The Lerma marshes also known as Chignahuapan, Chimaliapan and Chiconahuapan are natural protected areas and bird conservation zones. They are one of the priority hydrological regions, threatened and highly diverse in Mexico. However there is little updated information about their vegetation, even when they are in clear danger of disappear. This study identified the aquatic vascular flora and the changes in plant diversity of the three bodies of water by means of the collection of all species; additionally we applied the lineintercept method in 20 to 40 sites of each water body. We find 97 species of vascular plants included in 39 families and 65 genera. Fortysix percent are strict aquatic, 31% subaquatic and 20% tolerant. The families Cyperaceae, Asteraceae, Lemnaceae, Poaceae, Apiaceae and Polygonaceae are the better represented. The diversity of Lerma marshes is relatively high (Shannon index = 3.2) and presents particular variations in each wetland. Chiconahuapan shows the highest values and provides floristic heterogeneity to wetlands. The prevalence of terrestrial and aquatic weeds suggests a continued deterioration of these habitats; however, the presence of species characteristic of flooded areas and with restricted distribution suggests that the marshes still have significant plant diversity. Unfortunately these systems are subject to drastic and irreversible negative changes, if the conditions of disturbance, pollution and declining water levels are not reversed.
