Limnetica 37

Lake Marboré (or Ibón de Marboré) is a high-altitude (2612 m a.s.l.) alpine lake situated in the central area of the Pyrenees range (NE Spain), which forms part of Ordesa and Monte Perdido National Park. This emblematic lake of glacial origin is chiefly fed by snow and meltwater, and the outlet is the headwaters of the Cinca River. Given the extreme climatic conditions prevailing in the area and the difficulty of sampling and monitoring due to the thick layer of ice and snow that covers the cirque most of the year, this lake has been the subject of very little scientific research. Although some paleolimnological studies have been performed on sediment cores, no previous investigation has been carried out on the limnology (including limnochemistry, stratification dynamics and microbial ecology) of this lake. This work presents the main findings and conclusions that have emerged from a recent study conducted in the lake over the course of three summer campaigns (2013 to 2015, always in September). We report the stratification dynamics and water chemistry of this lake (including the nutrient availability and trace metal dynamics), as well as basic parameters related to the biological activity, such as O₂ consumption and redox chemistry. Our results show that Marboré is ultra-oligotrofic and hosts very low biological activity, typical of high mountain lakes. A slight stratification developed at the beginning of the ice-free period, with small vertical gradients in temperature (3-11 ºC) and electrical conductivity (60-100 µS/cm) between the bottom (situated at approximately 28 m depth) and the lake surface. The current pH (between 7.0 and 7.8, which is lower than previous measurements) is not compatible with the formation of calcite in the water column. This finding is coherent with the mineralogy of the sedimentary record. The most interesting geochemical finding, however, relates to the presence of significant concentrations of dissolved trace metals (Fe and Zn) and sulfate, along with suspended mineral particles (including sulfates), near the thermocline. These high concentrations suggest that local sulfide oxidation processes influence the lake chemistry.