1. Introduction
Temporary ponds (TPs) are globally known under different names, such as temporary aquatic habitats, seasonal wetlands, ephemeral wetlands, vernal pools, etc. [
1,
2], and their importance is related to the existence of several rare and endemic species [
3,
4]. Temporary ponds are naturally widespread in all geographical regions and rare habitats, in contrast to their ephemeral character, are amongst the most long-term of aquatic habitats [
5]. Within the EU, this habitat is mainly distributed in dry and sub-arid areas of its southern region.
European Union countries have performed an assessment of all freshwater habitats larger than 50 ha to meet the requirements of the Water Framework Directive 2000 [
6]. A set of indicators and multimetric indices has been developed for the purposes of biological, hydromorhpological and physico-chemical monitoring, such as morphology, hydrology, nutrient status, thermal conditions, salinity, pollutants and priority substances. Thus, such indicators have been developed for larger water bodies and they are still lacking for smaller wetlands, especially for temporary ponds which are typically smaller than 10 ha and can be as small as 1 m
2. This is in contrast with the conservation value, valuable ecosystem functions and the unique biodiversity of these systems [
7].
Temporary ponds in the Mediterranean regions of Europe have been studied more intensively and are under effective protection status, as a result of their identification as priority habitat (Annex I code 3170*) in the EU Directive 92/43/EEC (Habitats’ Directive) [
8]. Mediterranean Temporary Ponds (MTPs) are shallow water bodies with annual inundated and dry phases of varying duration and timing [
2,
9,
10]. The main ecological characteristic of the habitat is that the autumn-winter wet (aquatic) ecophase is followed by a spring-summer dry (terrestrial) ecophase. The typical species found in them are particular in the sense that they are often dwarf, “amphibious” species which have adapted to this alternation of ecophases [
1,
11,
12]. Thus, these seasonal water bodies, regardless of their small size, operate as biodiversity hotspots [
13,
14] maintaining gamma diversity and they host considerable diversity of flora and fauna species that are often rare and endemic occurring uniquely in this habitat [
2,
8,
10,
11,
12].
The distribution of MTPs reflects the Mediterranean climate zones [
1], though there is insufficient knowledge about their selective spatial distribution [
12]. These temporary ponds have often been unidentified by conservationists, in contrast to permanent water ecosystems, even within protected areas [
5]. More recently, it is a common concern of the scientific community that the delicately balanced hydrological regime of these temporary water bodies, might be exceptionally susceptible to various factors, including soil drainage for agriculture or urban development, high vulnerability to pollutants, lack of public awareness and climate change [
5]. These ponds have been reported in various Mediterranean countries, such as Greece where they are known under the names “arolithoi”, “rousies” and “kolympes” [
15].
Due to the seasonality and vulnerability of the MTPs it is important to identify the biotic and abiotic parameters during the dry and the inundated ecophase, in order to provide an integrated description of these systems and understand their existence and development. Altitude, the surface area of the wet system, pond size and disturbances, such as grazing by cattle, have been found to be very important for biota [
2,
15,
16]. Furthermore, different geological and lithological conditions [
12], timing and duration of inundated and dry period [
17,
18], soil and water physicochemical factors [
2,
12,
14], size and depth of the pond, water supply [
2,
15] and trophic status [
2] result in different ecological conditions forming various biotic communities [
18]. Specifically, geochemical indicators, such as pH, sediment mineralogy, concentrations of ions, nutrients and potential pollutants in the soil and water column determine the quality and allow to quantify the environmental stress impacting the ecosystem (ecological status) [
7]. In terms of ecosystem functions, these ponds are important biodiversity hotspots, in the sense of species composition and biological traits [
19]. Well-connected networks of ponds are vital in the provision of new climate space, and spatial planning should be encouraged in order to enable pond biota to adapt to climate change [
19]. However, threats to these habitats strongly depend on specific site conditions, including pasture, expansion of cropland, forestry, hunting and touristic activities [
20].
In Greece, there are 33 documented MTP sites—73% of which are located on the Aegean islands [
2,
15], and the rest in the mainland. According to Zacharias et al. [
2], 39% of the MTPs in Greece are located on silty and sandy formations, 17% on the calcareous substrate and the remaining 44% lays on siliceous and volcanoclastic sediments (sedimentary deposits mostly composed of rock fragments of volcanic origin). The most common threat to Greek temporary ponds relates to intensive agricultural activities, which either expand over the MTPs or pollute their water bodies with fertilizers [
2]. Thorough geological studies are of high importance, since they could set up the geo-environmental baselines and their results could be used as reference points at the future (e.g., Reference [
21]) in the view of climate change or prior to any conservation management actions. Moreover, the geochemical conditions that dominate in MTPs, form an individual environment, for each one, of high geochemical value, which has been little studied up to now.
This paper focuses on four small and independent MTPs of high altitude thatlie within the National Forest Park area at Mt. Oiti, Central Greece. They are the only MTPs known on Mt. Oiti and the surrounding mountain ranges of Central Greece, with the exception of three ponds on Mt. Kallidromo which have different vegetation types and host few representative flora species. The flora composition of these ponds includes a total of 83 plant species; nine of them are dominant and special to temporary ponds. These ponds include the endangered local endemic species
Veronica oetaea, which grows at the beginning of the dry period [
22]. The fauna composition includes nine specialized (one possibly endemic) crustaceans [
23]. The ecological status, classified in accordance with Annex V of the Water Framework Directive [
24,
25,
26], defines the quality of the structure and functioning of the pond. The current study adopts the evaluation method of Dimitriou et al. [
27].
The objective of this research is the determination of the geo-environmental characteristics of the MTPs on Mt. Oiti, Central Greece. This is the first integrated study which contributes to the understanding of the ecosystems of Greek MTP areas, by investigating a wide range of parameters. Our findings can determine not only the ponds’ geological background and ecological status, but can also contribute to a better understanding of the presence and fate of the temporary ponds in the Mediterranean region in general. Furthermore, the future monitoring of these geochemical factors will allow to detect changes and to help in early warning regarding qualitative and quantitative risks.
4. Discussion
Temporary ponds, despite their small size and simple community structure, are often considered as early warning systems of the impacts from the long-term variations to larger-scale aquatic systems [
11]. Moreover, the factor altitude is critical for the threatened dwarf plant species of
Veronica oetaea (
Figure 10), as it is restricted to the high-altitude ponds of Mt. Oiti, but not to the lower ones, such as the Louka pond. According to Salerno et al. [
38], since the 1980s there is a trend of the lower elevation ponds (2500 m a.s.l.) to disappear or to reduce their surface area.
The geophysical research results in combination with the satellite images processing, led to the determination of the surface and subsurface structure of the Mt. Oiti’s MTPs. Our interpretation of the ERT profiles acquired across the pond of Livadies, displayed a synform of high resistance rock that has been outlined exactly under the pond itself (
Figure 4), which probably contributes to the formation of the temporary pond [
29]. On the contrary, the topography does not appear to have adopted the synform, neither the erosion procedures have altered the open surface geomorphology. Overall, the water is collected on a morphological ridge. The results of the ERT sections at the pond of Louka (
Figure 5) have clearly reveal displacements of high resistivity basement rocks (alpine basement), which seem to be synthetic structures to the westernmost normal fault that forms the Louka graben. This basement structure is covered with sediments, characterized by lower resistivity. This pond differs from the one described at Livadies, as the topography is clearly related to a subsided basin like water collection landform and not a morphological ridge. Regarding the pond of Alikaina (
Figure 6), a geoelectrical synform has also been identified from the ERT profiles, but with a more progressive distribution of resistivity values, increasing from small depths to greater ones. No connection to any groundwater was identified. At the Greveno pond, the ERT survey (
Figure 7) has also revealed a high resistivity synform outlined partly under the pond. As a result, the ponds seem to be sealed from any groundwater aquifer. Furthermore, with the exception of the Louka pond, the topography prevents any runoff of surface waters to the ponds. Therefore, the origin of the water of those ponds is confined to precipitation and, thus, water chemistry may be controlled by biological activities and water–rock interaction in the ponds’ bottom sediments.
Abiotic indicators, such as physical and chemical indicators, are specific of soil and water conditions and can be used to specify various disturbances impacting the wetland ecosystem [
7]. In a permanent pond or lake, the OM accumulates at the bottom of the habitat, while in a temporary pond, there is no sediment build-up every year at its bottom [
5]. In temporary ponds, most of the OM originates from the wet phase and oxidizes during the dry phase, as OM turnover is greatly correlated with redox conditions in wetlands sediments [
39]. Due to their small size and water volume, ponds are particularly sensitive to climate conditions as both the organic and inorganic compounds of their sediments are subject to oxidative aerial processes during the dry periods resulting in quite low OM values. This aerial oxidation of the OM contributes to the timelessness of these ponds preventing filling by organic residues. Concerning the MTPs of Mt. Oiti, the correlation between SI and OM (
Supplementary Table S3), which is directly related to the flora presence, exhibits a strong correlation coefficient of 0.89. TDS concentration is extremely poor, regardless of seasonality.
The bottom sediments granulometry of the ponds varies around the MTPs area. Between SI and gravel content, there is a positive correlation coefficient of 0.79. The mineralogy of the bottom sediments of the ponds does not display any significant variations. Our analytical results suggest that no correlation can be determined between SI and SiO
2. However, an absolute correlation can be made between SI and MgO of 0.99 correlation coefficient. It is noted that MgO presence is related to the oxygen supply, which strengthens flora existence [
40].
Despite their absolute element concentrations, the bottom sediments of the ponds show similar upper-crust normalized element spidergrams patterns similar to Taylor and McLennan [
41] upper-crust composition (
Figure 11). An exception to this is the phosphorus content, which displays a slight difference between the Greveno–Alikaina and Louka–Livadies ponds and may be related to local biological activity. As barium substitutes both K in alkali feldspars and Ca in plagioclase, and plagioclase is absent in those sediments (
Table 3), Ba exhibits almost identical normalized values as K, which results in a slight negative anomaly to Ba. Moreover, Sr negative anomaly is attributed to the lack of Ca bearing minerals, such as calcite and plagioclase (
Table 3). The slightly positive anomaly to Ti may be related to clay minerals and amorphous Fe-Ti oxides, which originate from the flysch of the wider area’s bedrock [
33]. TDS, chlorine, boron and bromine values are extremely low, due to the origin of the water, mostly deriving from snow and rainfall.
The higher PO
43− content recorded in Alikaina water samples is well-matched with the higher P
2O
5 content in the bottom sediments of the pond (
Table 4 and
Table 5). The extremely low calcium and magnesium content of the water of all ponds can be explained by the absence of any carbonate minerals in the bottom sediments.
Kissoon et al. [
42] have worked on shallow lakes in Minnesota, and they have suggested the extent to which multiple elements in shallow lake waters and sediments were influenced by a combination of variables, including sediment characteristics, lake morphology and percent of land cover in watersheds. These data may be informative for illustrating the extent of functional connectivity between shallow lakes and adjacent lands within these lake watersheds. In contrast, the chemical results of the studied water samples from the MTPs of Mt. Oiti suggest that it is of meteoric origin, directly derived from precipitation (rain and snow), with very low mineralization (TDS < 1000 mg/L). During the two-years and five seasonal periods (wet/dry) of water sampling, the slight variations of pH values were not correlated with seasonality. Low concentrations of TOC, NO
3− and NO
2− indicate that fauna does not impact the quality significantly. However, increased amounts of nitrates during the dry period of 2013 may be attributed to the sporadic presence of animals. The most significant features, distinguishing Oiti’s pond waters from any other province water bodies, are the extremely low content of any studied cation. According to Vasilatos et al. [
43,
44] and Megremi et al. [
45,
46], TDS and major and trace cations concentrations of groundwater and river water samples from the neighboring area of eastern Sterea Hellas (central Greece) are significantly higher (by several orders) than the ponds samples from Oiti. It is notable that in the wider area of the Louka pond, there is an abandoned bauxite mine. However, the bauxite buried deposits do not affect the hydrochemistry of the pond. The low-temperature conditions of the studied areas may be unfavorable for active water–rock interaction [
45]. Furthermore, recent efforts for the establishment of the plant species at the pond of Louka have proved that the ecological niche of the endemic plant may include the bioclimate at altitudes as low as 1150 m. The fact that there is a trend of surface area loss for lower elevation ponds (under 2500 m) [
38] is promising for the endemic plant
Veronica oetaea.
The normalized element spidergrams of MTPs water samples from Mt. Oiti (
Figure 12) indicate that all elements exhibit lower values than the wetland water with the exception of K and Rb. The observed positive anomaly of those elements could be related to the mineralogical composition of the bottom sediments (
Table 3). K and Rb are very mobile elements in environmental conditions and exhibit similar geochemical behavior. It is noted that K may constitute the main nutrient ion for the performance of flora. The Sr, Fe and Si negative anomalies observed, may be attributed to water-rock interaction controlling the chemistry of the wetland water [
45] that has been used for normalization. That fact may be related to the enrichment of those elements in the wetland water; normalizing (dividing) with their values results the low quotients plotted in the diagram.
Piper plots of the ponds water major ions contents revealed that MTPs of Mt. Oiti have similar characteristics to typical shallow and fresh water [
47,
48,
49], mostly of calcium bicarbonate dominant (
Figure 13). The water samples of the ponds, as can been seen in Piper diagram plot, are of calcium-bicarbonate type except the pond of Louka water sample during the dry period which is of mixed-bicarbonate type (
Figure 12). According to Berner and Berner [
47], calcium bicarbonate dominant water type is a common type of meteoric common water. In the dry period the Mg
2+ trend is higher compared to the wet season, probably due to dilution in the wet season and evaporation during the dry season. Moreover, it is noteworthy that according to Shvartsev [
48] the wetlands around temperate humid climate zones in Europe, North America and Asia are of calcium-bicarbonate (to mixed) type, same as water samples from the MTPs of Mt. Oiti.
The geochemical and mineralogical study of bottom and water pond samples indicates that the MTPs of Mt. Oiti are not exposed to any significant pressures. However, during the dry period of 2013, the presence of pasture animals was identified, as increased nitrate values were determined at the water pond samples of Louka and Greveno. Moreover, slightly increased lead values in bottom sediments at the Louka pond are probably attributed to the hunting activities (e.g., Reference [
50]), as buckshot disposal was identified in situ. Furthermore, in all Oiti ponds in situ observations were conducted for evidence of tourism and other activities (
Table 6). Concerning the Louka pond, the evident trampling is mainly due to the fact that it is located among summer shepherd establishments, and secondarily due to the better accessibility of the pond area because of its lower altitude. Touristic activities, such as off-road vehicles crossing, have been noticed at the peripheral belt of the ponds. Trampling by off-road vehicles may change the geomorphological character of the ponds, sediment substrate and column and subsequently prevent the germination and the development of the endemic pond species [
11].
All the acquired geological, geochemical, hydrochemical and mineralogical data were combined in order to identify the pressures that affect the ecological status of MTPs of Mt. Oiti. According to the evaluation method of the ecological status by Dimitriou et al. [
27], the MTPs site of Mt. Oiti was classified in accordance with the four quality categories (excellent, good, medium and at-risk), given the situation of each pond (
Table 6). Herein, the ecological status represents the pressures imposed on the habitat, excluding the evaluation of flora composition data at each site [
22,
51]. The resulting ecological status is combined with the ecological status of the MTP sites in Greece by Dimitriou et al. [
27], as shown in
Table 6. Of the examined MTPs, Livadies, Alikaina and Greveno pond present an excellent eco-status meaning that these ponds tend to maintain their typical characteristics. The Louka pond presents a medium ecological status, indicating the degradation of pond area related to human and animal pressure activities.
The Louka pond is exposed to limited threats which may affect soil and water quality and the biota. Notably, these results are in agreement with the floristic study of the ponds by Delipetrou et al. [
22,
51]. The conservation status of the MTPs based on the floristic composition and structure (abundance and spatial distribution of the typical habitat species and intrusion of non-typical species) is excellent in Greveno, Livadies and Alikaina, but degraded in Louka where terrestrialization of the habitat is evident. Overall, the sound conservation status of the MTPs is related to the abundance and diversity of the typical habitat species which, being restricted to this habitat, increase gamma (landscape) diversity.
5. Conclusions
This study presents for the first time a full dataset of the geo-environmental parameters of the EU Priority Habitat Type “Mediterranean Temporary Ponds” (MTPs), along with their current ecological status in Mt. Oiti, Greece. These temporary ponds display unique ecological characteristics. Τhe results of this study suggest that they are at a sound conservation status and have good future prospects, although, one pond is exposed to limited threats that could affect sediment and water quality and also the biota. The main pressures identified for those habitats were grazing and touristic activities.
Mediterranean Temporary Ponds of Mt. Oiti correspond to shallow depths (5–20 cm), small size (112–607 m2) and high altitude (between 1000–2000 m a.s.l.). Electrical Resistivity Tomography measurements identified synforms outlined under the ponds. However, topography does not always adopt these synforms, mostly due to erosion procedures. The most significant features, distinguishing these pond waters from any other province water bodies are the extremely low content of all studied cations. The water bodies of the studied MTPs are of biocarbonate dominant type, and a fresh meteoric water origin is suggested. The bottom sediments of the ponds show similar element spidergrams patterns close to upper-crust composition. Neither bottom sediments nor water chemistry presented evidence for interaction between bedrock, sediments, and water—probably due to the low-temperature conditions of the area. Shape complexity (SI) of the MTPs of Mt. Oiti, which is an index for species richness, indicates the direct correlation of SI with organic matter, clay minerals and magnesium presence.
Since lakes and ponds are considered as early indicators of climate change, in which high altitude ecosystems are especially vulnerable, this study will allow the establishment of a geo-environmental baseline level of these habitats that can be used in future comparisons. The overall results contribute to a better understanding of the presence of temporary ponds and their development in Mediterranean environments.