Zooplankton community structure in relation to microcystins in the eutrophic Lake Zumpango (State of Mexico)

We studied the seasonal variation of rotifers, cladocerans and copepods in Lake Zumpango, Mexico City, from June 2010 to May 2011. Physical and chemical variables of the water were measured at three sites, while 50 L of water was filtered from these sites to study the zooplankton abundance, diversity and biomass. The water temperature ranged from 14 to 23 °C, and nitrate and phosphate concentrations from 0.2 to 1 and 4 to 14 mg L–1, respectively. Microcystis sp. Dolichospermum sp. and Planktothrix sp. were present throughout the year. Microcystin concentrations were between 0.06 to 11.7 μg L –1, but for six months the concentrations exceeded 1 μg L–1 (1.1 to 11.7 μg L –1), with the highest concentrations observed in August and October. We recorded 33 species of rotifers, including Brachionus angularis (1 to > 11000 ind. L–1) and Brachionus havanaensis (1 to 6683 ind. L –1), while the most abundant cladoceran was Moina micrura (about 10 ind. L–1). The species diversity ranged from 0.8 to 2.9 bits ind–1, but small (< 250 μm) zooplankton (rotifers and copepod nauplii) dominated the community.

Competition between Brachionus calyciflorus and Brachionus angularis (Rotifera) in relation to algal food level and initial population density

Intensive interspecific competition for limited resource often can result in the exclusion of inferior competitors, decrease the species diversity and alter the structure of the zooplankton community. Competitive experiments between Brachionus calyciflorus and Brachionus angularis were conducted at three Scenedesmus densities (0.5 × 106, 1.0 × 106 and 2.0 × 106 cells ml−1) and four initial inoculation densities (numerically, 100% B. calyciflorus, 75% B. calyciflorus and 25% B. angularis, 50% each of the two species, 25% B. calyciflorus and 75% B. angularis, and 100% B. angularis). The results showed that at the low food level, B. angularis outcompeted B. calyciflorus and vice versa at the high food levels. At the intermediate food level, B. angularis was displaced by B. calyciflorus at nearly all the initial inoculation densities except for 75% B. angularis, at which both species coexisted until the termination of the experiment. When grown alone at 0.5 × 106, 1.0 × 106 and 2.0 × 106 cells ml−1 of Scenedesmus, B. calyciflorus reached the peak abundance values of 34 ± 4, 69 ± 5 and 101 ± 9 individuals ml−1 and had population growth rates of 0.608 ± 0.032, 0.654 ± 0.033 and 0.518 ± 0.039 d−1, respectively. The corresponding values for B. angularis were 265 ± 8, 330 ± 30 and 802 ± 87 individuals ml−1 and 0.623 ± 0.020, 0.770 ± 0.036 and 0.871 ± 0.013 d−1. The results suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.