Seasonal changes in the zooplankton abundances of the reservoir Valle de Bravo (State of Mexico, Mexico)

Abstract

Valle de Bravo is one of largest drinking water reservoirs in Mexico, serving nearly 12% of Mexico City's population of 26 million. We studied the monthly variations in the zooplankton density and diversity for one year (November 2004 to October 2005) at five different depths (2, 4, 8, 12, and 20 m) at each of the five sites of the reservoir. The water body was generally alkaline; pH decreased with increasing depth. Dissolved oxygen also decreased drastically with depth, from an average of 8 mg/L to less than 1 mg/L. Chlorophyll a concentration varied considerably (2.6 ± 0.06 to 12.5 ± 2.8 μg/L) both seasonally and with depth. Secchi depth averaged 1.65 m and varied between 0.63 and 3.21 m along the seasons but exhibited minimal differences among sites. Differences in zooplankton abundance and composition were small among sites, supporting the theory that anthropogenic disturbances affect the reservoir as a whole. The mean total zooplankton density was about 400 ± 293 ind./L, mostly composed of rotifer species, particularly Keratella cochlearis, Polyarthra vulgaris, and Trichocerca similis. Densities of K. cochlearis at 2-m depth exceeded 1800 ind./L during the rainy season (May-July). Average density of P. vulgaris was 200 ± 133 ind./L, while that of T. similis was half that amount. The dominant cladoceran taxa were Bosmina longirostris, Chydorus sphaericus, and Daphnia laevis. An inverse relationship between depth and zooplankton abundance and depth and diversity was observed. In contrast, we observed a direct relation between the mean rotifer and cladoceran density. Shannon-Wiener's species diversity index ranged between 1.00 and 4.09. Fourteen of the rotifer species found are new records for Valle de Bravo and another 13 species observed earlier were not encountered in the present study. The overall trends in both the densities and dominance of rotifer species of the reservoir did not essentially change during the last five years. Our data point toward the need for an integrated management of Valle de Bravo.

Key words:

• annual cycle
• reservoir
• Rotifera
• vertical distribution