EFFECTS OF CYANOBACTERIA, Microcystis spp. ON THE POPULATION GROWTH AND REPRODUCTIVE CAPACITY OF A CLADOCERAN, Moina Micrura Kurz 1984

To determine the effects of Microcystis spp. on a tropical cladoceran, Moina micrura was collected and isolated from a pond in the Universiti Putra Malaysia, Selangor, Malaysia, ad mass cultured in the laboratory for use in population growth study and chronic bioassays (>10 days). Moina micrura was cultured and maintained in 20L aquarium using filtered-sterilized pond water (0.45 μm fiberglass filters), pH 7.8 ±2 at room temperature 27.0 ± 2.0 °C under a photoperiod of 12 h light and 12 h dark (120 ±2 µmoles photon m^-2 sec^-1 of light intensity) and fed ad libitum with Chlorella vulgaris (concentration of 4 × 10⁶ cells ml⁻¹). For the population growth study, experiment was conducted at a stocking rate of 200 female l^−l under the same condition as above until the population began to decline. They were fed once in the morning every day with three different species of microalgae; Microcystis aeruginosa, Microcystis viridis, and C. vulgaris as a control. Algae cells were harvested at late exponential growth phase for feeding regimes. Our preliminary study showed that the food concentration of 1 × 10⁵ cells ml^-1 was the optimum concentration to initiate biological responses of M. micrura towards all three microalgae species. Each treatment was conducted in triplicates. 25 ml of the well-mixed culture was subsampled and examined in a petri dish under a dissecting microscope. For chronic bioassay, neonates (< 24h) were individually reared in 20 different glass vials containing culture medium and kept under 30 ±4 µmoles photon m^-2 sec^-1 of light intensity. All the glass vials were checked daily (at 12h intervals) to determine age at first reproduction (day), fecundity (no of eggs female^-1), total offsprings (no. of offsprings female^-1) and longevity (no. of days). Both Microcystis spp. were toxic to M. micrura. The mortality of M. micrura subjected to M. aeruginosa and M. viridis was significantly higher (p≤0.05) than the control treatment after 72h and 144h of exposure, respectively. After 96h of exposure to M. aeruginosa, M. micrura showed 100% mortality. For M. viridis, 100% mortality of M. micrura occurred at 168h of exposure. For the mean body size, M. micrura exposed to M. aeruginosa did not reach maturity as their mean body size only reached 627.80 SE±31.4µm compared to M. micrura fed with C. vulgaris (814.94 SE±21.84µm) and M. viridis (914.21 SE±12.64µm). The population growth rate of M. micrura fed with C. vulgaris was 0.18 day^-1 while growth rates were negative when fed with M. aeruginosa (-0.046 day^-1) and M. viridis (-0.023 day^-1). In the chronic bioassay, the exposure of M. micrura to M. aeruginosa resulted in delayed production of M. micrura's first offspring, which only occurred on day 6 compared to M. micrura fed with C. vulgaris which produced their first offspring earlier on day 3. This study showed that exposure of M. micrura to both Microcystis spp. reduced the fecundity, total offsprings production and longevity of M. micrura compared to those fed with C. vulgaris.