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Increasing the understanding of high mountain lake dynamics is essential to use these remote aquatic ecosystems as proxies of global environmental changes. With this aim, at Sabocos, a Pyrenean cirque glacial lake or tarn, this study shows the main results of a morphological and catchment characterization, along with statistical analyses of its hydrochemical trends and their concomitant driving factors from 2010 to 2013. Dissolved oxygen, water temperature stratification, and its snow and ice cover composition and dynamics have been also investigated. According to morphological analyses, Sabocos can be classified as a medium-large and deep lake, having a circular contour and a long water retention time as compared to Pyrenean glacial lake average values. Sabocos hydrochemistry is mainly determined by very high alkalinity, pH and conductivity levels, and high Ca(2+), Mg(2+), and SO4(2-) content, coming from the easily weatherable limestone-dolomite bedrock. Thus, lake water is well buffered, and therefore, Sabocos tarn is non-sensitive to acidification processes. On the other hand, the main source of K(+), Na(+), and Cl(-) (sea salts) and nutrients (NH4(+), NO3(-), and phosphorous) to lake water appears to be atmospheric deposition. Primary production is phosphorous limited, and due to the N-saturation stage of the poorly developed soils of Sabocos catchment, NO3(-) is the chief component in the total nitrogen pool. External temperature seems to be the major driver regulating lake productivity, since warm temperatures boot primary production. Although precipitation might also play an important role in lake dynamics, especially regarding to those parameters influenced by the weathering of the bedrock, its influence cannot be easily assessed due to the seasonal isolation produced by the ice cover. Also, as occurs in the whole Pyrenean lake district, chemical composition of bulk deposition is highly variable due to the contribution of air masses with different origin.

This study presents the key hydrochemical characteristics and concentration levels of major (Ca, Mg, Na, Si, K, Sr, Fe) and trace (Ba, Sc, Cr, Mn, Al, As, Li, Co, Cu, U, Pb, Hg, Au, Sn, Zn, Cd, Ag, Ni) elements in the water mass of four selected Pyrenean cirque glacial lakes (Sabocos, Baños, Truchas and Escalar tarns) with different catchment features, between 2010 and 2013. Resulting data set is statistically analyzed to discriminate between the natural or anthropic origin of the elements. Analyses indicate that in all cases, the main source of most major and trace elements is geological weathering, being thus individual bedrock composition the main driver of differences between lakes. Several anthropogenic sources of airborne Cu, Sc, Co, and Cr must be also considered. The shallowness of the lake is also a factor that may influence element cycling and concentration levels in its water mass. Concentrations of anthropogenic elements were low, comparable to those reported in other glacial lakes, way below the WHO, US EPA, EC, and Spanish legal limits for drinking water quality, indicating the absence of serious pollution. Toxic heavy metals Cd, Pb, Hg, and Zn were not detected in any of the tarns.

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Baños tarn is a low-altitude Pyrenean glacial lake with both geothermal and surface influxes and significant local anthropic stressors. Its water was seasonally sampled from 2010 to 2013 to study its dynamics and evaluate the main factors affecting its hydrochemical trends. Main surface and thermal streams draining into the water mass were also assessed and a detailed characterisation of the study site was made. The large catchment area of the Baños tarn implies a huge water input, a short water retention time and a high water mass mixing that reduces the natural stratification of the lake. The hydrochemical features of the Baños tarn are primarily determined by a slightly alkaline pH and fluctuating medium–high ionic and nutrient contents. Hydrological changes affecting lake tributaries are the foremost factors driving seasonal changes in the ionic composition of the lake. Resuspension of organic matter and nutrients trapped in floor sediments by a century-long history of untreated waste water inputs is supposed to be the main nutrient source to the Baños water mass. Other anthropic activities, such as tourism, dam and withdrawal pipe management, as well as shoreline reforestation, also affect the trophic dynamics of the Baños tarn. These results present herein are scientifically novel because of the uncommon features of the Baños tarn and may broaden our understanding of factors affecting glacial lake dynamics on a global scale.