Landscape ecology and geochemistry of high altitude lakes-insight from the Central Pyrenees

Resumo

High elevation lakes are promising sentinels of global environmental changes. Yet many of the fundamental processes at these sites are still poorly understood. In this tesis I examined how environmental factors shape ecotope and ecosystem development at high altitude lakes and identified potential environmental hazard sources from the local geology. The study area spans a roughly 80 km E-W linear gradient in the central Pyrenees and is characterised by a high lake density. Data were collected during several field surveys and included a complex pool of landscape, ecological, geochemical and climate variables which helped answer fundamental questions regarding ecological and geochemical processes at high elevation lakes. I found that ecotope development at high altitude waterbodies is largely driven by a particular combination of hydrological, geo-morphological and topographical forces. This can be interpreted as major evidence of postglacial landscape evolution which created different physical niches for biota setting. Complex interactions among these factors were found to shape the riparian vegetation structure and determined their species co-occurrence patterns. Major littoral zoobenthic communities showed significant sensitivity to external ecotope factors such as topography formation (though its effects on catchment type, shore and catchment snow coverage and connectivity with other lakes) and hydrodynamics (waterbody size, type and discharge of input/output), as well as to the riparian vegetation structure. These communities formed eurytopic associations, i.e. associations present in a variety of habitats. Both riparian and littoral ecosystems also responded considerably to large scale gradients such as altitude, latitude and longitude, possibly a condition of major climates converging in the Central Pyrenees. Generally altitude watherbodies are poor in nutrient resources. The present results showed that their riparian and littoral ecosystems have a high capacity to reflect a particular set of attributes from the surrounding terrestrial system. Relatively high levels of trace metals were found in the sediment and waterof Respomuso lake catchment, at the contact zone between granite and metamorphosed sedimentary bedrock. The origin of these metal was the metal-rich geology. The sediment-bound trace metals constitute a considerable metal burden in the catchment, with the concentrations of As, Cd and Ni exceeding the sediment quality guidelines for the protection of aquatic life. With respect to arsenic- a highly toxic element of concern, the results revealed a relatively high degree of natural enrichment in the sediments, due to mobilization from source areas upstream wich are dominated by quartzite and slate elements. The arsenic concentrations in the water also exceeded the guideline value for the protection of aquatic biota at a number of sites. The findings indicate that this most likely resulted from its higher mobility from the sediments or surrounding metal-rich bedrock under the oxic condition of the streams. This is a significant result as these concentrations may increase ir the environmental/climate conditions change. Nonetheles, one of the most singnificant findings resulted from the paleogeochemical analysis of a sediment core. The results uncovered the potential of climate change, particularly the elevation of the freezing line, a general increase in the frequency of drier periods and a reduction of snow cover in the last three decades to enhance trace metals mobilizations from mountain exposed topography. Among the metals, arsenic and nickel,two hazardous elements, crossed their safe concentrations for the protection of aquatic life in the sediments deposited in recent years, pointing out to a potential threat to the wider environment. While this finding may raise as many questions as it answers, no doubt it has the potential to open a new directionin the challenging field of climate change research. These achievements have potential implications for the use of mountain waterbodies as sensors and integrators in the global monitoring of the environment.