" Lago de Izabal, the largest lake of Guatemala, is a unique freshwater body in which geology, land use, hydrology, and climate have combined to produce anomalous concentrations of heavy metals in the sediments and an ongoing eutrophication process. The lake has been monitored over a period of one year generating nearly 9000 water chemical analyses and 900 sediment analyses. Lake-surface waters were sampled for a full year at 17 sites. Spatial and temporal variations were examined. A hydrologic analysis, including rainfall-runoff relationships for the watershed as well as determination of the water budget of the lake, has been conducted. The pH is uniform during the entire year with an average of 7.8. The lake becomes isothermal during the low rainfall season and weakly stratified during the high rainfall season. The lake is well oxygenated throughout the year but anoxic conditions dominate the bottom waters with a drop in pH from 8 to 7 when the lake develops thermal stratification.^ The controversial issue of saline incursions from the Atlantic Ocean has been analyzed. The average K, Cl, and TDS content of Lago de Izabal indicates no penetration of saline waters into the lake. TDS values calculated from resistivity profiles conducted between San Felipe, the outlet of Lago de Izabal, and the ocean show that saline water penetrates areas located halfway between the lake and the ocean.^ Chemical weathering and subsequent soil erosion are major processes controlling water quality in the lake. Heavy metals in sediment, including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb, have been analyzed with the Factor of Enrichment (FE) and the Index of Geoaccumulation (I-geo). The FE shows that the sediments are enriched with respect to average crust of the earth in Cr, Co, Ni, and Pb. The I-geo indicates that the sediments of the lake present moderate contamination by Ni and Cr, and moderate to strong contamination by Co and Pb. Outcrops of ophiolites in the north shore have been identified as the major source of Cr, Co, and Ni. Pb-tailings, located at the edge of the Cahabon watershed, are a potential source of Pb, but anthropogenic input is also suggested.^ Extensive deforestation and subsequent agricultural runoff has accelerated the nutrient input into the lake. Total P, total N, chlorophyll, transparency, and primary productivity indicate eutrophic conditions. Trophic state indices suggest hypereutrophic conditions based on chlorophyll and eutrophic conditions based on Total P and transparency. The Vollenweider model shows that the annual P loading to the lake has been strong enough to initiate the process of eutrophication. Diatoms constitute the major phytoplanktonic community with green algae and bluegreen algae as secondary communities. The lake behaves as a sink for $rm Hsb4SiOsb4.$ This causes the high Si:P ratio responsible for the abundance of diatoms. Rios draining the ophiolites have the highest silica content of the area suggesting that chemical weathering is an important source of silica "
Classement: Ecologie et environnement, Amérique centrale et de l'Amérique du Nord.
Machorro Sagastume, Rudy Antonio. 1996. "Water quality at Lago de Izabal, Guatemala: Geochemical characterization and assessment of trophic status". ETD Collection for University of Texas, El Paso (crc06783) (résumé)