Ecological restoration of the Peñalara Lake

At the beginning of last century nineties, the Peñalara Lake had a deplorable state of conservation. The area accumulated garbage, the lake suffered recurrent eutrophication episodes and the banks were tremendously eroded. In addition, an exotic fish specie, the Arctic char, had been introduced and caused the disappearance of many aquatic species living in this lake.

All these environmental problems were the result of a disproportionate and inappropriate use of this enclave, initiated from the seventies. In 1990, however, there is a turning point in the progressive deterioration of this high mountain wetland. In this date occurs the reclassification, from Natural Site of National Interest to the Natural Park (which would be later integrated into the current Sierra de Guadarrama National Park). With reclassification came the first emergency measures as to amend the detected environmental problems, more specifically the prohibition of bathing and camping.

As of that date, a number of limnological studies were conducted in the lake, which remains in limnological monitoring. These studies have served as a base to a series of actions aiming the ecological restoration of the lake.

Residues removal

For nearly two decades, there was in this high mountain area a remarkable accumulation of residues (plastic, bottles, batteries, aluminum foil, remains of clothing, canned food and soft drinks, organic waste, etc.). Perhaps the most obvious impact to the casual visitor was aesthetic, with an evident degradation of the quality of the landscape. However, the accumulation of residues had a much greater impact for its contribution to the eutrophication process.

The progressive implementation of different public use, and above all, the increase in the level of awareness of the visitors in general, has managed to significantly reduce this problem. However, it was necessary to carry out an intensive cleaning campaign that included removal of large debris from the bottom of the lake by divers.

Apart from the recommendations on residues that are made to the visitors, the National Park maintains a cleaning team which is responsible for removing any garbage.

Control of eutrophication

Eutrophication is the entry and enrichment in nutrients, primarily phosphorus and nitrogen in the water having in natural state very low concentration of these. Phytoplankton algae need these nutrients, along with sunlight and carbon dioxide (CO2) to develop. In the lake, they have light and CO2 in abundance, so that their growth is limited only by the availability of nutrients. When their concentration level increases artificially, there is a disproportionate growth of algae, so the water gets a characteristic green color. Ultimately, all the physico-chemical characteristics and virtually all the aquatic communities are affected.

The Peñalara Lake has had very low concentrations of phosphorus and nitrogen for thousand years. However, the massive influx of visitors since the end of the 70 triggered a progressive eutrophication process. The accumulation of garbage that has been commented previously contributed to this. The lake banks eroded and caused much of these soils organic substances, another source of nutrients, dropping into the lake. In addition, bathing in the lake, a very common activity before the nineties, promoted the resuspension of nutrients stored in the sediment. The introduction of the arctic char could have contribute to the recycling of nutrients within the lake and therefore to the stimulation of the eutrophication.

After the ban on bathing and camping at the beginning of the nineties, the hydro-chemical analysis revealed that in only 3 or 4 years the nutrient concentration reduced to the levels expected in this type of lakes (oligotrophy). For example, in 1990 the amount of phosphorus in the lake reached of 427 μg P/l and an annual average of 242 μg P/l, while from 1995, the maximum phosphorus annual total has not exceeded the 150 μg P/L, with an average of 24 µg P/l.

Erosion Control

The excessive influx of visitors was also the cause of the lake banks erosion by the continuous trampling. In a certain area of the moraine, there was a soil decrease of almost a meter thick, only in a period of less than 10 years, which is equivalent to almost half of the height the moraine reaches over the lake water level. It is important to note that, given that the estimated rate of soil formation in mountain areas is around 0.5 - 1 mm/year, the decapitation and loss of soil in the lake moraine meant the disappearance, in just one decade, of soil formed during centuries.

On the other hand, the banks erosion has resulted in a significant increase of the sedimentation rate in the lake. Aquatic ecosystems evolve over thousands of years, filling up with sediment until closing. However, as part of the eroded material in the the lake banks dropped bottom and sediment, the erosion accelerates the lake ecosystem natural aging process.

In 1995 the perimeter of the lake was fenced as to avoid visitors trampling. This fence consists of a steel cable on picket lines, in which advices, explaining the reason for this limitation of access, have been placed. In addition, the moraine that closes the lake, the most severely affected, was fenced with an electric shepherd to prevent the entry of livestock.

In only a few months, there was a remarkable recovery of grazing vegetation in the moderately eroded areas, as well as an improvement condition of the soil. However, in the most affected area there was no spontaneous recovery and several systems had to be tested as to retrieve the vegetative cover. The most successful method was the matgrass (Nardus stricta) nursery production, from seed collected in the area. These plants were used in the moraine to form small “islands”, and the rest of the area has been colonized by the effect of vegetative growth.

The effectiveness of these actions is under evaluation, with the following-up to the lake eroded material sedimentation rate, by means of sediment traps. These traps allow to know the eroded material existing in the basin and transported into the lake, getting an indicator of the degree of erosion in the basin. The evolution of the sedimentation rate obtained with sediment traps has been changing in recent years, with a decreasing trend of sedimentation. From 1.9 Kg/m² in 1997, to 0.83 Kg/m² in 1998 and to less than 0.56 kg/m² since that moment.

Eradication of Arctic char

The arctic char (Salvelinus fontinalis), is a salmonid native to the NE of North America which was introduced around 1970 in the Peñalara Lake. Previously there was no fish in this lake, which due to their greed led to the disappearance of numerous species of aquatic organisms inhabiting the lake.

After a series of studies to determine the extent of the impact, will be held on a plan for the eradication of this specie with the purpose of allowing the recovery of the aquatic populations affected. The eradication project was carried out between summer 1999 and summer 2004, although with different effort in different years. Five gill nets were used (30 m long) composed of 12 panels of 2.5 m x 1.5 m, with a 5 to 55 mm mesh size. The last scan was conducted in the spring of 2002, counting from the start of the eradication a total of 550 individuals, aged up to 4 years.

The eradication has been a remarkable change in the populations of aquatic organisms, with a rapid settlement of new species. As well, it has gone from 9-13 aquatic family organisms with arctic char living in the lake, to 23-26 families following the eradication project. All these species appear in ponds, which have served as recolonization farms.