PARENT SESSION
Poster Session #23: Fish, Lakes, Streams, and Wetlands.
Wednesday, August 8, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall
67
Impact of deforestation in tropical volcanic lakes .
Vazquez, Gabriela1, Martinez, Luisa1, 1
ABSTRACT- Land use has an important influence on the state of the health conditions of freshwater ecosystems. Because of their small watershed, volcanic lakes offer a unique scenario to test this hypothesis. In the Biosphere Reserve of Los Tuxtlas, Mexico, we selected three volcanic lakes with different land use in their cones. The lakes are 35-50m deep and are warm monomictic. In each, vegetation structure on the cones was described, and in the water, physical, chemical, biological (chlorophyll a), and the composition of the phytoplanktonic community were analyzed. The cone of "Manantiales" remains in a pristine state, and is covered by tropical rain forest (TRF) species. In "Majahual" there are fractions of pastureland immersed in the TRF, while "Chalchoapan" is almost totally deforested and covered by croplands, with a few patches of isolated remnant TRF. The Trophic State Indices calculated for transparency and chlorophyll indicate that "Manantiales" and "Majahual" are mesotrophic while "Chalchoapan" is eutrophic. Furthermore, the concentration of suspended solids and ammonium were always significantly higher in "Chalchoapan". These physical changes affected the phytoplanktonic community: in "Manantiales" and "Majahual" green algae and diatoms were predominant (Oocystis, Pandorina, Staurastrum, Synedra, Epithemia) while blue-green algae (Microcystis, Merismopedia, Anabaena) were most abundant in "Chalchoapan". Phytoplanktonic species indicate good and poor health conditions respectively. The conservation of freshwater ecosystems requires an integrated approach where the surrounding terrestrial ecosystems should also be preserved.
KEY WORDS: volcanic lakes, phytoplankton, impact of deforestation, Mexico