Water quality in Lake Xochimilco, Mexico: zooplankton indicators and Vibrio cholerae

Lake Xochimilco is a eutrophic water body in Mexico City used by the local population for aquaculture and agriculture. Water level is maintained with inputs of partially treated waste water from the Cerro de la Estrella treatment plant. In this study we analysed the water quality at two sites of Lake Xochimilco, Lake Xaltocan and the Santa Cruz Canal using various indicators such as zooplankton diversity, saprobic indices, bacterial concentrations and physico-chemical variables. Eighty litres of water was filtered from Lake Xochimilco from each site, once a month from March to October of 2012, and the rotifers, cladocerans and copepods were enumerated and identified. Physico-chemical parameters such as temperature, pH, Secchi depth, water depth, nitrogen and phosphorus and chlorophyll a concentrations, and bacterial densities were measured. During the study we recorded 33 species of rotifers, the most abundant being Brachionus angularis, B. calyciflorus and B. havanaensis.  Among the microcrustaceans the most abundant were the cladoceran Moina micrura and the copepods Acanthocyclops americanus and Arctodiaptomus dorsalis. The species diversity was around 2 bits/ind. and the saprobic index between 1.5-2.0, indicating that both sites were β meso-saprobic.  At both sites nitrogen was <1 mg/L and phosphorus ranged between 2.5-7.8 mg/L. Chlorophyll a concentrations were between 66-136 µg/L. The toxigenic (Vibrio cholerae No-O1/No-O139) and the non-toxigenic (Vibrio cholerae No-O135) strains of the bacterium were recorded, closely associated with littoral rotifers and cladocerans particularly Brachionus quadridentatus and  Alona sp. All variables indicate that these sites in Lake Xochimilco are eutrophic and highly contaminated and that the water quality needs to be improved.