Environmental comparison, through sedimentary records, between pre-Hispanic and current conditions of a lacustrine system

Two moments of the environmental history of San Pedro lacustrine system are compared through sedimentary, pollen and diatom records. The first one cor responds to the pre-Hispanic period, representing the pristine conditions o r background of the system. The second period, more recently, represents th e human disturbance conditions during the late ten years. To achieve the ab ove mentioned, sediment cores were collected from the bottom of two lakes t hat conform the San Pedro lacustrine system (Laguna Grande and Chica). Chro nological (C-14), pollen and diatom analysis of superior and inferior layer s of the cores were carried out. The results indicate that the San Pedro la custrine system has undergone drastic changes since Spanish settlement. Evo lving from a natural vegetation, predominantly native, to an intense pressu re of forestry use. In the same way, the qualitative water state presents a variation in its trophic features, showing a evolution from oligotrophic t o eutrophic waters in Laguna Grande and from oligotrophic to mesotrophic in Laguna Chica. Finally, it is suggested the incorporation of sedimentary re cords analysis in the study of environmental impact and management of lakes . In this way, it will be possible to know the background (the existing pre -Hispanic conditions) and the response of aquatic ecosystem to the human in fluences.