Abstract

Laguna del Maule is a volcanic lake (36°S-70°N, 2160m) in the Cordillera de Los Andes, 250 km S of Santiago de Chile, area: 45 km² and maximum depth of 50 m. Salmonids and myriophylum macrophytes are abundant and it has sectors for trekking, mountaineering and off road. In 2011 and 2012 (pioneer surveys) six cores were obtained with a gravity corer (4, 43, 50 and 31m depth), Pb210/Cs137 studies were carried out. Cores were opened, photographed and described sedimentologically. Core LEM11LEMAL-1G-1A (54cm long) showed two volcanic tephras and was volumetrically subsampled every 5cm, aliquots were taken for diatomic analysis, processed according to standard methods, permanent slides were mounted and observed under an optical microscope. We use diatoms as bioproxies to infer past environmental conditions because they are highly sensitive to changes on rate and nutrients supplies and are powerful tools to quantify eutrophication. The first 5cm of the core were studied, understanding that the surface sediments represent an integrated sample from the lake diatoms, both spatially and temporally. 38 genera were identified belonging to Bacillariophyceae and presence/absence analysis of diatoms associations showed high species richness: 70spp and let infer mesotrophic water conditions, because of the presence of Asterionella Formosa and Fragilaria crotonensis (planktonic) to eutrophic: Aulacoseira granulata and Stephanodiscus sp. (planktonic). These results are indicative of strong natural eutrophication events (high levels of phosphorus and nitrogen) in recent times as a result from prolific of spawning salmonids and deep water anoxia with subsequent release of phosphorus from sediments.