the high molecular mass (HMW) fraction. The second peak, whose maximum was at retention times of 11–12 min and of 15–16 min, was assigned to smaller DOM molecules and named the humic substances (HS) fraction.


Table 1. Sediment samples numbering and depths down the profiles with dating values for Lake Peipsi and Lake Rõuge Tõugjärv.

fraction with larger molecules and was operationally named the high molecular mass (HMW) fraction. The second peak, whose maximum was at retention times of 11–12 min and of 15–16 min, was assigned to smaller DOM molecules and named the humic

Period Sample no Depth, cm Age dating, y Period Sample no Depth, cm Age dating,y **I** 1 0/-1 2006 **I** 1 0/-1 2006

**II** 14 -13/-14 1970 14 -13/-14 1981

**III** 34 -33/-34 1880 34 -33/-34 1917

Table 1. Sediment samples numbering and depths down the profiles with dating values for

Lake Peipsi and Lake Rõuge Tõugjärv.

35 -34/-35 1877 35 -34/-35 1914 36 -35/-36 1875 36 -35/-36 1911 37 -36/-37 1872 **IV** 37 -36/-37 1905 38 -37/-38 1870 38 -37/-38 1899 39 -38/-39 1867 39 -38/-39 1893 40 -39/-40 1865 40 -39/-40 1887 41 -40/-41 1862 41 -40/-41 1881 42 -41/-42 1857 42 -41/-42 1875 43 -42/-43 1852 43 -42/-43 1869

> 44 -43/-44 1864 45 -44/-45 1858

15 -14/-15 1966 15 -14/-15 1979 16 -15/-16 1962 16 -15/-16 1976 17 -16/-17 1958 17 -16/-17 1974 18 -17/-18 1954 **III** 18 -17/-18 1971 19 -18/-19 1950 19 -18/-19 1968 20 -19/-20 1945 20 -19/-20 1964 21 -20/-21 1940 21 -20/-21 1961 22 -21/-22 1934 22 -21/-22 1958 23 -22/-23 1928 23 -22/-23 1954 24 -23/-24 1922 24 -23/-24 1951 25 -24/-25 1916 25 -24/-25 1948 26 -25/-26 1911 26 -25/-26 1944 27 -26/-27 1907 27 -26/-27 1941 28 -27/-28 1904 28 -27/-28 1938 29 -28/-29 1901 29 -28/-29 1934 30 -29/-30 1897 30 -29/-30 1931 31 -30/-31 1892 31 -30/-31 1928 32 -31/-32 1887 32 -31/-32 1924 33 -32/-33 1882 33 -32/-33 1921

2 -1/-2 2005 2 -1/-2 2005 3 -2/-3 2004 3 -2/-3 2005 4 -3/-4 2003 4 -3/-4 2004 5 -4/-5 2002 5 -4/-5 2002 6 -5/-6 2000 6 -5/-6 2000 7 -6/-7 1997 7 -6/-7 1998 8 -7/-8 1994 **II** 8 -7/-8 1996 9 -8/-9 1990 9 -8/-9 1994 10 -9/-10 1986 10 -9/-10 1991 11 -10/-11 1982 11 -10/-11 1989 12 -11/-12 1978 12 -11/-12 1986 13 -12/-13 1974 13 -12/-13 1984

Lake **Peipsi Rõuge Tõugjärv**

substances (HS) fraction.

Fig. 2. Multi-wavelength HPLC chromatograms of porewater samples from Lake Peipsi sediment core at different depths: (a) 4 cm, (b) 37 cm, (c) 41 cm.

HPLC Fingerprints of Porewater Organic Compounds as Markers for Environmental Conditions 319

All chromatograms of L. Peipsi sediment *pw*s were very similar, consisting of two main peaks representing HMW and HS fractions. The intensities and positions of the peaks (*i.e.* fractions) changed in different sediment layers reflecting age-related changes in the concentrations and transformation of organic constituents. The area of the HMW fraction was always smaller (~3% of the total area) than the second HS fraction (~97% of the total area). The calculated molecular masses for the HMW fraction varied between 200 and 270 kDa. The HMW fraction was absent from samples dating from the 1990s and from older samples from the nineteenth century. The HS fraction, with molecular masses between 700 and 3,700 Da, was dominant. The calculated average Mw was 1,500 Da, which is characteristic for aquatic humic and fulvic acids. The profiles of the determined chemical

The L. Rõuge Tõugjärv *pw*DOM was also separated into two peaks. The components of the second peak eluted as a broad distribution and sometimes with a partially resolved subshoulder. Possibly, the composition of the *pw*DOM from those sediment layers where the sub-shoulder appeared (some layers from the 1980s and 1960s) might have been somehow different from the major DOM composition. According to DAD spectra, components eluted with the first peak contained proteinaceous material, while the second peak spectra were characteristic of HS. The retention times of both peaks remained stable down the core. The calculated molecular masses for the HMW fraction varied between 800 and 1,000 kDa (Mw). The HMW fraction varied between 6 to 13% of the total peak area and was thus present in a significantly higher amount than in L. Peipsi sediment *pw*s. Possibly, HMW material might have been formed from some proteins encapsulated into HS aggregates or micelles. The ability of HS to aggregate into large supramolecules has been reported previously (Havel & Fetsch, 2007; Piccolo, 2001). Generally, average Mw values of the analysed L. Rõuge Tõugjärv sediment *pw* HSs slightly exceeded 1,000 Da, and Mn was close to 400 Da. Molecular mass values of HS were in good agreement with molecular mass distributions reported for

The depth profiles for both lake cores (Fig. 4) indicated corresponding changes in Mw and Mn values. The molecular mass values for HS from L. Peipsi were slightly higher: 1,500 Da *vs.* 1,000 Da. The high fluctuations in HS molecular masses during 1870–1930s were not detected for L. Rõuge Tõugjärv. The down-core profiles of the chromatogram total peak

The Mw/Mn ratio, or polydispersity, which is a measure of the homogeneity of organic matter, was mostly stable down the core, varying from 2.3 to 3.5 for L. Rõuge Tõugjärv and from 1.9 to 3.0 for L. Peipsi. This indicated the relatively homogeneous HS fraction in both lakes studied. The results showed that the molecular mass distribution and the polydispersity of DOM from L. Peipsi and L. Rõuge Tõugjärv were quite similar to those of sediment *pw*DOM from other lakes from Estonia and other regions studied by HPSEC (Fu et al., 2006; Leeben et al., 2008a;

The absorbance ratio of DOM at wavelengths of 250 and 360 nm (A250/A360) indicates the source of organic matter in the sediments (Peuravuori and Pihlaja, 1997). A higher ratio is related to autochthonous organic matter, which is produced within the lake, and the substances of smaller size and lower aromaticity are present in DOM molecules. Lower values of absorbance ratio reflect a higher aromaticity with an extent of allochthonous organic matter that originates outside the lake and is carried into the lake by inflows (McKnight et al., 2001). The ratio for L. Peipsi core samples was constant until the 1960s, with an average value close to 4.0. Thereafter, up to the present, it increased to 6.5, meaning that the origin of the organic matter changed to autochthonous. Constant values were also obtained for the L. Rõuge

areas and HS fraction areas were similar and exactly followed the changes in DOC.

Lepane et al., 2004, 2010a; Makarõtševa et al., 2010; O'Loughlin & Chin, 2004).

characteristics and HPLC variables are presented in Fig. 4.

aquatic fulvic acids (Klavinš, 1997; Lepane et al., 2004).

Fig. 3. Multi-wavelength HPLC chromatograms of porewater samples from Lake Rõuge Tõugjärv sediment core at different depths: (a) 6 cm, (b) 13 cm, (c) 27 cm, (d) 42 cm.

(a)

(b)

(c)

(d) Fig. 3. Multi-wavelength HPLC chromatograms of porewater samples from Lake Rõuge Tõugjärv sediment core at different depths: (a) 6 cm, (b) 13 cm, (c) 27 cm, (d) 42 cm.

All chromatograms of L. Peipsi sediment *pw*s were very similar, consisting of two main peaks representing HMW and HS fractions. The intensities and positions of the peaks (*i.e.* fractions) changed in different sediment layers reflecting age-related changes in the concentrations and transformation of organic constituents. The area of the HMW fraction was always smaller (~3% of the total area) than the second HS fraction (~97% of the total area). The calculated molecular masses for the HMW fraction varied between 200 and 270 kDa. The HMW fraction was absent from samples dating from the 1990s and from older samples from the nineteenth century. The HS fraction, with molecular masses between 700 and 3,700 Da, was dominant. The calculated average Mw was 1,500 Da, which is characteristic for aquatic humic and fulvic acids. The profiles of the determined chemical characteristics and HPLC variables are presented in Fig. 4.

The L. Rõuge Tõugjärv *pw*DOM was also separated into two peaks. The components of the second peak eluted as a broad distribution and sometimes with a partially resolved subshoulder. Possibly, the composition of the *pw*DOM from those sediment layers where the sub-shoulder appeared (some layers from the 1980s and 1960s) might have been somehow different from the major DOM composition. According to DAD spectra, components eluted with the first peak contained proteinaceous material, while the second peak spectra were characteristic of HS. The retention times of both peaks remained stable down the core. The calculated molecular masses for the HMW fraction varied between 800 and 1,000 kDa (Mw). The HMW fraction varied between 6 to 13% of the total peak area and was thus present in a significantly higher amount than in L. Peipsi sediment *pw*s. Possibly, HMW material might have been formed from some proteins encapsulated into HS aggregates or micelles. The ability of HS to aggregate into large supramolecules has been reported previously (Havel & Fetsch, 2007; Piccolo, 2001). Generally, average Mw values of the analysed L. Rõuge Tõugjärv sediment *pw* HSs slightly exceeded 1,000 Da, and Mn was close to 400 Da. Molecular mass values of HS were in good agreement with molecular mass distributions reported for aquatic fulvic acids (Klavinš, 1997; Lepane et al., 2004).

The depth profiles for both lake cores (Fig. 4) indicated corresponding changes in Mw and Mn values. The molecular mass values for HS from L. Peipsi were slightly higher: 1,500 Da *vs.* 1,000 Da. The high fluctuations in HS molecular masses during 1870–1930s were not detected for L. Rõuge Tõugjärv. The down-core profiles of the chromatogram total peak areas and HS fraction areas were similar and exactly followed the changes in DOC.

The Mw/Mn ratio, or polydispersity, which is a measure of the homogeneity of organic matter, was mostly stable down the core, varying from 2.3 to 3.5 for L. Rõuge Tõugjärv and from 1.9 to 3.0 for L. Peipsi. This indicated the relatively homogeneous HS fraction in both lakes studied. The results showed that the molecular mass distribution and the polydispersity of DOM from L. Peipsi and L. Rõuge Tõugjärv were quite similar to those of sediment *pw*DOM from other lakes from Estonia and other regions studied by HPSEC (Fu et al., 2006; Leeben et al., 2008a; Lepane et al., 2004, 2010a; Makarõtševa et al., 2010; O'Loughlin & Chin, 2004).

The absorbance ratio of DOM at wavelengths of 250 and 360 nm (A250/A360) indicates the source of organic matter in the sediments (Peuravuori and Pihlaja, 1997). A higher ratio is related to autochthonous organic matter, which is produced within the lake, and the substances of smaller size and lower aromaticity are present in DOM molecules. Lower values of absorbance ratio reflect a higher aromaticity with an extent of allochthonous organic matter that originates outside the lake and is carried into the lake by inflows (McKnight et al., 2001). The ratio for L. Peipsi core samples was constant until the 1960s, with an average value close to 4.0. Thereafter, up to the present, it increased to 6.5, meaning that the origin of the organic matter changed to autochthonous. Constant values were also obtained for the L. Rõuge

HPLC Fingerprints of Porewater Organic Compounds as Markers for Environmental Conditions 321

The statistical analysis of data was performed to reveal periods in the characteristics of separated DOM fractions. Based on DOC and absorbance ratio data, the L. Peipsi sediment core was divided into three age/depth periods: (I) 0–13 cm of sediment core depth, dated to 2006–1974; (II) 14–33 cm core depth, dated to 1970–1882; and (III) 34–43 cm core depth, dated to 1880–1852. L. Rõuge Tõugjärv sediment core was operationally separated into four age/depth periods: (I) 0–7 cm, dated to 2006–1998; (II) 8–17 cm, dated to 1996–1974; (III) 18– 36 cm, dated to 1971–1911; and (IV) 37–45 cm, dated to 1905–1858 (Table 1). The mean values of the analysed variables divided into three or four periods with 95% confidence

The HMW fraction data (peak area, molecular masses, and polydispersity) were statistically similar down the core, as was the HS fraction polydispersity, and therefore did not allow the differentiation of sediment layers (Fig. 5). The DOC, total chromatogram peak area, and HS peak area changed similarly, thus proving the suitability of peak areas as semi-quantitative characteristics of DOM. The 1880–1852 dated samples had elevated DOC values. The upper 0–13 cm sediment DOM had statistically relevant differences in comparison to period III as revealed by DOC and HS molecular masses. The recent DOM accumulating into sediments has lower molecular masses and the highest absorbance ratio in comparison with preceding sediment layers. The obtained results indicate that recent *pw*DOM in L. Peipsi is more aliphatic and contains lower average molecular mass organic compounds which are likely of autochthonous origin. This might be the result of the microbial degradation of labile organic matter constituents such as carbohydrates (Zaccone et al., 2009). The absorbance ratio in L. Peipsi *pw*s shows significant differences throughout the sediment profile and can thus serve

As in the first lake sediment core studied, the polydispersity of HMW and HS fractions did not show any particular trend along the L. Rõuge Tõugjärv core profile (Fig. 6). The DOC, total chromatogram peak area, and HMW and HS fraction peak areas changed similarly. The obtained results indicated a general increase in all those variables with depth. However, it was not possible to differentiate between periods II and III (*i.e*. corresponding to years 1996–1911) by using DOC and semi-quantitative chromatographic data. Also, in the case of this lake the highest *pw*DOC was registered in the deepest layer 37–45 cm. The molecular masses of both HMW and HS of this undisturbed sediment core show different trends down the profile in comparison with L. Peipsi core. Similarities were found between the most recent and the oldest layers (dated to 2006–1998 and 1905–1858, respectively) and differences were found between the intermediate ones (periods II and III, dated to 1996– 1974 and 1971–1911, respectively). Thus, the upper sediment layer (0–7 cm) variables indicate decreased DOM input with the characteristic high molecular mass compounds. The molecular mass data variations may reflect the influence of the watershed but also the seasonal climatic factors, like in-lake primary production. The observed distinct increase in absorbance ratio that was synchronous with a decrease in the DOC content possibly indicates the enhanced algal productivity and eutrophication of the lake, but also the lower

**3.3 Age-related changes in DOM characteristics of sediment cores** 

as an excellent variable for revealing the changes in sediment core.

contribution of allochthonous organic matter into the lake.

limits are shown in Figs. 5 and 6.

**3.3.2 Lake Rõuge Tõugjärv** 

**3.3.1 Lake Peipsi** 

Tõugjärv core until the 1940s, indicating higher degree of allochthonous organic matter in the lake. In the mid-twentieth century the ratio slightly increased, which coincided with the period when the lake sediments received decreased proportions of allochthonous organic compounds due to the decline in rural land-use practices and decreased sub-soil erosion. However, since 1980s there was a sharp increase in the absorbance ratio of the DOM up to 8, which also indicated the dominance of a more aliphatic autochthonous organic matter.

Fig. 4. Profiles of general chemical characteristics and variables by HPLC of Lake Peipsi and Lake Rõuge Tõugjärv sediment porewaters. The year denotes the year of sediment deposition.

#### **3.3 Age-related changes in DOM characteristics of sediment cores**

The statistical analysis of data was performed to reveal periods in the characteristics of separated DOM fractions. Based on DOC and absorbance ratio data, the L. Peipsi sediment core was divided into three age/depth periods: (I) 0–13 cm of sediment core depth, dated to 2006–1974; (II) 14–33 cm core depth, dated to 1970–1882; and (III) 34–43 cm core depth, dated to 1880–1852. L. Rõuge Tõugjärv sediment core was operationally separated into four age/depth periods: (I) 0–7 cm, dated to 2006–1998; (II) 8–17 cm, dated to 1996–1974; (III) 18– 36 cm, dated to 1971–1911; and (IV) 37–45 cm, dated to 1905–1858 (Table 1). The mean values of the analysed variables divided into three or four periods with 95% confidence limits are shown in Figs. 5 and 6.

#### **3.3.1 Lake Peipsi**

320 International Perspectives on Global Environmental Change

Tõugjärv core until the 1940s, indicating higher degree of allochthonous organic matter in the lake. In the mid-twentieth century the ratio slightly increased, which coincided with the period when the lake sediments received decreased proportions of allochthonous organic compounds due to the decline in rural land-use practices and decreased sub-soil erosion. However, since 1980s there was a sharp increase in the absorbance ratio of the DOM up to 8, which also

**DOC, mg L-1 A250/A360 Area Total, AU\*s Area HS, AU\*s**

**Area HMW, AU\*s Mw of HS, Da Mn of HS, Da Mw/Mn of HS**

0 500 1000 1500

0 500 1000 1500

0 1000 2000

0 1000 2000

0 500 1000 1500

0 500 1000 1500

indicated the dominance of a more aliphatic autochthonous organic matter.

Lake Rõuge Tõugjärv sediment porewaters. The year denotes the year of sediment

0 2000 4000

0 2000 4000

Fig. 4. Profiles of general chemical characteristics and variables by HPLC of Lake Peipsi and

**Lake Peipsi**

**Lake Rõuge Tõugjärv**

**Lake Peipsi**

**Lake Rõuge Tõugjärv**

deposition.

**Year**

**Year**

100

100

**Year**

**Year**

0 10 20 30

0 10 20 30

The HMW fraction data (peak area, molecular masses, and polydispersity) were statistically similar down the core, as was the HS fraction polydispersity, and therefore did not allow the differentiation of sediment layers (Fig. 5). The DOC, total chromatogram peak area, and HS peak area changed similarly, thus proving the suitability of peak areas as semi-quantitative characteristics of DOM. The 1880–1852 dated samples had elevated DOC values. The upper 0–13 cm sediment DOM had statistically relevant differences in comparison to period III as revealed by DOC and HS molecular masses. The recent DOM accumulating into sediments has lower molecular masses and the highest absorbance ratio in comparison with preceding sediment layers. The obtained results indicate that recent *pw*DOM in L. Peipsi is more aliphatic and contains lower average molecular mass organic compounds which are likely of autochthonous origin. This might be the result of the microbial degradation of labile organic matter constituents such as carbohydrates (Zaccone et al., 2009). The absorbance ratio in L. Peipsi *pw*s shows significant differences throughout the sediment profile and can thus serve as an excellent variable for revealing the changes in sediment core.

#### **3.3.2 Lake Rõuge Tõugjärv**

As in the first lake sediment core studied, the polydispersity of HMW and HS fractions did not show any particular trend along the L. Rõuge Tõugjärv core profile (Fig. 6). The DOC, total chromatogram peak area, and HMW and HS fraction peak areas changed similarly. The obtained results indicated a general increase in all those variables with depth. However, it was not possible to differentiate between periods II and III (*i.e*. corresponding to years 1996–1911) by using DOC and semi-quantitative chromatographic data. Also, in the case of this lake the highest *pw*DOC was registered in the deepest layer 37–45 cm. The molecular masses of both HMW and HS of this undisturbed sediment core show different trends down the profile in comparison with L. Peipsi core. Similarities were found between the most recent and the oldest layers (dated to 2006–1998 and 1905–1858, respectively) and differences were found between the intermediate ones (periods II and III, dated to 1996– 1974 and 1971–1911, respectively). Thus, the upper sediment layer (0–7 cm) variables indicate decreased DOM input with the characteristic high molecular mass compounds. The molecular mass data variations may reflect the influence of the watershed but also the seasonal climatic factors, like in-lake primary production. The observed distinct increase in absorbance ratio that was synchronous with a decrease in the DOC content possibly indicates the enhanced algal productivity and eutrophication of the lake, but also the lower contribution of allochthonous organic matter into the lake.

HPLC Fingerprints of Porewater Organic Compounds as Markers for Environmental Conditions 323

I II III IV

Area HMW

I II III IV

HS Mn

I II III IV

HMW Mn

I II III IV

Fig. 6. Plots describing mean values of Lake Rõuge Tõugjärv DOM semi-quantitative (areas), molecular, and spectroscopic characteristics arranged into four age/depth periods (see text).

0 0.2 0.4 0.6 0.8 1 1.2 1.4

**Mean**

**Mean**

**Mean**

I II III IV

HS Mw/Mn

I II III IV

HMW Mw/Mn

I II III IV

Area Total

A250/A360

**Mean**

**Mean**

**Mean**

**Mean**

I II III IV

Area HS

I II III IV

HS Mw

I II III IV

HMW Mw

I II III IV

For abbreviations see Fig. 5 legend.

**Mean**

**Mean**

**Mean**

**Mean**

DOC

Fig. 5. Plots describing mean values of Lake Peipsi DOM semi-quantitative (areas), molecular, and spectroscopic characteristics arranged into three age/depth periods (see text). Red bars indicate confidence limits at the 95% level. DOC, mg L-1; A250/A360: absorbance ratio at respective wavelengths; Mw/Mn: polydispersity; Mw and Mn: weight – and number-average molecular masses, respectively, Da; Area Total: total chromatogram peak area; Area HMW and Area HS: HMW and HS fraction peak areas, respectively, mAU\*s.

I II III

Area HMW

I II III

HS Mn

I II III

HMW Mn

I II III

> 0 0.5 1 1.5 2 2.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

**Mean**

**Mean**

**Mean**

I II III

HS Mw/Mn

I II III

HMW Mw/Mn

I II III

Area Total

A250/A360

Fig. 5. Plots describing mean values of Lake Peipsi DOM semi-quantitative (areas), molecular, and spectroscopic characteristics arranged into three age/depth periods (see text). Red bars indicate confidence limits at the 95% level. DOC, mg L-1; A250/A360:

absorbance ratio at respective wavelengths; Mw/Mn: polydispersity; Mw and Mn: weight – and number-average molecular masses, respectively, Da; Area Total: total chromatogram peak area; Area HMW and Area HS: HMW and HS fraction peak areas, respectively,

**Mean**

**Mean**

**Mean**

**Mean**

**Mean**

**Mean**

**Mean**

mAU\*s.

**Mean**

I II III

Area HS

I II III

HS Mw

I II III

HMW Mw

I II III

DOC

Fig. 6. Plots describing mean values of Lake Rõuge Tõugjärv DOM semi-quantitative (areas), molecular, and spectroscopic characteristics arranged into four age/depth periods (see text). For abbreviations see Fig. 5 legend.

HPLC Fingerprints of Porewater Organic Compounds as Markers for Environmental Conditions 325

dendrograms are shown in Fig. 7. The aim was to identify the subgroups within the HPLC dataset and relate them to environmental changes. The L. Peipsi data allowed the layers to be grouped into two major groups. According to the results, the DOM from 2006–1994 formed a homogeneous subgroup that was in the same cluster as samples from 1982, 1928, and 1897–1857. The second major group was also divided into two subgroups. The first included sediment layers from years 1880–1872, 1911–1901, 1922 and 1940. The second subgroup covered mainly the sediment layers from 1990–1934, excluding the 1982 layer. Palaeolimnological studies state that anthropogenic impact on the lake has increased since the 1950s. Until that time, the lake was considered mesotrophic (Leeben et al., 2008b). Biomanipulation of the lake was carried out in 1993–1994 and was reported to have improved the lake ecosystem. This event can be seen in HPLC data considering the grouping results of organic compounds. Sedimentary pigment analysis indicated and thus

L. Rõuge Tõugjärv experienced anthropogenic catchment disturbances up to the beginning of twentieth century, as indicated by extensive farming and increased drainage. During the first part of the twentieth century the development of efficient agricultural practices and reforestation improved the water quality. During the second part of the twentieth century the cultivated area declined and reforestation continued but the widespread use of mineral fertilizers caused an increase in primary production. After old agricultural practices stopped in the 1990s the lake was recovered and is reported to be mesotrophic today. The anthropogenic activities can be tracked by sediment investigations. L. Rõuge Tõugjärv sediments were annually laminated and thus possessed records with calendar year chronology. Thus, changes in this lake ecosystem and climate could be resolved seasonally. The L. Rõuge Tõugjärv HPLC organic matter data enabled the sediment layers to be classed into two major homologous groups. The recent sediment layers (2006–1998) formed a separate subgroup and were included in the same cluster as samples from 1951–1911 and some separate layers from 1991, 1986, 1974, and 1964. The second major group was also divided into two subgroups: the first one was similar to period II (1996–1976) and the second was similar to period IV (1905–1864), together with some separate layers from years 1971–1968, 1958–1954, 1934, 1921. The organic matter characteristics from period IV samples may reflect long-term agricultural impact because the lake has been mediated by human activity over hundreds of years (Heinsalu & Alliksaar, 2009). The massive utilization of fertilizers led to increased primary production in the 1960s–1980s (Alliksaar et al., 2005). Thus, one of the major clusters might reflect the eutrophication of L. Rõuge Tõugjärv. Since the 1990s the lake has been classified as mesotrophic with a decrease in diatoms and very good water quality. Historically, the same is reported for the time period 1920–1940. The above-described periods correlate well with the major cluster that included the most recent

confirmed the eutrophication of the lake since the 1980s (Leeben et al., 2008a).

organic matter data together with data from the first part of the twentieth century.

**4. Conclusion** 

The obtained results for both lakes show quite good agreement with some common eutrophication indicators (diatoms, fossil pigments) and thus confirm the suitability of organic compounds data for the assessment of the ecological state of the water bodies.

The results presented in the present study allowed the changes in the sediment porewater organic compounds to be assessed and related to the environmental conditions of the studied lakes. The applied HPLC method with multi-wavelength detection did not alter the

Fig. 7. Cluster analysis of Lake Peipsi and Lake Rõuge Tõugjärv sediment core samples from different depths (numbers indicate sediment depth in centimetres).

#### **3.4 Tracking environmental change in organic compounds records**

During the second half of the nineteenth and early twentieth centuries, L. Peipsi had a stable ecosystem similar to natural reference conditions as indicated by low autochthonous productivity. During the second half of the twentieth century, the ecological conditions of L. Peipsi worsened constantly. In the 1960s the lake was classified as mesotrophic. Eutrophication is the major environmental issue in the L. Peipsi basin due to the nutrient load to the lake. The main source of nutrient pollution of L. Peipsi is agriculture and municipal wastewaters. The decline in agriculture during the 1990s caused pollution to decrease and the quality of waters to improve. The lake area has been in a period of transition for more than decade.

Cluster analysis of *pw*DOM data was performed to reveal periods with similar characteristics in the studied L. Peipsi and L. Rõuge Tõugjärv sediment cores. The dendrograms are shown in Fig. 7. The aim was to identify the subgroups within the HPLC dataset and relate them to environmental changes. The L. Peipsi data allowed the layers to be grouped into two major groups. According to the results, the DOM from 2006–1994 formed a homogeneous subgroup that was in the same cluster as samples from 1982, 1928, and 1897–1857. The second major group was also divided into two subgroups. The first included sediment layers from years 1880–1872, 1911–1901, 1922 and 1940. The second subgroup covered mainly the sediment layers from 1990–1934, excluding the 1982 layer. Palaeolimnological studies state that anthropogenic impact on the lake has increased since the 1950s. Until that time, the lake was considered mesotrophic (Leeben et al., 2008b). Biomanipulation of the lake was carried out in 1993–1994 and was reported to have improved the lake ecosystem. This event can be seen in HPLC data considering the grouping results of organic compounds. Sedimentary pigment analysis indicated and thus confirmed the eutrophication of the lake since the 1980s (Leeben et al., 2008a).

L. Rõuge Tõugjärv experienced anthropogenic catchment disturbances up to the beginning of twentieth century, as indicated by extensive farming and increased drainage. During the first part of the twentieth century the development of efficient agricultural practices and reforestation improved the water quality. During the second part of the twentieth century the cultivated area declined and reforestation continued but the widespread use of mineral fertilizers caused an increase in primary production. After old agricultural practices stopped in the 1990s the lake was recovered and is reported to be mesotrophic today. The anthropogenic activities can be tracked by sediment investigations. L. Rõuge Tõugjärv sediments were annually laminated and thus possessed records with calendar year chronology. Thus, changes in this lake ecosystem and climate could be resolved seasonally. The L. Rõuge Tõugjärv HPLC organic matter data enabled the sediment layers to be classed into two major homologous groups. The recent sediment layers (2006–1998) formed a separate subgroup and were included in the same cluster as samples from 1951–1911 and some separate layers from 1991, 1986, 1974, and 1964. The second major group was also divided into two subgroups: the first one was similar to period II (1996–1976) and the second was similar to period IV (1905–1864), together with some separate layers from years 1971–1968, 1958–1954, 1934, 1921. The organic matter characteristics from period IV samples may reflect long-term agricultural impact because the lake has been mediated by human activity over hundreds of years (Heinsalu & Alliksaar, 2009). The massive utilization of fertilizers led to increased primary production in the 1960s–1980s (Alliksaar et al., 2005). Thus, one of the major clusters might reflect the eutrophication of L. Rõuge Tõugjärv. Since the 1990s the lake has been classified as mesotrophic with a decrease in diatoms and very good water quality. Historically, the same is reported for the time period 1920–1940. The above-described periods correlate well with the major cluster that included the most recent organic matter data together with data from the first part of the twentieth century.

The obtained results for both lakes show quite good agreement with some common eutrophication indicators (diatoms, fossil pigments) and thus confirm the suitability of organic compounds data for the assessment of the ecological state of the water bodies.

#### **4. Conclusion**

324 International Perspectives on Global Environmental Change

**Lake Peipsi**

transition for more than decade.

different depths (numbers indicate sediment depth in centimetres).

**3.4 Tracking environmental change in organic compounds records** 

Fig. 7. Cluster analysis of Lake Peipsi and Lake Rõuge Tõugjärv sediment core samples from

During the second half of the nineteenth and early twentieth centuries, L. Peipsi had a stable ecosystem similar to natural reference conditions as indicated by low autochthonous productivity. During the second half of the twentieth century, the ecological conditions of L. Peipsi worsened constantly. In the 1960s the lake was classified as mesotrophic. Eutrophication is the major environmental issue in the L. Peipsi basin due to the nutrient load to the lake. The main source of nutrient pollution of L. Peipsi is agriculture and municipal wastewaters. The decline in agriculture during the 1990s caused pollution to decrease and the quality of waters to improve. The lake area has been in a period of

Cluster analysis of *pw*DOM data was performed to reveal periods with similar characteristics in the studied L. Peipsi and L. Rõuge Tõugjärv sediment cores. The

**Distance**

**Distance**

**Lake Rõuge Tõugjärv**

The results presented in the present study allowed the changes in the sediment porewater organic compounds to be assessed and related to the environmental conditions of the studied lakes. The applied HPLC method with multi-wavelength detection did not alter the

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**17** 

 *Australia* 

**Management Strategies for Large River** 

Large river basins are the origin of ancient civilization (Barbier & Thompson, 1998; Sadoff & Grey, 2002). Floodplain lakes, situated adjacent to large river systems, are connected with river channel networks. The connectivity between river channels and wetlands makes the "boom" and "bust" ecology following the drought and flood events that continues to support diverse floral and faunal communities in the floodplains lake systems (Jenkins & Boulton, 2003). Rich biodiversity and occurrence of macro-invertebrate drifts in the Upper Paraguay River-Floodplain-System, parts of the Pantanal (Brazil) Wetland System, and dense microphyte community with regularly supplied allochthonous nutrient inputs and moderation of physical extremes in the billabongs of the Murray-Darling River Floodplain-System Australia are some examples of highly productive floodplains lake ecosystems in the world (Shiel, 1976; Wantzen et al., 2005). Being a productive ecosystem, people living across the large river basins have been greatly benefited from the resources generated by these wetlands for generations (Bright et al., 2010). For example, the indigenous people of the Orinico River Basin, South America, and Murray Darling Basin, Australia have been harvesting the specialised fish community that are adapted to the floodplains wetland systems over several centuries in the past (e.g., Lundberg et al., 1987; Humphries, 2007). Since the productivity of the large river floodplains lake ecosystems is dependent on naturally occurring riverine flood events, any alternation of the hydrological patterns of rivers can have strong impacts on nutrient dynamics, biological diversity and assemblages of these lakes (Fisher et al., 2000). Over the past few decades the large river systems and its adjacent wetland habitats have undergone rapid environmental changes. Anthropogenic activity across the river basin has increased substantially. River regulations such as construction of dams, irrigation channels, dykes and weirs, and catchment land use activities such as deforestation, agriculture and cattle ranching and introduction of exotic flora and fauna are increased (Power et al., 1996; Kingsford, 2000, Bunn & Arthington, 2002). Rapid climate warming is further intensifying the conditions of ecosystems including thechanges in hydrology and water quality of rivers and lakes (Carpenter et al., 1992; Lewis et al., 2000; Palmer et al.; 2008). The coupled human-climate disturbances have led to an increased habitat heterogeneity and complexity of ecosystem processes of majority of floodplains lake systems worldwide (Tockner et al., 2000). Consequently, the people who

**1. Introduction** 

**Floodplain Lakes Undergoing Rapid** 

*School of Science & Engineering, University of Ballarat, Victoria* 

**Environmental Changes** 

Giri Kattel and Peter Gell

