Effect of Sub-Oligohaline Levels of NaCl Levels on the Zoo-Plankton Community Structure: A Laboratory Microcosm Study

Rotifers, cladocerans, and copepods constitute the bulk of zooplankton biomass in freshwater ecosystems. An increase in salinity levels due to evaporation from fresh-water bodies affects species richness and zooplankton abundance. We exposed zoo-plankton in mesocosms to sub-oligohaline salt concentrations (0, 50, 100, or 200 µg/L of NaCl) for 18 days. We also measured selective physicochemical water parameters. Our results showed that pH, phosphates and chlorophyll a levels of the medium in the test jars decreased with time. However, conductivity values did not show any clear trends with time. The initial zooplankton composition consisted of 13 rotifer, 1 cladoceran and 2 copepod species. After being exposed to 200 mg/L salinity level for three weeks, only 5 rotifer species and 1 cyclopoid species survived, and in terms of densities, naupliar stages dominated. The densities of Brachionus calyciflorus, Keratella cochlearis, and Polyarthra vulgaris decreased with increasing salinity level and expo-sure time, while Brachionus angularis and Brachionus budapestinensis were eliminated. The low levels of salt used in this work had also adverse effects on species richness and abundances of zooplankton, over just a few days. Therefore sub-oligohaline lev-els of salinity may pose a threat to species richness and abundances of zooplankton in natural waterbodies.