A palaeoecological evaluation of environmental degradation in Lake Mikri Prespa, NW Greece

Biological Conservation 57 (1991) 89-109

A Palaeoecological Evaluation of Environmental Degradation in Lake Mikri Prespa, NW Greece A. C. S t e v e n s o n Department of Geography, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK & R. J. F l o w e r Palaeoecology Research Unit, Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, UK (Received 25 May 1990; revised version received 20 September 1990; accepted 23 October 1990) ABSTRACT Sediment cores from Lake Mikri Prespa, N W Greece were examined to determine the extent and possible causes of environmental degradation in the wetland. While diatom analyses of a z 10pb_dated sediment core demonstrate that the lake does not appear to have undergone any significant changes in water quality within the last seventy years, significant variations in the sediment accumulation rate were discovered. These changes appear to be linked to changing anthropogenic pressures on the landscape involving river diversion, rural depopulation and abandonment of terracing cultivation and development o f an extensive irrigation network in the catchment. It is concluded that the application of palaeoecological methods has an important role to play in the management o f wetland ecosystems by providing a temporal context in which the timing, trends and nature of environmental degradation can be established and evaluated.

INTRODUCTION Lake Mikri Prespa (Fig. 1) is an internationally important site for breeding waterfowl including Dalmatian pelican Pelicanus crispus, white pelican 89 Biol. Conserv. 0006-3207/91/$03"50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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Pelicanus onocrotalus and pygmy cormorant P h a l a c r o c o r a x p y g m e u s (Pyrovetsi & Crivelli, 1988). Although the site was declared a National Park in 1974 no management plan has been implemented and no control of landuse activities within the catchment has occurred. This has led to claims that Mikri Prespa has become eutrophic by runoff of agricultural fertilisers and untreated raw sewage entering the lake (Pyrovetsi et al., 1984; Pyrovetsi & Gerakis, 1987; Koussouris et al., 1989). These assertions have been supported by the few limnological studies of the lake (Koussouris & Satmadjis, 1987; Koussouris et al., 1987) which report high phosphorus and nitrate loadings, suggesting that lake Mikri Prespa is meso-eutrophic/ eutrophic. In addition, Koussouris et al. (1989) use a hydrological desk study, together with an assessment of phosphorus loadings from the catchment, to conclude that Lake Mikri Prespa is meso-eutrophic and lying close to a eutrophication threshold. However, the published nutrient data quoted in these papers are inconsistent and to date no study presents evidence of historical changes in the algal populations within the lake which could corroborate the inferred eutrophication. A second perceived threat to the lake is that agricultural intensification within the catchment has led and will lead to increased erosion and consequent reduction in open lake area (Friends of Prespa, 1988). The only way to resolve some of these issues is to apply palaeolimnological techniques to assess if recent ecosystem change has occurred. Such techniques have proved essential in evaluating the r61e of anthropogenic activity in causing both the eutrophication of many lakes in temperate regions (e.g. Battarbee, 1978, 1986) and increased sediment accumulation rates as a result of catchment erosion (e.g. Flower et al., 1989). In this study, diatom and chemical analyses of a short 2xOpb_dated sediment core from the lake are carried out to assess the environmental impact of catchment disturbance and water enrichment. In addition, the extent of contamination by atmospheric pollution is assessed by geochemical analyses.

The site

Lake Mikri Prespa (40°41'N, 21 °37'E) is a high-altitude lake (853m) located in the northwestern part of Greece on the borders of Yugoslavia and Albania. It is a shallow (8"0m depth), karstic basin (Jakovljevic, 1935), separated from Lake Megali Prespa by a narrow alluvial isthmus. Mikri Prespa has a total surface area of 4735 ha, of which 4235 ha belong to Greece and 500 ha to Albania (Table 1). Two small islands occupy the Greek part of the lake (10 ha)--Agios Achillios and Vidronessi. The lake's hydrology is dominated by surface inflows and groundwater additions from underground seepage. The lake is normally drained by a single artificial outflow,

92

A. C. Stevenson, R. J. Flower

TABLE 1 Lake Characteristics Rainfall Area Volume Maximum depth Mean depth

752"1mm 4 735"0ha 324 x 106m3 8.4m 4.1 m

Altitude Shorelinedevelopment Maximumlength Maximumwidth

853 m 2-25 13.6kin 6'1 km

Data from Scoullos and Hatzianestis (1989) and Koussouris et al. (1987). the Koula channel, to Lake Megali Prespa and unquantified losses by underground seepage to both Megali Prespa and Lake Ochrid. However, in the early 1960s both Megali and Mikri Prespa stood at 855.7 m asl and at that time they were effectively one lake (Hollis et al., 1989). Moreover, since they maintained equality of water level for 14 months at these high stages, it seems likely that there was an overflow to the River Devoll in Albania. This is in fact described as the outflow from the system in Broikos (1962). Water chemistry

Table 2 presents the results of all the available water chemistry for Mikri Prespa obtained from a variety of sources (for comparison phosphorus levels for three eutrophic lakes are included). The data from Koussouris et al. (1987) have been omitted since the values quoted for available phosphorus and nitrate are far higher than for any eutrophic lake in the UK. Since it is unclear whether standard methodologies have been used in all analyses the values contained within this table should be treated with caution. For instance, the results of the Land Reclamation Survey Laboratory are a factor of ten higher than the others, especially for nitrogen compounds. For nitrate it appears that an insensitive analytical method is being used since the detection limit is 440 #g litre- 1. The very high mean nitrate figures for 1985 and 1986 lie way outside previous and current measured maxima (c. 340 pg litre- 1) produced by the other studies. Similar conclusions can be drawn for nitrate and ammonia, with current maxima being of the order of c. 2.3 #g litre- ~ and 30-50 #g litre- ~, respectively (Mourkides et al., 1978; Koussouris et al., 1989) compared to the results of the Land Reclamation Laboratory of 20-30/~g litre- 1 and 160/~g litre- 1, respectively. Phosphorus results from all workers appear to be consistent and on this basis the lake would be classified as meso-eutrophic (Wetzel, 1975). Climate, geology and soils

The climate shows the characteristics of a hot and dry Mediterranean type during summer (July 23.6°C) and Mid-European type during the winter with

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A. C. Stevenson, R. J. Flower

long periods of high rainfall (752"1 mm), snow, increased cloudiness and low temperatures (January 0-8°C). Under an Emberger bioclimatic classification the area is typical of a humid Mediterranean type (Emberger et al., 1963). The catchment geology is dominated by limestones and dolomites on the western side of the lake, while the eastern side is predominantly granites and gneisses. Soils are formed mainly of alluvia and colluvia on the agricultural land and are characterised by medium pH (6-0-6"5) and low amounts of phosphorus and potassium. The soil types of the forest zone are poorly known but consist of calcareous rendzinas and podsolised brown earth forest soils (Pavlides, 1985).

Vegetation The fringes of the lake are dominated by extensive beds of Phragmites australis (nomenclature follows Tutin et al. (1964-1980)), especially around Koula, the eastern shores and into the Albanian section of the lake. Associated with Phragmites, especially in Vromolimni, are Scirpus laeustris, Typha angustifolia and Iris pseudaeorus (Pavlides, 1985). Submerged macrophyte populations are dominated by Ceratophyllum demersum, C.

submersum, Potamogeton perfoliatus, P. crispus, P. pusillus, Myriophyllum spieatum, M. verticillatum, Najas marina and Vallisneria spiralis (Pavlides, 1985). Floating aquatic communities in sheltered areas are characterised by

Nymphaea alba, Nymphoides peltata, Nuphar lutea, Potamogeton natans, P. lueens and P. gramineus. The catchment vegetation belongs to a Balkan mountainous form and can be divided into four major zones. The coastal alluvial plain is dominated by agricultural activities, especially cultivation of the white bean Vicia alba while around the villages lombardy poplars Populus nigra and walnuts Juglans regia are frequently planted. The remainder of the catchment comprises two major forest zones: a low-altitude forest dominated by Carpinus orientalis/Ostrya earpinifolia/Quereus frainetto, and a highaltitude one dominated by Quercus petraea/Fagus sylvatica/Abies alba. In parts of the alpine zone an extensive alpine grassland is developed and in most locations is probably the result of anthropogenic activity, e.g. grazing and terracing cultivation.

METHODS Short sediment cores (lengths c. 20 cm) were taken from four locations, fixed using conventional compass triangulation, within the lake (Fig. 1) using a gravity-driven Kajak sampler. Cores were extruded at a temporary field base

Palaeoecology of a Greek lake

95

into water-tight polythene bags, at half-centimetre intervals, and taken back to the laboratory for subsequent analyses. After comparison of all cores and concluding that all showed similar physical and microfossil trends, core MIP-1, taken from the deepest point in the lake (c. 8.0m), was used for further analysis. Sediment subsamples were prepared for diatom, geochemical, pollen and carbonaceous particle analyses using standard methodologies (Stevenson et al., 1987; Rose, 1990). RESULTS Sediment characteristics

The lake sediment was typical for an upland lake in a limestone catchment: rich in calcium carbonate, grey to black in colour with a fairly coarsegrained texture. Percentage dry weight, loss on ignition, wet density and

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A.C. Stevenson, R. J. Flower

96

CaCO 3 profiles for MIP-1 show little variation, indicating fairly homogeneous sediment throughout the core (Fig. 2). Percentage dry weight is < 20% throughout the core with a slight trend to lower values towards the core top. Organic content indicated by loss on ignition values is low at < 3 0 % throughout the core and shows a slight trend to higher values towards the core top. There is a small decline in organic content localised around 7 cm depth. Wet density shows very little change over the length of the core. The constancy of these measurements indicates that the source of sediment has probably changed little during the time period spanned by the core. The slight loss on ignition decline could indicate either a small increase in catchment soil erosion at the time corresponding to c. 7 cm depth or a decrease in organic productivity. Low organic content of lake sediments is typical for lakes in subhumid/Mediterranean environments and indicates relatively high rates of soil erosion and/or calcite precipitation or a high rate of mineralisation from the water column rather than low lake productivity p e r se (cf. Flower et al., 1984, 1989). TABLE 3 CRS Model 2X°pb Chronology for the MIP-! Core

Depth (cm)

0"00 1"00 2-00 3-00 4-00 5"00 6"00 7"00 8-00 9-00 10"00 I1"00 12"00 13-00 14"00 15"00 16"00 17-00 18'00 19"00 20"00

Chronology date

Sedimentation accumulation rate

(Ao)

(g cm- 2 )'ear- 1)

(cm ),ear- t)

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Palaeoecology of a Greek lake

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Dating Sediment from core MIP-1 was analyzed for 21°pb, 226Ra, 134Cs, 137Cs and 241Am by gamma spectrometry (Appleby et al., 1986). High values of i a4Cs demonstrate the presence of Chernobyl fallout and confirm that the core top represents sediment deposited within the last two years (Fig. 3). However, this Chernobyl spike now lies some 2.5cm below the present surface, indicating a high sediment accumulation rate. The earlier 1963 1a 7Cs weapons peak is unfortunately masked by this later Chernobyl peak of 137Cs but the 241Am record in the sediment (Fig. 4) provides a convenient marker for the 1963 bomb fallout peak of radionuclides and is located at 8.75 cm. Figure 4 presents the sediment chronologies according to the CRS model of sediment accumulation (Appleby & Oldfield, 1983), with the 13~Cs- and 241Am-dated levels overlain and there is a good agreement between the independent chronologies. In order to produce the final chronologies the CRS model of sediment accumulation tied to the known fixed dates from the 137Cs and 241Am results was used (Fig. 4 and Table 3). The results clearly show that sediment accumulation rates have been moderately high for the time period covered by the core while sediment accumulation rates have

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Palaeoecology o/a Greek lake

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almost doubled from 0.229cm year-1 in the 1920s to a current 0.527cm year -1. This general trend masks periods of fluctuating sediment accumulation; rates initially rise from 0"229 cm year- 1 in the late 1920s to a peak of 0.413 cm year-1 by the early 1940s; they then decline steadily until 1968 (0.274 cm year- ~), apart from a small rise in the late 1950s. From 1970 accumulation rates rise steadily to the current rate of 0.527 cm year-1.

Diatom analysis Sediment subsamples were prepared for diatom analyses (Battarbee, 1986) from selected levels in the core and about 300 valves were counted for each level. The concentration of sedimentary diatoms was estimated by the latex microsphere method of Battarbee and Kneen (1981). Diatom taxa were identified using standard floras (e.g. Hustedt, 1930-1966). Only 32 taxa (including the few unknown species) were observed in the core. Preservation of diatom valves was variable with many being intact and others partially dissolved. Since the degree of valve dissolution did not appear to increase down the core it is probable that dissolution occurred before or during valve incorporation in the sediment. Although overall representivity may be impaired it is thought that changes in the sedimentary diatom record do indicate changes in past diatom populations rather than differential preservation.

Diatom stratigraphy The summary percentage diatom diagram (Fig. 5) shows that the planktonic diatom Cyclotella ocellata has dominated the sediment diatom assemblage throughout the period spanned by the core. Over the upper 12cm of sediment this diatom forms more than 70% of the total assemblage. Below this depth it declines and its frequency abundance is about 50% at the core base (21 cm). At 12 cm depth and above several diatom taxa make their first appearance in the diagram, including Aulacoseira granulata, its variety angustissima and Nitzschia fonticola v. pelagica. Of the diatoms which decline in abundance towards the core top ,4. granulata v. muzzenensis is initially the most abundant (c. 20% at 21 cm depth) but declines to less than 10% above 12 cm depth. Several Fragilaria species show less regular changes in percentage abundance but tend to decline above 7 cm depth, as does an unknown species of,4chnanthes. The variation in diatom concentration with sediment depth is shown in Fig. 5. There is a continuous decline in cell concentration from the core base to c. 10cm depth as the concentration falls by about 50%. Above this depth cell concentration increases irregularly to the core top.

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Interpretation The percentage diatom frequency diagram provides no evidence of significant phosphorus enrichment in the lake over the period spanned by the core (AO 1926-1988). The most common diatom is the planktonic Cyclotella ocellata, a diatom that is characteristic of high pH, calcium-rich freshwater lakes (Gasse & Tekaia, 1983). This species is also dominant in the pre-enrichment flora of Crosemere in the U K (R. W. Battarbee, pers. comm.), and in early Holocene sediments in Lake Lucerne (Lotte, 1988), for example, its occurrence is thought to indicate low nutrient concentration. Small increases in less common diatoms noted above 12 cm depth (hO 1951) in the Mikri Prespa core, such as Aulacoseira granulata and Cyclotella pseudostelligera, are, however, indicative of slight nutrient enrichment. They occur in the most recent period and are possibly a response to use of agricultural fertilisers within the catchment.Unfortunately little is known about the ecology of A. granulata v. muzzenensis, the diatom that declines most strongly in the lower section of the core, other than that it is planktonic in several large European lakes (Hustedt, 1930--66). The concentration decline in total diatom cell abundance found between the core base and c. 10 cm depth suggests an increase in erosion of catchment soils at this time. Since the sediment characteristics do not change over this depth an accelerated supply of sediment from the same source, probably calcareous soil, is most likely. However, the overall cell concentration does not drop below 10 6 cell g- 1 so that the degree of erosion inwash is relatively minor. The rate of sediment accumulation is not particularly high for a lake in this type of catchment (cf. Flower et al., 1989) and, as inferred from the diatoms, has not caused any major change in water chemistry.

Chemistry--major cations The major cations show few long-term trends throughout these cores (Fig. 6). All cations follow the same pattern characterised by concentration peaks at 10, 7 and 2 cm with minor peaks at 18 and 14 cm. While this pattern could reflect a reduction in the sediment accumulation rates at these points it is noticeable that the concentration peak at 7 cm is also associated with a depression in diatom and lead concentrations and suggests an erosional origin for these peaks (Mackereth, 1966; Engstrom & Wright, 1984). Since this level dates to approximately 1969, just after the two lakes had been recorded at their highest level ever, the associated floods could have provided the necessary erosional forces. Calcium levels are extremely high, reflecting the karstic nature of the basin. Furthermore, previous studies of the lake have also shown that calcium carbonate is being actively

A. C. Stevenson, R. J. Flower

102

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precipitated in the water column, especially in the shallower waters where temperature and organic matter production is high enough to promote calcite formation (Stumm & Morgan, 1970; Scoullos & Hatzianestis, 1989). Trace metals

The trace metal profiles are shown in Fig. 7. Unlike many oligotrophic, northern European lakes (Battarbee et al., 1988) the trace metal profiles do not all show distinct elevations that may be associated with industrial pollution. Of the four trace metals examined only lead shows any elevation of its concentrations from 17 #g g- 1 between 21 and 14 cm to approximately 25 #g g-1 at 14 cm (AD 1947) sediment depth. Similar results were found by Scoullos and Hatzianestis (in press). Most of the lead within the core is likely to be of an atmospheric origin since the lead profile, bound to the organic fraction, is the mirror image of that of the major cations. Overall, however, lead values are a factor of ten lower than would be found in the uppermost sediments of most northern European lakes where atmospheric contamination has been demonstrated (Battarbe¢ et al., 1988) and, given the relatively low sediment accumulation rates, must reflect low levels of industrial contamination in this area of southern Europe. The other three trace metal profiles show no industrial contamination

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104

A. C. Stevenson, R. J. Flower

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Carbonaceous particle concentration in the MIP-1 core.

21

DISCUSSION A N D CONCLUSIONS Sediment accumulation rate

The dating, physical and diatom analyses of the core demonstrate that the early part of the core represents a period of relative ecological stability. Sediment accumulation rates increase steadily from the core base to 15"5 cm (AD 1943) and this rise is most likely related to the continued flow of the Agios Germanos torrent at high flows into Lake Mikri Prespa and the continued cultivation of the easily eroded soils of the upper catchment. The decline in rates from 1943 onwards could relate to the final diversion of the Agios Germanos torrent into Lake Megali Prespa (AD 1945) and the cessation of the cultivation of the upper parts of the catchment associated with a 78% decline in the population of the area from 1940 to 1951 (Pyrovetsi et al., 1984). The more recent increase in sediment accumulation rates from 7cm (AD1970) is probably a consequence of agricultural intensification within the catchment associated with the construction and implementation of the irrigation-drainage system in 1971 and its intensification in the early 1980s (Pyrovetsi et al., 1984) resulting in increased sediment delivery to the lake. The long-term water quality implications of these increased sediment accumulation rates appear not to be significant since the diatom flora has been fairly stable throughout the timespan of the core. As to the problem of the lake gradually infilling, this cannot be realistically ascertained unless further multi-core studies are implemented and further studies are conducted on the nature of tectonism in the region and any rates of subsidence/uplift measured. However, it is clear that sediment accumulation rates have risen continuously since 1970 at the coring site and the implementation of the agricultural improvement scheme.

Palaeoecology of a Greek lake

105

Water quality The dominance of Cyclotella ocellata in the core suggests that the lake is, as yet, little affected by increased nutrient loadings and is still mesooligotrophic in nature. The relatively high concentration of sedimentary planktonic diatoms indicates that the periods of increased catchment disturbance have had little impact on the lake water quality, at least during the growing season of the diatoms. Overall, the diatom evidence shows only slight eutrophication of the lake from the 1950s, possibly associated with the agricultural development of the alluvial plain to the east of the lake. These conclusions contradict the earlier work of both Pyrovetsi et al. (1984) and Koussouris et al. (1989), who report that phosphorus concentrations in Mikri Prespa are relatively high and suggest that the lake may be more eutrophic than that indicated by the diatom results. Moreover, the desk study by Koussouris et al. (1989) calculates phosphorus loadings from point-source estimates and shows the lake to be in a critical state with regard to phosphorus loading and eutrophication. By using the Vollenweider critical phosphorus loading model they calculate that the lake receives c. 0-24 P g m - 2 year- 1 more than the recommended permissible P loading figure (Vollenweider, 1975). This figure could, however, be an overestimation. Table 4 compares phosphorus loadings (estimated from P supply, spring water column total-P and lake residence time) in Mikri Prespa with Lough Neagh, Northern Ireland, a lake known to be highly enriched from point-source P pollution (Wood & Gibson, 1973). The data in the table show clearly that whilst there is good agreement in Lough Neagh between phosphorus loading calculated from the mean spring concentration of total-P and total-P from point sources this is not so for Mikri Prespa. Here the same calculations show a discrepancy of more than 50%, with total-P loading calculated from point sources being the greatest. Moreover, in the lake-based calculation of phosphorus loading used here for Mikri Prespa a maximum total-P concentration of 42 #g litre- 1 rather than the mean was used, so the value of 1-7 mg P m - z will be an overestimate of the true figure. In addition, the annual phosphorus loading calculated from point sources in Mikri Prespa is almost three times the areal loading figure for eutrophic Lough Neagh (Wood & Gibson, 1973) and between double and three times the loadings given for a series of eutrophic lakes in Canada (Dillon & Rigler, 1974). This high phosphorus loading for Mikri Prespa could result from an overestimation o f loading because a simplistic assumption made by Koussouris et al. (1989) is that all the phosphorus deposited within the catchment is transferred to the lake. In reality, much of the phosphorus produced within the catchment will be immediately immobilised within the calcium-rich soils, leaving only a small fraction available for ultimate transfer to the lake (Wood & Gibson, 1973).

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The areal phosphorus loading figures calculated by Koussouris et al. (1989) are almost certainly strongly overestimated, and erroneous conclusions have been drawn about the trophic status of Mikri Prespa. In addition to immobilisation of phosphorus within the catchment-soil system other mechanisms may be at work in reducing phosphorus availability in the lake water. Studies of both phosphorus and nitrate runoff from agricultural land, entering the lake system through reed fringes, have shown that nutrient concentrations are reduced markedly through the reed-bed (S. Penson, pers. comm.). Secondly, phosphorus availability may be reduced by the formation of biologically unavailable detrital phosphorus as phosphate reacts with calcium in calcium-rich soils and hard waters (Boyer & Wheeler, 1989). Thirdly, and related to the second mechanism, is the coprecipitation of phosphorus with calcite formation in the lake and its rapid transfer to the sediment (Murphy et al., 1983a, b). The necessary conditions for authigenic calcite formation are known to occur in the lake (Scoullos & Hatzianestis, in press) and studies of other calcareous lake systems have found this to be a powerful mechanism that can remove up to a third of the total phosphorus (Kleiner, 1988). These considerations, when combined with the evidence from the oligotrophic diatom flora, do not support the hypothesis that Mikri Prespa is a seriously enriched lake. This conclusion is further supported by the limited lake chemistry data (Table 2), which demonstrate that available phosphorus values are generally low. Moreover, the values of total-P show little change over the ten years of available chemistry data despite agricultural intensification within the catchment. TABLE 4 Annual P Loading in Mikri Prespa Estimated from Point Sources of P in the Catchment, Total-P in the Water Column and Lake Residence Time (Koussouris et al., 1989) Basis o f P loading calculation

Method of calculation

Annual P loading Total P (tonnes)

(rag P m - 2)

Catchment supply of total-P

Summation of point sources

14"5 (301-368)

2-7 (0"8-1-0)

Total-P in lake water

Mean spring total P/ residence time

9"3 (324)

1-7 (0"9)

For reference, comparable figures for Lough Neagh are shown m parentheses.

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In conclusion, the lake is still fairly pristine in nature but while the threat from eutrophication has been overstated a catchment m a n a g e m e n t plan is necessary to prevent future increases in fertiliser use and consequent e u t r o p h i c a t i o n . F u r t h e r m o r e , sediment dating, albeit of one core, demonstrates that erosion from the catchment has increased and that sediment accumulation rates are now at their highest in the core's history. With no effective outflow from the lake, measures should be taken to reduce soil erosion from the catchment.

ACKNOWLEDGEMENTS The authors would like to t h a n k Sheila Bradford, Alain Crivelli, Jiorgos Catsadorakis and Professor A. Gerakis for their help in the field. Neil Rose provided the soot analysis and Peter Appleby provided the 21°pb dating. Rick Battarbee and Ted Hollis are thanked for their c o m m e n t s on earlier drafts of this manuscript. This work was performed under contract 3535 f r o m the World-Wide F u n d for Nature (International).

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