Spatiotemporal Variations in Physico-Chemical Parameters and Their Impact on Zooplankton Dynamics in River Shivana

The abundance of zooplankton was seasonal with high numbers in summer and a gradual transition from monsoon to winter. The monsoon season had low abundance due to increased river discharge, nutrient dilution and physical removal of plankton from the river downstream. There have been similar decreases in monsoon rainfall over the Narmada, Tapi, and Krishna rivers (Gupta et al., 2019). The opposite is true during the summer, when water is more stable, more retained, and more productive for phytoplankton, with subsequent rapid generations of zooplankton. Many freshwater systems in the tropics have also been found to have similar seasonal trends (Jeppesen et al., 2011). Declines in water quality and increases in pollution-tolerant zooplankton evidence the use of zooplankton as an ecological indicator. Higher levels of BOD, COD, nitrate and phosphate were associated with more abundant rotifers and protozoans, and higher tolerance species were associated with lower dissolved oxygen. The results are in line with the ecological indicator paradigm of Sládeček (1983), which showed that the organisation of the zooplankton community was an indicator of trophic level and pollution in lentic freshwater systems. The study results indicate that most of the standard physico-chemical parameters are within acceptable limits, but the River Shivana is being subjected to ever-increasing anthropogenic pressure, as it has high concentrations of conductivity, nutrients, BOD and COD. A marked shift from clean-water species to pollution-tolerant zooplankton species is also indicative of gradual eutrophication and declining water quality. Nevertheless, there were many copepods and cladocerans, indicating that the river was relatively ecologically resilient. This means that frequent monitoring of zooplankton and water quality parameters is necessary to predict ecological damage in rivers and to formulate a sound management and conservation strategy. Novel finding: The current study revealed that physicochemical parameters significantly influenced the size and structure of the zooplankton community in the River Shivana across different seasons. Increased organic enrichment and nutrient inputs are indicated by summertime increases in temperature, electrical conductivity, biological oxygen demand, chemical oxygen demand, nitrate, and phosphate, along with a drop in dissolved oxygen. At the same time, the most abundant zooplankton group in the summer season was Rotifera, a group found in eutrophic conditions. The presence of pollution-tolerant organisms such as Brachionus, Asplanchna, and Keratella in certain sections of the river also indicated eutrophic conditions. However, it was found that multiple species of cladocerans and copepods are present, indicating that the river is not severely damaged. In general, the study could help affirm the usefulness of zooplankton in assessing the health status of freshwater systems and show the close connection between zooplankton and seasonal changes in water quality.

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