We analyzed nutrient, chlorophyll, and ionic concentrations in 17 lakes along a strip of land located in the rainy Andes Mountains and the ecotone with the dry Patagonian steppe, from 36 to 44° S latitude. Most of the sampled lakes had the ionic composition of Andean lakes and showed ultraoligotrophic conditions, while a few with the ionic composition of Patagonian Plateau lakes showed higher nutrient concentrations. Chlorophyll was correlated to P and N concentrations, but chlorophyll/TP ratio was lower than reported in previous worldwide surveys. A lithology dominated by igneous rock together with the remoteness and pristine character of the basins contribute to the oligotrophic status of the lakes. Two other factors appear to be important in maintaining the oligotrophic condition. Andisols, allophanic soils developed over volcanic ash deposits in the rainy areas, have large P retention capability and therefore decrease the load of P to the lakes. Natural dense Nothofagus spp. forests, with a geographical distribution coincident with that of the andisols, are efficient mechanisms for nitrogen retention through nitrogen resorption from senescent leaves.Instituto de Limnología "Dr. Raul A. Ringuelet
