Seasonal variations of zooplankton abundance in the freshwater reservoir Valle de Bravo (Mexico)

Information on the density and diversity of zooplankton from drinking water reservoirs in Mexico is meagre. This is important not only from the point of view of lake management but also for providing clean drinking water for human populations. In the present work, we provide quantitative information on the seasonal variations of zooplankton and selected physico-chemical variables from Valle de Bravo, a large man-made reservoir in the State of Mexico. Based on the nutrient data, this reservoir can be regarded as mesotrophic. However, we found a high density of phytoplankton. Among Cyanophyceae, Anabaena, Microcystis, Nostoc and Oscillatoria were encountered, particularly during the warmer months. Microcystis blooms were observed from June to September. Diatoms dominated the phytoplankton during the remaining months of the year. Among zooplankton, Rotifera comprised the highest number of species. The most common species occurring throughout the year were Keratella chochlearis, Polyarthra vulgaris, Trichocerca capucina, Trichocerca similis, Asplanchna priodonta and Synchaeta pectinata. The highest abundance of K. cochlearis was observed in May at site 5 (340 ind l(-1)). P. vulgaris occurred in high numbers throughout the year. Both Trichocerca capucina and T. similis were encountered in low numbers (mean annual average: 10 ind l(-1)). Among the crustacean zooplankton, Bosmina longirostris occurred in higher numbers (up to 105 ind l(-1)) regardless of the time of the year. Copepods were predominantly from the genera Mesocyclops and Megacylops; calanoids were rare. Naupliar stages were dominant during winter months (up to 670 ind l(-1)). These results have been discussed from the point of view of Microcystis control in water bodies.