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.
Abstract:To monitor the stage in turbid reservoirs with a sloping bank, it has been proposed to install a near-infrared Lidar on the bank and to orient it so that it points at the water surface with a large incidence angle (between ≈ 30 • and 70 • ). The technique assumes that the Lidar can detect suspended particles that are slightly below the water surface. Some laboratory results and the first long-term assessment (>2 years) of the technique are presented. It found that: (1) although the test Lidar provides erratic distance data, they can be easily filtered according to the intensity of the received signal; (2) the Lidar provides reliable data only when the water is very turbid (Secchi depth smaller than ≈ 1.0 m); and (3) the reliable data can be used to estimate daily stage values (after a simple field calibration) with an uncertainty better than ±0.08 m (p = 0.95). Although the present form of the technique is not very accurate, it uses an inexpensive instrument (≈1500 USD) which can be easily installed in a safe place (such as is the roof of a building). It is argued that the technique could be also used to monitor the stage and the sub-surface velocity in others turbid water bodies, such as some coastal areas (a recent field of application) and flooding rivers.
The Valle de Bravo reservoir provides 30% of Mexico City’s drinking water. Unfortunately, human activities in the watershed have affected the water quality in this reservoir. Its trophic state changed from oligotrophic in 1980 to mesotrophic in 1987. Research conducted from March 1992 to February 1993 defined the main limnological characteristics of the water body and identified the most highly polluted subbasins in the Amanalco River Watershed. For this purpose, water, sediments and vascular plants were sampled in the waterbody. In the Amanalaco River Watershed land use and nutrient loadings in each subbasin were quantified. The study dam is warm and monomictic, with a nine-month period of stratification and an overturn in December. Bottom anoxia occurred during the stratified phase. The reservoir was eutrophic during summer and mesotrophic the rest of the year. In the Amanalco basin, the highest phosphorus and nitrogen loadings (P, N in t y-1) entered by way of the Becerra (4.9, 47.2), Pipioltepec (8.6, 61.9) and Candelaria (8.5, 69.5) subbasins. A good agreement was found in a comparison between the export coefficients estimates in the Amanalco River Watershed and the measured amounts of phosphorus and nitrogen transported to 38 US waterbodies. Nevertheless, the phosphorus export coefficients for the forested subbasins within the Amanalco River Watershed were rather high in relation to the typical values found in the literature, and the values for agricultural land were low in relation to the same source. For the first time in Mexico, export coefficients for phosphorus and nitrogen were estimated, both of which will be useful in assigning priorities in land management. The export coefficients estimates presented in this study can be applied to other scenarios in Mexico which have the same temperate semihumid climate.
Zimapán Reservoir is located in the central plateau of Mexico. Its main purpose is the generation of electric energy, and it has a high mean depth (52.40 m). The reservoir has two tributaries: the Tula River, which receives the wastewater of Mexico City, and agricultural run-off; and the San Juan River, with municipal and industrial wastewater. The population growth together with the dry climate are generating pressure on the water resource. The purpose of this study is to characterize the trophic state of Zimapán Reservoir and its trend on a five year study. It is warm with the presence of thermocline between 8 and 14 m. It was found hypereutrophic for the nutrient concentration (average TP and TN of 1,381 and 5,977 microg.L(-1), respectively), and eu-hypereutrophic in case of the Secchi disk (1.94 m) and the chlorophyll concentration (38 microg.L(-1)). The trend analysis showed and increment of nutrients in the period, but a decrement in Secchi disk and chlorophyll results. Nutrients are present in a very high concentration. The reservoir is gradually increasing its trophic state condition.
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