Wetlands of the Basque Country (VI): inland wetlands: glacier-origin lakes

2000/10/01 Estonba Mintxero, Mikel Iturria: Elhuyar aldizkaria

• Ekologia

Along the Quaternary, all the summits had long glacial periods. As a result, the huge amounts of accumulated snow originated a large amount of glaciers of great erosive capacity. In the high areas of the valleys, where the glaciers accumulate most of the snow, melted and constantly frozen in the cracks of the stones, caused the first erosion: the stones were reduced and dragged down. As a result of this first erosion, in these high areas of the valleys, depressions called circuses emerged. In addition, as the glaciers moved through the valleys, they were transported by stones and rock fragments, with an enormous modelling effect. Because on this road the erosion of the glaciers found numerous obstacles, the induced excavation was not always the same, so in these valleys of glacial origin, of typical "U" form, depressions of various sizes and shapes were created. When the substrate was waterproof in circuses, valleys and in any other depression originated by the glacier, when the glaciers disappeared, water was accumulated and high mountain lakes were formed.

Most of the lakes originated by the erosion of glaciers are very deep and with steep shores. For this reason, the phenomena within the water mass are more important than those of dry water interaction, so these high mountain lakes cannot be considered wetlands.

However, by concentrating the sediments of the basin in the lakes, these tend to fill and will move from lakes to lagoons of little depth. In the last phase of this evolution the vegetation will spread throughout the area of the lakes and will become peatlands.

The process of filling, as the depth and volume of water decreases, increases the proportion between surface and volume, and the relationships and exchanges between the water mass and its physical environment are increasingly important. The greater relationship with the shores in this process and the absence of problems for the sun to reach the entire column of water make their characteristics vary: cold, transparent primary waters, little mineralized, low alkalinity, saturated with oxygen and with a considerable annual cycle, the new conditions make them not have annual cycle and have more nutrients, becoming high mountain ponds. These are precisely the ones that interest us, since they can be considered wetlands.

Although these ponds are more eutrophic than lakes, for various reasons they cannot be very rich. On the one hand, we should not forget that in these places the climatic conditions are very hard, since in addition to the cold temperatures, they are frozen and under the snow between four and eight months a year, so the ideal season for life is very short. On the other hand, it must be taken into account that in the high mountains the soils are thin and little developed, since it cannot be forgotten that the soil is the most important place for the fixing of the nitrogen and accumulates phosphorus, magnesium and other elements. Therefore, if the soil is thin, their wetlands will host poor communities.

However, the extreme conditions imposed by the harshness of the climate make these pools true islands of nature that characterize their geographical environment. There, several northern species that expanded along the glaciers to the south, after the ice recession, have been isolated. It is said that these species are of Boreoalpine distribution, since in addition to appearing in the northern territories, we find them in the high mountains of the southern upper hemisphere. These lakes and lagoons were formed during the glaciations so they are relatively young. For this reason, their communities of living things are of low diversity and are constituted mainly by cosmopolitan species.

Vegetation vegetation vegetation vegetation vegetation vegetation vegetation

As has already been mentioned above, the scarcity of food in the pools of glacial origin and the shortage of cold temperature and adequate season make there a little life, but the communities that develop in these harsh conditions are quite peculiar and can only be seen in these areas.

It is formed by species of broad-based algae adapted to the life of phytoplankton in oligotrophic waters, beginning with those that live in free waters. The most abundant are the crisofiae of the genera Ochromonas and Dinobyron and the dinoflagellates of the genera Peridium and Ceratium, but also the diatoms and the cyanophytes are common. The latter are of vital importance, since their potential for fixing atmospheric nitrogen allows them to enrich these means, relatively poor in food.

As for macrophytes, in these high degree of filling ponds are organized in vegetable belts. From the inside out, three and five meters deep, we only find on the stones the chlorophageal algae Nitella (often together with the cyanobacteria of the genus Nostoc). From there, the aquatic plant belt is developed. Among the deepest species of this belt stand out Ranunculus trichophyllus and Miriophillum alternifolium. Above live the species of ferns Isoetes lacustris and Isoetes velatum and the species Subularia aquatica, Ranunculus aquatilis and Sparganium angustifolium. Finally, we find species that live a few centimeters deep, such as Carex rostrata or Menyanthes trifoliata. Outside the water, but on the shores of lakes with high soil humidity, are formed juncales formed by species of genera Carex, Juncus and Eriophorum. Inside, peatlands are created in areas with clayey sediments.

Fauna and fauna Fauna of the Andes

These wetlands that host poor plant communities cannot be very rich in fauna. An example of this is the low density of zooplankton in the waters of these lakes. As with phytoplankton, it is mainly formed by cosmopolitan and boreoalpine species, such as Cyclops abyssorum copepodo, Daphnia longispina cladozero or Asplachna priodonta, Polyarthra vulgaris and Kellicotia longispina errotifero.

As for the benthic communities, in addition to the larva of insects filtered in the sediment and between the stones, we find cladoceros Biapertura affinis and Euryceros lamellatus or several species of non-planktonic oysters, cnidarios of the genus Hydra or red oligoquets of the genus Tubifex. On the stones, however, larvae of various insects (efemeropterans, tricopterans and plekopterans) appear that inhabit cold and oxygenated waters; moreover, due to the scarcity of calcium carbonate, live gastropods of the genus Lymnaea with very thin and soft and bivalvious shells of the genus Pisidium.

Other macroinvertebrates found in these lakes are the feared predatory coleoptera of the genus Dytiscus, which form a bubble in the back to breathe, the heteropterans of the genera Notonecta and Gerris or larvae of chickpeas.

As for vertebrates, they do not constitute large populations in these high mountain lakes. With regard to fish, the populations of benthic invertebrates are scarce and the common trout and ezkail populations found in these wetlands are very few. In the case of amphibians, two species of urodelos and two of anuros live here. The urodelos are the pale triton (omnipresent species) and the pyrenean triton (endemic and high mountain species). However, the amuros are the red wild frog and the txantxikua.