Spatially explicit modelling of the impacts of land-use and land-cover change on nutrient inputs to an oligotrophic lake

Land-use and land-cover has a relevant role in nutrient fluxes at the watershed scale. Rivers are the natural bridges between terrestrial and aquatic systems where water, nutrients, and sediments, among others, are transferred through land-margin and margin-aquatic interactions. In this regard, it is necessary to gain further understanding of the influence of land-use and land-cover changes (LULCCs) on aquatic systems characterized by high quality standards but whose watersheds are subject to recent LULCC. In this study, a spatially explicit model (N-SPECT) was applied with the purpose of assessing whether, in southern-central Chile, the recent LULCC could alter the nutrient input to oligotrophic lakes (total nitrogen (TN) and total phosphorus (TP)). We hypothesize that maintaining the LULCC would force a significant increase in nutrient inputs, mainly due to deforestation and agricultural expansion into areas of greater slopes and altitudes currently covered by temperate forest. The results showed degradation and deforestation with annual rates of loss of old-growth forest close to 1%. Similarly, significant clearance for agricultural purposes near rivers and coastal areas was noted. This condition will cause an increase in nutrient inputs from tributary rivers, and it is estimated that by 2020, under current LULCC trajectories, the TN loads entering the lake will be 1.11 times higher than those registered in the mid-1980s. The results indicate that to preserve and/or restore the quality of aquatic south-central Chilean ecosystems, it is necessary to modify the current LULCC: (1) to fundamentally stop the temperate forest degradation and (2) to regulate the expansion of agricultural land in steeper and higher areas, where important forest still exists.