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Introduction
Peat-bogs are ecosystems with a high degree of organization, which leads to spatial heterogeneity and characteristic structures consisting of water pools alternating with hummocks of Sphagnum (Margalef). The nutrient input to these ecosystems is limited and consists essentially of minerals dissolved in rainwater. In general, peat-bogs are characterized by low primary production, but have considerable accumulation of organic matter. Nevertheless, this accumulation of elements is not passive, because the considerable circulation and redistribution of water in peat-bogs usually create diverse micro-gradients.
Peat-bogs are highly suitable habitats for algal flora and assemblages (Bunt, 1954; Ettl, 1968, 1970; Hickman & Vitt, 1973; Kitner et al., 2004; Neustupa et al., 2009; Nováková & Poulíčková, 2004; Skuja, 1948, 1956, 1964; Šťastný, 2009). These studies focused on flora and ecological gradients, too (Poulíčková et al., 2003).
Various aspects of the aquatic systems in the Pyrenees mountains have been tackled, mainly by studies of lakes (Bartumeus et al., 2006; Catalán, 1987; Catalán et al., 1993, 2006; Felip et al., 1995, 1999; Gacia et al., 1994; Pla, 1999, 2001; Sanz et al., 2002; Vilaseca, 1978), but also of springs and rivers (Sabater & Roca, 1990, 1992).
In addition to lakes and rivers, Sphagnum peat-bogs form another widespread Pyrenean freshwater ecosystem. Interest in Sphagnum vegetation is readily explained by the fact that this group of mosses is the most ecologically dominant and economically important worldwide (Andrus, 1986).
In the Spanish Pyrenees, Sphagnum peat-bogs are usually restricted to, lake shores or wet soils situated between 1,800 and 2,200 m a.s.l. (Casas et al., 1994). These peat-bogs cover only small areas. In general, they are ecologically important and are sometimes considered vulnerable ecosystems. This vulnerability is especially relevant in the Pyrenees because there are few bogs and these are exposed to persistent negative influences that can severely disturb or even destroy them (e.g. drought, tourist activities, draining, overgrazing by cows and sheep).
Several studies have examined the algal flora of Pyrenean peat-bogs (Allorge & Manguin, 1941; Cambra, 1998, 2010; Cambra & Roura, 1995; Cambra & Hindák, 1998; Carter, 1970; González-Guerrero, 1927; Massanell, 1966; Margalef, 1946, 1948, 1952, 1956). However, additional data on the biodiversity of these ecosystems are still required. Sphagnum peat-bogs host a wide variety of algae, which usually form highly diverse populations. Sphagnum mosses reduce the pH of a given site (Glime et al., 1982) and may alter its environmental variables (Poulíčková et al., 2004). This capacity explains why species distribution differs along environmental gradients, e.g. from wet to dry Sphagnum (Andrus, 1986). However, little attention has been paid to the distribution of Sphagnum in peat-bogs in the Pyrenees. Here, we studied algal communities along an ecological gradient in a peat-bog in the Bassa Nera lake.
Materials and Methods
The study was carried out (summer 2010) at the Bassa Nera lake, located in the Aigüestortes National Park, NW Catalan Pyrenees (Fig. 1). This water body can be considered a relict lake with extended Sphagnum vegetation. The lake is located 1,890 m a.s.l., below the Gran Tuc de Colomers mountain (GPS 42º 38' N, 0º 55' E). The substrate is schist, the lake has an average depth of almost 4 m and its water is dark brown (Bassa Nera means 'Black Pond'). The catchment covers approximately 37 ha. The lake is oval (102 x 64 m), with a surface area of just over 4,500 m2. A flat peat-bog in the littoral zone holds a massive layer of Sphagnum that has a thickness of 4 m and covers almost 7,000 m2.
To study how the composition of algal assemblages changes over gradients, three parallel, linear transects were done from the open water into the Sphagnum peat-bog (Fig. 2). Each transect was 10 m long. Algal samples were collected every 2 m along each transect. A total of 12 samples were collected (4 samples per transect) from the lake shore of the Sphagnum zone (T1, T2, T3), from the dry-Sphagnum "hummock" zone (T4, T5, T6) and from the Sphagnum peat-bog zone surrounding the lake (T7 to T12). All samples were collected by squeezing a similar biomass of Sphagnum heads. Samples were then preserved in the field with formaldehyde to a final concentration of 4%. In addition, measurements for water temperature, pH (CRISON-pH), electrical conductivity (CRISON-Conductivimeter) and water content (% moisture out of Sphagum mass) were taken along the transects.
Figure 2 Way and sites where the algal samples were collected along the transect in Bassa Nera Lake, Spain.
Algae were studied under an OLYMPUS BX-51 microscope. Each sample was homogenized before being pipetted onto previously prepared slide mounts. Diatoms were cleaned following Tomàs (1988) and mounted with naphrax. For taxonomic identification, several monographs were used Anagnostidis & Komárek, 1988; Bourrelly, 1968, 1970, 1972; Desikachary, 1959; Gonzalves, 1981; Komárek & Anagnostidis, 1990, 2000, 2005; Komárek & Fott, 1983; Krammer, 1997, 2002, 2003; Krammer & Lange-Bertalot, 1985, 1986, 1988, 1991a, 1991b; Lange-Bertalot 1993, 1996, 1999a, 1999b, 2001, 2002, 2003, 2004; Krieger, 1937; Lange-Bertalot & Krammer, 1989; Lenzenweger, 1996, 1997, 1999, 2003; Mrozińska, 1985; Popovský & Pfiester, 1990; Printz, 1964; Starmach, 1966, 1972).
Cells were counted in random microscopic fields until a total of 300-400 valves/cells were reached. The relative abundance of each taxon was then calculated. From these data, we calculated the Shannon & Weaver's diversity index (H') for each sample. In addition, a Canonical Correspondence Analysis (CCA) was performed with the PAST statistical package. This statistical analysis was done to identify algal assemblages along the gradient, as well as to check the relationship between certain physico-chemical parameters and the species.
Results
The water temperature of the lake ranged between 21.3 and 27.4 ºC, while the water temperatures in the Sphagnum peat-bog were narrower, ranging between 26 and 27.4 ºC (Table 1). The pH of the lake was between 3.7 and 5.62, while in the Sphagnum peat-bog it was lower, between 3.7 and 4.02. The conductivity values in the water lake were 50-121 μS/cm, while in the Sphagnum peat-bog they were 67.2-121 μS/cm.
Table 1 Data of water temperature (ºC), pH, Conductivity (mS/cm), Shannon diversity (H') and water contents along the Bassa Nera transect.
A total of 110 algal taxa were identified (Table 2), of which desmids (37.6%) and diatoms (33%) were the dominant groups. The algal flora in the Bassa Nera lake had ubiquitous algal taxa, e.g. Achnanthidium minutissimum (Kützing) Czarnecki, Cocconeis placentula Ehrenberg or Pinnularia viridis (Nitzsch.) Ehrenberg. We also observed numerous species which are considered acidophilous, e.g. Chroococcus turgidus (Kützing) Nägeli, Cosmarium rectangulare Grun. (Fig. 3F), Desmidium swartzii Agardh ex Ralfs (Fig. 3B), Euastrum insigne Hassall ex Ralfs, E. pulchellum Brébisson. (Fig. 3C), Netrium digitus (Ehrenberg ex Ralfs) Itzigs. & Rothe, Oocystis solitaria Wittr., Penium polymorphum (Perty) Perty, Xanthidium armatum (Brébisson) Rabenh. ex Ralfs (Fig. 3E) and Staurastrum striolatum (Nägeli) Arch. (Fig. 3D). Several species are of particular interest, as they have rarely been recorded in Spain, such as Cystodinium cornifax (Schilling Klebs), Euastrum crassum Kützing, Euastrum insigne Hassall ex Ralfs (Fig. 3A), Euglena mutabilis Schmitz, Gymnodinium fuscum (Ehrenberg) Stein, Monomastix pyrenigera Skuja, Sphaerocystis stellata Her. and Woloszynskia neglecta (Schilling) Thompson.
Table 2: List of algae and cyanoprokaryota taxa found in the Bassa Nera Lake at the Pyrenean peat-bog, Spain.
Figures 3a-f Desmids of Bassa Nera at spanihs Pyrenees. a) Euastrum insigne Hassall ex Ralfs (Segment = 20 mm); b) Desmidium swartzii Agardh Ex Ralfs (Segment = 10 mm); c) Euastrum insulare (Wittr.) Roy (segment = 10 mm); d) Staurastrum striolatum (Nägeli) Arch. (Segment = 10 mm); e) Xanthidium armatum (Brébisson) Rabenh. ex Ralfs (Segment = 50 mm); f) Cosmarium rectangulare Grun. (Segment = 10 mm).
Lake vegetation were, dominated by Potamogeton alpinus, Myriophyllum alterniflorum, Utricularia sp. and Equisetum fluviatile. The vegetation covered 3,900 m2, which accounts for 85% of the lake's surface. The presence of other species on the Sphagnum peat-bog was also notable (e.g. Drosera rotundifolia, Drosera anglica, Eriophorum sp., Menyanthes trifoliata, Parnassia palustris and Potentilla palustris).
The first two axes in the results from the CCA (Fig. 4) explained 83.93% of the data variability. The first CCA axis had more weight and we considered that it expresses a combined pH and moisture gradient (54.47% of the variability). The pH was lower at T4, T5, T6, T7-T12 and higher at T1, T2 and T3. For the second CCA axis, conductivity had more weight (29.46% of variability). Points T4, T5 and T6 had greater conductivity. Moreover, these sampling points were aerophytic environments. In contrast, points T7-T12 corresponded to the transition from an aquatic to a terrestrial environment and the largest expansion of Sphagnum biomass.
Figure 4 Correspondence Canonical Analysis (CCA) of the studied samples along the transect in Bassa Nera peat-bog at spanish Pyrenees.
Three groups of algal communities were clearly differentiated along the gradient. Nevertheless, in spite of the spatial segregation of three environments in the peat-bog, some taxa were distributed throughout the whole transect, tolerating the small variations in pH, conductivity and temperature, e.g. Frustulia crassinervia (Ehrenberg) Lange-Bertalot & Krammer, Navicula subtilissima Cleve and Tabellaria flocculosa (Roth) Kützing.
Group I: The lake shore Sphagnum zone (T1, T2, T3) had relatively acidic water and the algal assemblage was dominated mainly by Encyonema minutum (Hilse) Mann, Pinnularia maior (Kützing) Rabenh., Closterium costatum Corda ex Ralfs, Cosmarium conspersum Ralfs and Actinotaenium turgidum (Brébisson ex Ralfs) Teiling. However, the quantitative data showed that, in terms of relative abundance, diatoms and flagellated dinophytes (especially Woloszynskia neglecta (Schilling) Thom.) were the dominant taxa in this zone. The species included in Group I corresponded to a mixture of taxa, a characteristic of peat-bogs, together with representatives of phytoplankton dystrophic lakes.
Group II: The dry Sphagnum "hummock" zone, with less water. This environment had subaerial conditions, which was the main ecological factor. In general, acidophilous taxa and aerophilous species were dominant, consisting mainly of Eucapsis alpina Clements et Schantz, Oocystis solitaria Wittr., Gomphonema gracile Ehrenberg, Euastrum insulare (Wittrock) Roy, Cylindrocystis brebissonii (Ralfs) de Bary, Nostoc kihlmani Lemmermann and Euglena mutabilis Schmitz, while the other algal groups were quantitatively irrelevant.
Group III: The Sphagnum peat-bog zone surrounding the lake (T6, T7, T8, T9, T10, T11, T12) had more acidic water and higher water temperature. The Sphagnum peat-bog was dominated by the diatoms Cymatopleura elliptica (Brébisson) Smith, Diatoma mesodon (Ehrenberg) Kützing, Epithemia sorex Kützing, Fragilaria brevistriata Grun., Sellaphora pupula (Kützing) Mereschkovsky, Pinnularia microstauron (Ehrenberg) Cleve and by other algae, such as Cylindrospermum stagnale (Kützing) ex Bornet & Flahault, Hapalosiphon fontinale (Agardh) Bornet et Flahault, Euastrum insigne Hassall ex Ralfs, Closterium intermedium Ralfs and Actinotaenium cucurbita (Brébisson) Teil. These algal groups were quantitatively and qualitatively dominant, but in this case the assemblage was formed by sphagnophilous species. Another relevant aspect was the increase (quantitatively and in species richness) in desmid taxa along the transect, reaching their highest relative abundance in this zone.
Species richness and diversity increased towards the water of the lake (Fig. 5). The diversity index (H') was between 1.71 and 2.83 along the transect. The lakeshore Sphagnum zone (T1, T2, T3) had higher diversity (H'=2.37-2.83 bits) than other points of the transect. In contrast, samples T4 to T12 showed slightly lower values (H'= 1.71-2.1 bits).
Discussion
Peat-bog systems are characterized by low nutrient concentrations and substantial amounts of organic matter. In general, peat-bog algal assemblages consist of a mixture of acidophilous and sphagnophilous taxa, which are widespread along micro-gradients in these habitats (Cambra & Hindák, 1998; Borics et al.,1998; Kol, 1970).
The Bassa Nera lake is sheltered from the wind by terrestrial vegetation (mainly Pinus uncinata) and hydrophyte cover, thus reducing significant water turbulence. In these undisturbed lentic environments, organisms that do not show effective movement sink to the sediments (Borics et al., 2003). This might explain why the dominant algae in the Sphagnum zone of the lakeshore were flagellated dinophytes like Woloszynskia neglecta while Hemidinium nasutum Stein or Gymnodinium fuscum (Ehrenberg) Stein were less abundant. Moreover, periphytic algae are relatively abundant in such habitats. In the Bassa Nera lake these algae were represented by Achnanthidium minutissimum (Kützing) Czarnecki, Encyonema gracile Ehrenberg, Gomphonema gracile Ehrenberg and Oedogonium sp.
As a result of the dryness of the Sphagnum "hummock" zone (Group II), the establishment of many desmid species was hindered, while the presence of diatoms was also limited because of the extremely low pH (Poulíčková et al., 2004; Lederer,1999). This pattern was reported in peat-bogs in Hungary by Uherkovich (1984), who concluded that desmids are more sensitive to drying out than diatoms. Consequently, flora impoverishment starts with the disappearance of desmids. This is precisely what we observed for desmid species richness in the dry Sphagnum "hummock" zone. However, the relative abundance of desmids in Group II was higher than in the lakeshore Sphagnum zone. Thus, although Group II contained only a few desmid species, they were sometimes relatively abundant. On the basis of this observation, we believe that desmids, such as Cylindrocystis brebissonii (Menegh. ex Ralfs) de Bary, Penium polymorphum (Perty) Perty and Euastrum insulare (Wittrock) Roy, are relatively well adapted to dry peat-bog habitats. Our results are consistent with data reported by Poulíčková et al. (2004) and suggest that the poor algal diversity of Group II in Sphagnum hummocks is attributable to dry and acidic conditions.
The Sphagnum peat-bog zone (Group III) was inhabited by typical bog algae, namely mainly desmids and diatoms. The zone's samples show affinities with Micrasterieto jenneri-Euastretum insignis, described by Margalef (1955) in Galicia (NW Spain). This finding greatly extends the known distribution area in Spain of this community, which is probably widespread in acidic peat-bogs of northern Spain, reaching the sub-alpine zone, as occurs in the Pyrenees. However, during our visits to more than one hundred peat-bogs in this mountain range over several years, we did not detect this desmid community in any lake. Its presence in the Bassa Nera lake is highly relevant for the Aigüestortes National Park and allows it to be classified as a relict peat-lake.
With regard to the diversity index (H') of these communities, the greatest diversity was observed in Group I, thereby indicating that this algal community is diverse. However, in this case, this environment hosts not only algae that live among Sphagnum thalli, but also diverse forms of phytoplankton, some of which are possibly from the periphyton, which coexist with sphagnophilous species.
When dealing with small-scale gradients like the one addressed in this study, the spatial heterogeneity of algae must be examined to check for small changes over very short distances. Our results confirm those reported by Nováková (2002), in which the spatial heterogeneity of algal communities of particular microbiotopes varied more than between various localities (peat-bogs). Moreover, we observed that species dominance increased with decreasing moisture, results that are consistent with the findings reported by Poulíčková et al. (2003) and Krenková (2001).
Our results confirm the high algal biodiversity in Sphagnum peat-bogs (even in southern European countries). The recent decrease in species diversity in European peat-bogs reflects the long dry period that started in the early eighties (Borics et al., 2003). On the basis of these considerations and our results, we conclude that peat-bog algal assemblages could serve as indicators of anthropogenic disturbances at local levels, as well as climate changes on a global scale.
