Limnological aspects of a high-mountain lake in Mexico

Abstract

El Sol is a high-mountain crater lake situated in the caldera of Nevado de Toluca Volcano, Mexico. The climatic conditions at El Sol are intermediate between those of high plains (paramo) and arid tablelands (puna). The volcanic environment of the lake basin, its small drainage area and the high percentage oxygen saturation favor the maintenance of highly transparent water. A combination of tropical conditions with high altitude produce thermal characteristics intermediate between temperate and tropical lakes. Of these characteristics, a low average temperature is outstanding, despite the large solar radiation input to the system. Freezing frequency is intermediate between the annual freezing of temperate lakes at high latitudes and the sporadic freezing of equatorial high-mountain lakes. The radiation income is favored by water transparency and generates persistent masses of hot water in the lake's deepest parts whose temperatures differ up to 2.5 °C from those at higher levels. Rain is the principal contributor of solids to the lake through erosion of the crater walls. The drop in temperature during winter produces an important supply of ammonia from decaying organisms. The relationship N:P < 16 shows N to be limiting a primary productivity which is mainly due to benthic algae. The peak in phytoplankton chlorophyll coincides with the nutrients maximum in August, the middle of the rainy season. El Sol has various characteristics in common with other high-mountain lakes, such as polymictic, oligotrophic, soft waters susceptible to acidification.