Javiera Abarca, Hugo Nicolás Ulloa , Yarko Niño

Patagonian lakes are one of the most unexplored aquatic environments on Earth, and little is known about their thermo-hydrodynamics and current trophic state. Meanwhile, increasing urbanization and industrialization in their catchments compromise their health. Here, we investigate Lake Llanquihue, one of the Earth's great freshwater bodies in Northern Patagonia, Chile. Still considered a pristine environment, Llanquihue has experienced recent contamination events along its littoral, whose impacts remain unknown. In response, public and private agencies have started to develop tailor-made monitoring plans to survey water quality. However, without comprehensive knowledge of the lake's functioning, it is impossible to determine the fate and effects of contaminants in the water body. Here, we characterize via numerical simulations the basin-scale hydrodynamics of Lake Llanquihue, crucial information for diagnosing the transport of dissolved and suspended matter within its basin. Aiming to delimit the regions impacted by contaminated discharges, we pose a fundamental question that applies to any lake: What is the physical connectivity between two zones of interest within a lake? To address this question, we introduce a framework that characterizes the preferential pathways and quantifies the timescale tracers take to stream from one zone to another within an aquatic system. This framework is applied to investigate the physical connectivity among the main urban and rural settlements connected to the littoral zone of Lake Llanquihue. Our results show that the physical connectivity between long-distance littoral regions is primarily controlled by a time-persistent, large-scale gyre and seasonal mesoscale gyres. The above information will enable lake managers to monitor the health of the water basin, identify and notify potential risk zones impacted by polluted effluents, and advance toward a more sustainable relationship with Lake Llanquihue.