PALEOENVIRONMENTAL RECORD FROM TWO HIGH MOUNTAIN TROPICAL LAKES IN MEXICO

There are only two high mountain lakes in Mexico: El Sol and La Luna. Located inside the crater of the Nevado de Toluca volcano (4, 200 m.a.s.l.), they are shallow lakes (<15m) with low mineralization and are considered to be sensitive and vulnerable ecosystems. While La Luna is in relative pristine conditions, trouts have been introduced into El Sol. The comparison of the diatoms that characterize each lake is of special interest in the understanding of environmental and anthropogenic changes in a high altitude, low latitude setting. The present study describes the diatom distribution in a sedimentary sequence from each lake. Chrysophycean stomatocysts, palynomorphs, pollen, carbon particles and magnetic susceptibility were also analyzed. The sediment cores were date whit ¹⁴C and ²¹⁰Pb. A total of 61 and 50 diatom taxa were identified in the Sol and La Luna respectively. The Pennales were the dominant group while the Centrales were represented only by Aulacoseira distans and Discotella stelligera. The main species are benthic-periphitic and characteristic of low mineralization and oligotrophy, which is consistent with their trophic level, shallowness and high transparency. Differences in species composition between the lakes have been interpreted in terms of pH differences but probably they also indicate that both systems are changing and giving warning signals of an increasing trophic level, the case of lake El Sol being more serious with the appearance of the Discotella stelligera and the increased abundance of phytoplankton remains in the most recent sediments (1990), reflects the positive effects on nutrient recycling operated by the stocked fish. Along the core the interval between AD 1500 to 1900 is identified as a time of recurrent droughts as there are signals of forest fires and reduction in water level in the lakes (Pinus pollen decrease, carbon particles increase and diatom assemblage changes).