Lakes Of Greece Research Articles

Fecundity of silver-phase eels (Anguilla anguilla) from different habitat types and geographic locations

Abstract The reproductive ecology of European eel (Anguilla anguilla) is poorly understood, though information on life-history traits such as fecundity is necessary for appropriate management of this critically endangered species. During the growth period of the life cycle, yellow-phase eels can be resident in different habitat types (e.g. salinity ranging from freshwater to marine) over a wide geographic distribution. Consequently, maturing silver-phase eels migrating back to the oceanic spawning grounds can display considerable variation in certain life-history traits depending on their habitat and/or geographic origin. Few fecundity estimates are available from wild A. anguilla, and no investigation of the effects of habitat type (i.e. salinity) or geographic location on variation in fecundity has been undertaken. To assess the former, we examined silver-phase eels which had predominantly been resident in freshwater (Lake Mälaren, Sweden) or brackish (Baltic Sea) habitats. Despite Lake Mälaren eels generally being larger, there was no difference in the fecundity–body size relationship between these salinity zones. Second, we determined if fecundity differed between silver-phase eels sampled from different geographic locations: Lake Mälaren and the Baltic Sea outlet in Sweden, the River Shannon in Ireland (northwest Europe) and Vistonis Lake in Greece (eastern Mediterranean). Our results showed that silver-phase eels from Sweden (Lake Mälaren and Baltic Sea outlet) and Greece were significantly more fecund at a given body size than those from Ireland. Spawner models and conservation policies should therefore take account of eel geographic origin in particular when incorporating this life-history parameter.

Reproductive potential of silver European eels (Anguilla anguilla) migrating from Vistonis Lake (Northern Aegean Sea, Greece)

The European eel (Anguilla anguilla), once abundant throughout much of Europe and North Africa, has recently been classified as critically endangered. Information on its biology from the eastern Mediterranean is lacking, especially in relation to spawner quality. Therefore, silver eels were sampled during their seaward spawning migration from Vistonis Lake in Greece. Characteristics linked to reproductive output and success (i.e. body size and condition, sex ratio, silvering, Anguillicola crassus infection, fecundity and oocyte diameter) were examined. The lake produced large (687–1138 mm), exclusively female silver eels, 61.7% of which were infected by A. crassus. Silver eel fecundity, the first estimates from the southern part of the species range, was positively related to body length (R2 = 0.693; P < 0.001) and body weight (R2 = 0.731; P < 0.001). Fecundity did not differ between A. crassus infected and uninfected silver eels, but Greek silver eels were significantly more fecund than those in north-west Europe. The reproductive potential of Vistonis Lake silver eels and their contribution to the A. anguilla spawning stock is discussed.

Trophic State Evaluation of a Large Mediterranean Lake Utilizing Abiotic and Biotic Elements

The trophic state of a freshwater ecosystem reflects its environmental quality. This is why several trophic indicators have been developed for such water bodies based on chemical, physical and biological parameters. Apart from that, there are several biotic elements which can be used in accessing the environmental condition of a freshwater ecosystem. Zooplankton organisms are important elements of the structure and function of lakes and are considered useful indicators of alterations in their trophic dynamics and ecological state related to changes in nutrient loading and climate. In accordance to the above, the present study is an attempt to assess the trophic condition of the largest lake in Greece (Lake Trichonis) through the investigation of the physicochemical elements, along with the biotic indications provided by a three-year study of the lake’s zooplankton. The present results, compared with previous studies conducted between 15 and 25 years before, showed that there was an increase in the maximum values of the concentrations of chlorophyll-α and nutrients, while there was a decrease in water transparency. The implementation of Carslon’s trophic state index (TSI) revealed that Lake Trichonis still remains an oligo-to mesotrophic ecosystem as it was in the past. However, although the zooplankton investigation showed several features that are common in oligotrophic lakes, there are certain eutrophic characteristics of the zooplankton community (e.g. abundance variation pattern, indicator species, seasonal succession of cladocerans) pointing out a different state of the ecosystem in comparison to the past. In conclusion, the use of a biotic element like zooplankton revealed that Lake Trichonis is experiencing a transitional condition towards the eutrophic state and points out the necessity for constant inspection and monitoring of this ecosystem.

Influence of hydrophyte abundance on the spatial distribution of zooplankton in selected lakes in Greece

Submerged hydrophyte vegetation consists of a highly important biotic component of maintaining lake ecosystems towards a “clear water” ecological status. Aquatic macrophytes are well known to play a significant multidimensional role in lakes by competing with phytoplankton growth, stabilising sediment and offering refuge to fishes, macro-invertebrates and littoral zooplankton, amongst others. Zooplanktons that are associated with macrophyte beds, in particular, may act as a positive feedback mechanism that contributes to maintaining a clear-water state. Although there are several studies investigating the relationships between macrophytes and zooplankton in European lakes, few have yet been carried out in Greek lakes. Seasonal field sampling was conducted from spring 2006 to autumn 2008 in four lakes of northwestern Greece. Zooplankton samples were collected from within hydrophyte beds in each lake to estimate their relative abundance and species density. Hydrophyte abundance and composition was recorded on a five-point scale. Moreover, water samples were analysed to determine nutrient and chlorophyll-a concentration. Pearson correlations between zooplankton density and key physicochemical variables were conducted to distinguish significant abiotic variables related with major zooplankton groups. Kruskal–Wallis non-parametric analysis was used to test for significant differences in zooplankton composition and environmental variables amongst the five hydrophyte abundance classes. In addition, Canonical correspondence analysis was used to distinguish possible correlations amongst the macrophyte and zooplankton species. Zooplankton density was significantly higher in dense macrophyte vegetation. Small-sized species (e.g. Rotifera) dominated the zooplankton community, indicating the eutrophic nature of the lakes. Large Cladocera were present in low abundance and were mostly littoral. The current research contributes to a better understanding of relationships between biotic groups in selected Greek lakes.

A natron source at Pikrolimni Lake in Greece? Geochemical evidence

The geochemical conditions that are responsible for the formation of “Chalastraion nitron” in the basin of Lake Pikrolimni (Northern Greece) were investigated in this study. The goal of the study is to confirm Pliny's description: “at Clitae in Macedonia it is found in abundance the best, called soda of Chalestra…” The ion-specific interaction model based on Pitzer's equations showed that the lake brine samples are saturated versus carbonate–bicarbonate sodium and sodium–chloride–sulfate minerals. The results of individual X-Ray Diffraction (XRD) analyses on the evaporative deposits showed that the salts consist mostly of trona, burkeite, thenardite and halite confirming Pliny's description.

Molecular ecology of zebra mussel invasions

The invasion of the zebra mussel, Dreissena polymorpha, into North American waters has resulted in profound ecological disturbances and large monetary losses. This study examined the invasion history and patterns of genetic diversity among endemic and invading populations of zebra mussels using DNA sequences from the mitochondrial cytochrome oxidase I (COI) gene. Patterns of haplotype frequency indicate that all invasive populations of zebra mussels from North America and Europe originated from the Ponto-Caspian Sea region. The distribution of haplotypes was consistent with invasive populations arising from the Black Sea drainage, but could not exclude the possibility of an origin from the Caspian Sea drainage. Similar haplotype frequencies among North American populations of D. polymorpha suggest colonization by a single founding population. There was no evidence of invasive populations arising from tectonic lakes in Turkey, while lakes in Greece and Macedonia contained only Dreissena stankovici. Populations in Turkey might be members of a sibling species complex of D. polymorpha. Ponto-Caspian derived populations of D. polymorpha (theta = 0.0011) and Dreissena bugensis (one haplotype) exhibited low levels of genetic diversity at the COI gene, perhaps as a result of repeated population bottlenecks. In contrast, geographically isolated tectonic lake populations exhibited relatively high levels of genetic diversity (theta = 0.0032 to 0.0134). It is possible that the fluctuating environment of the Ponto-Caspian basin facilitated the colonizing habit of invasive populations of D. polymorpha and D. bugensis. Our findings were concordant with the general trend of destructive freshwater invaders in the Great Lakes arising from the Ponto-Caspian Sea basin.

Phylogeography and systematics of zebra mussels and related species

The genus Dreissena includes two widespread and aggressive aquatic invaders, the zebra mussel, Dreissena polymorpha, and the quagga mussel, Dreissena bugensis. This genus evolved in the Ponto-Caspian Sea basin, characterized by dynamic instability over multiple timescales and a unique evolutionary environment that may predispose to invasiveness. The objectives of this study were to gain insights into the demographic history of Dreissena species in their endemic range, to reconstruct intraspecific phylogeographic relationships among populations, and to clarify systematics of the genus, using DNA sequences from the mitochondrial cytochrome oxidase I (COI) gene. We found four deeply diverged clades within this genus, with a basal split that approximately coincided with the Cretaceous-Tertiary boundary. Divergence events within the four base clades were much more recent, corresponding to geographically disjunct sets of populations, which might represent species complexes. Across all taxa, populations of Dreissena shared a common pattern of genetic signatures indicating historical population bottlenecks and expansions. Haplotype diversity was relatively low in Ponto-Caspian drainages relative to more stable tectonic lakes in Greece, Macedonia, and Turkey. The phylogeographic and demographic patterns in the endemic range of Dreissena might have resulted from vicariance events, habitat instability, and the high fecundity and passive dispersal of these organisms.

Lake Koronia, Greece: Shift from autotrophy to heterotrophy with cultural eutrophication and progressive water-level reduction

Lake Koronia, a Ramsar site, is shallow, polymictic, hypertrophic and until recently was aerially the fourth largest lake in Greece. Although exceeding 5 m in the past, lake depth has declined progressively from 3.8 m in 1980 to < 1 m in 1997, reducing surface area and water volume by 50% and 80%, respectively. Specific conductivityincreased from 1300 μS cm −1 in 1977 to >6000 μS cm −1 in 1991. Increased phosphate concentrations from the late 1970's (8–45 μg L −1) to the late 1990's (100–1000 μg L −1) document that the previously eutrophic system with a limited littoral zone switched to hypertrophy dominated by massive cyanobacteria blooms. Oxygen saturation of the water column increased progressively from about 80% in 1983 to full saturation about 1993, after which it decreased progressively to only 20% saturation in 1997. In spite of cyanobacteria dominance, community metabolism of the lake switched from progressively increasing autotrophy to rapidly advancing heterotrophy associated with progressive water-level reduction leading to fish extirpation in the lake.

Limnological survey of the warm monomictic lake Trichonis (central western Greece); I. The physical and chemical environment

The physical and chemical status of Trichonis – the largestanddeepest natural lake in Greece – is examined over two annualcycles (1985–86 and 1988–89). A correlation between lakenutrientpatterns and phytoplankton biomass is attempted. Limnologicalfeatures are compared with data from other warm and temperatelakes.

Contribution to the knowledge of the genus Dina Blanchard, 1892 (Hirudinea: Erpobdellidae)

This paper deals with the genus Dina of the Erpobdellidae. A comprehensive comparison of all known species of this genus is given. A lectotype for Dina absoloni Johansson, 1913, is selected. A new species, Dina latestriata n. sp. from Prespa Lake in Greece is described. Data concerning the evolution of Dina are given.

Climatic controls on Holocene lake-level changes in Europe

Sensitivity experiments with a simple water-balance model were used to constrain the possible climatic causes of distinct Holocene patterns of lake-level variation in different regions of Europe. Lakes in S Sweden were low at 9 ka, high around 6.5 ka, low again around 4 ka and are high now. Lakes in Estonia show similar but weaker trends. Lakes in S France were highest around 9 ka, lowest around 4 ka, intermediate now. Lakes in Greece were also high around 9 ka but continued rising until 7.5 ka, then fell gradually from 5 ka with a brief high phase around 3 ka, and are low now. The model was forced with insolation anomalies deduced from orbital variations, temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone (inferred from reconstructed changes in the equator-to-pole temperature gradient), in order to evaluate the effects of resultant evaporation changes on catchment water balance. The resulting simulated changes in runoff (precipitation minus actual evapotranspiration) were slight, and frequently opposite to the observed trends. Larger changes in precipitation are plausible and are required to explain the data. The required precipitation increase in N Europe from 9 ka (low) to 6 ka (high) is suggested by GCM experiments to have been a consequence of interacting insolation and residual ice-sheet effects on the atmospheric circulation over the North Atlantic. The explanation of other observed changes, including the drying trend during the Holocene in S Europe, has not been provided by GCM experiments to date. Explanations may lie in changes in mesoscale circulation, sea-surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way.

Biological Conservation Implications of Water-level Fluctuations in a Wetland of International Importance: Lake Kerkini, Macedonia, Greece

Kerkini Lake in Greece, a reservoir maintained principally for agricultural irrigation purposes on the Strymon River, has been classified as a ‘Wetland of International Importance’ according to the Ramsar Convention. Hydrological conditions favoured the formation of a shallow, very productive lake, rich in species diversity and biotic abundance. The importance of the wetland lay in its complex zonation of various habitats which mainly consisted of: shallow open water with sand-strips, beaches, and mud-flats; shallow-water areas with communities of emergent vegetation; a lowland hardwood riparian forest mainly ofSalixandTamarixspecies; a shrub-swamp; and extensive wet meadows. The diversity of habitats supported a rich waterbird fauna for breeding, stationing during migration, and wintering.Nevertheless, in 1982 a new high-dam was constructed, higher embankments along the banks of the Lake were established, and other works were carried out (to increase the water capacity of the reservoir), in response to irrigation demands. No consideration was given to the wetland resources. Operation of the new dam resulted in changes in the flooding etc. regime, with severe impact on the biotic resources of the wetland, especially by waterlevel increases of more than 5 m in less than 4 months during spring — the critical growing- and breedingseason. Wetland heterogeneity and the mosaic structure of water-land-vegetation were thereby lost.Now the riparian forest is diminishing in area and declining in productivity, the reed-swamp with emergent macrophytes has been lost, and wet meadows have also disappeared. Deep flooding and the loss of these habitats is reflected in marked decline in the densities of migratory waterbird populations and, most important, in drastic shrinkage in the populations of breeding species. Similar impact is observed in the dramatic decline of fish species diversity and abundance. In general, nowadays, Kerkini presents an alarming example of loss of biodiversity and accelerating impoverishment of its former biotic resources.In view of the projected plans to increase further the water-storage capacity of the Lake by raising its waterlevel, measures are urgently needed: (a) for preventing such actions; (b) for restoring and maintaining marsh conditions, and (c) for ecologically sound management and sustainable use of the wetland resources.

Comparison of conservation attitudes among fishermen in three protected lakes in Greece

Sustainable utilization of wetlands is determined by the perceptions/attitudes of fishermen who are the main users of the lake resources. Successful implementation of conservation policies, management measures and environmental education programmes requires consideration of those attitudes and resolution of the conflicts between humans and the natural environment. Lakes Vistonis, Kerkini and Mikri Prespa in Greece, although wetlands of international importance, suffer severe conservation problems with a continuous deterioration of their resources. The aims of this paper are: (1) to compare the conservation attitudes of commercial fishermen of those lakes; (2) to explain those differences through variations in the socio-demographic and fishing technology/practices characteristics, and (3) to suggest proper education and management measures in order to resolve inherent conflicts. The opinion/attitude survey, carried out with a sample of 110 commercial fishermen, revealed that those in lake Vistonis differ significantly in their attitudes and perceptions towards wetland ecosystems, birds and more specifically pelicans from fishermen in lakes Mikri Prespa and Kerkini. The former expressed the most negative attitude to the environment. The conflict identified was towards the fish-eating birds, particularly cormorants preying heavily on the fish ponds they have constructed. However, fishing practices of these same fishermen are positive towards the resource they harvest. This attitude may result from the form of organization they have adopted in a fishermen's co-operative where strict regulations towards sustainable utilization of fisheries are enforced. On the other hand, in lakes Kerkini and Prespa, fishermen are harvesting individually the maximum days permitted, using small mesh nets and illegal fishing methods, thus imposing intensive pressure on the fisheries. As a result, strong competition for fisheries prevails between them. An education/information programme should be designed and implemented in the three lakes aimed at highlighting the values of wetlands, of the various ecosystem elements and interrelationships. The programme should also emphasize the interdependence of economic and environmental interests. Proper management measures should be taken to abate fish-eating birds' depredations in Vistonis.

Unusual characteristics in a Meromictic lake in Greece

Abstract The studied lake, a meromictic shallow lagoon, in Peloponissos, presents unusual physical and chemical features which derive mainly from its mineral springs, and the sea water that invades into the lake. The form of vegetation and the abundant organic matter of the mud together with the sulfate salts of the water create conditions which are responsible for the production of gases such as H2S and CH4. Vertical profiles of temperature, salinity and oxygen show that a large area of the lake is well stratified, at least periodically and no marked mixing occurs, between the upper less saline water and the lower saline water. In the halocline the vertical gradient of salinity is large. Mesothermic and poikilothermic conditions were also observed, whereas large temperature differences were recorded in water column.

A note on the frequency of occurrence of severe winters as evidenced in monastery and historical records from Greece during the period 1200?1900 A.D.

Monastery and historical records for Greece during the period 1200–1900 A.D. have been used to obtain the number of “severe” winters which occurred during this time period. A winter was considered as being “severe” if long duration of snow cover and/or freezing of lakes in Greece were archived. Evidence is presented that the coldest periods in Greece during the period 1200–1900 A.D. occurred in the first half of the 15th century, in the second half of the 17th and in the 19th century. Geographical coherence is indicated with other areas in Europe, e.g., with The Netherlands, where historical records exist over the same period. The number of severe winters in 50-year intervals does show a positive correlation with the volcanic dust veil index.