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Most Mexicans live in the arid and semiarid regions that represent two‐thirds of the Mexican territory, where water is scarce. Natural, as well as human, causes are favouring the degradation of Mexican lakes. There is a clear need to develop and implement sustainable water‐use programmes at a catchment scale. However, the accelerated degradation rate of the Mexican lakes means that there will not be enough time to perform whole‐basin evaluations to establish sustainable water‐use programmes before the lakes dry up. The case of the Valle de Santiago crater‐lakes clearly illustrates the declining trend that Mexican inland aquatic resources follow. Vegetation clearance, overgrazing, abatement of phreatic waters and salinization have induced severe erosion and overall desertification (land degradation) in the basin for what, it seems, a long time (i.e. prehispanic times). In this way, human activities could be provoking at least the following negative consequences: a hotter and drier local climate, water scarcity, dust storms and soil salinization. The aquatic (surface and groundwater) resources of the Valle de Santiago basin have been seriously threatened. Two of the four crater‐lakes have already dried up and phreatic mantle abatement reaches up to 2.5 m per year. In spite of these facts, no sustainable water‐use programme has been established yet. The future scenery of this Mexican basin looks alarmingly like many other basins in the central and northern Mexican territories.

Guadalupe Dam is a reservoir located into the Metropolitan area of Mexico City, which had been infested with water hyacinth (Eichhornia crassipes) for more than 12 years. In 1993 a management program was conducted in the reservoir. The main activities included the use of aquatic herbicides and mechanical control. The goal of this study was to monitor the composition and fluctuation of the planktonic community during the chemical control program. Five sampling stations were selected. Six samplings were made during the period of herbicides applications (July to November 1993) and one sampling more, used as control, was performed four months after the last application (March 1994). Herbicides diquat and 2,4-D amine were used in the chemical control program. Variables measured included temperature, dissolved oxygen, pH, chlorophyll a and numbers of phytoplankton, ciliates, rotifers, cladocerans and copepods. The reservoir is eutrophic, with high concentrations of chlorophyll a and low values of Secchi disc depth. Thermally, the dam seems to be warm monomictic. The infestation of aquatic weeds at the beginning of the program reached 95% of surface area. Two main effects of the herbicide application were observed. The first was the direct toxic effect of the chemicals on the growth and density of phytoplanktonic species, which lead to a reduction in zooplankton density related to food shortage. The second effect was indirect caused by the decomposing hyacinths which depleted dissolved oxygen concentrations. The oxygen deficit greatly affected the biological community. Although the chemical control program of Guadalupe Dam water hyacinth had a notable effect on the planktonic community, it recovered promptly. After the weed control program, the Guadalupe dam remained eutrophic. Although water hyacinth was eliminated, it was replaced by a large phytoplankton bloom.