Water Induced Changes in Aquatic Biodiversity in Monsoon-Dominated Rivers of Western Himalayas in Nepal: Implications for Environmental Flows

The headwaters of these basins are free from modern urbanization, with minimal organic pollution and sewage influx. The Mahakali River flows from the northern part of India, with 34% of its basin lying within Nepal, while the Karnali basin has a catchment area of 127,950 sq. km, with 55% falling in Nepal. Approximately 14% of the total basin is protected as national parks, wildlife reserves, and conservation areas. The region is considered an arid zone, with less than half the rainfall of Eastern Nepal, resulting in a lack of riparian vegetation and canopy coverage in the rivers. The majority of the annual rainfall occurs in four months from June to September. The study area was divided into three eco-regions: High-Mountain, Mid-Hills, and Lowland, with sampling sites distributed in rivers with road accessibility. All sites were mostly free from direct industrial or sewage influxes and waste dumping. A total of 33, 41, and 40 river reaches were sampled for post-monsoon, baseflow, and pre-monsoon seasons, respectively, in the headwaters of Mahakali and Karnali River basins in 2016 and 2017. The sampling sites were selected based on their accessibility and the presence of suitable habitats for macroinvertebrates. The sites were screened for impacts of organic pollution using a protocol consisting of four components: sensory features, ferrosulfide reduction, algae and periphyton coverage, and richness and abundance of benthic macroinvertebrates. The study aimed to assess the ecological consequences of water diversion projects, particularly irrigation projects, water mills, and micro-hydropowers, on river health. The researchers anticipated that the increasing degree of water abstraction would result in reduced taxonomic richness and abundance of indicator taxa. To test this hypothesis, the responses of macroinvertebrate assemblages in abstracted stream reaches were compared to those at natural (reference) sites. Macroinvertebrates were sampled during post-monsoon, baseflow, and pre-monsoon seasons using a standardized protocol. The samples were analyzed using Non-metric Multidimensional Scaling (NMDS) and Redundancy Analysis (RDA) to visualize clustering of sites according to percentage of water abstractions and to explore environmental variables that explained variation in macroinvertebrate community composition. The results showed a significant pattern of macroinvertebrates across the water abstraction categories, with sites clustered into three clumps: "none to slight water abstraction (< 30% - Class 1)", "moderate water abstraction (> 30% to < 80% - Class 2)", and "heavy water abstraction (> 80% - Class 3)". The study also found that water abstraction varied seasonally in the region, with a significant difference in water abstraction between the post-monsoon and baseflow seasons. The RDA plot indicated that certain taxa, such as Acentrella sp., Paragenetina sp., Hydropsyche sp., Glossosomatinae, Elmidae, Orthocladiinae, and Dimesiinae, were positively correlated with water velocity, while other taxa, such as Torleya sp., Caenis sp., Cinygmina sp., Choroterpes sp., Limonidae, and Ceratopogoniidae, were found in sites with high proportions of pool sections and relative high temperature induced by flow reduction. The study concluded that macroinvertebrate abundance was a more sensitive metric than taxonomic richness in the abstracted sites. The results highlighted the importance of understanding the relationship between flow alterations induced by water abstractions and changes in macroinvertebrate composition to determine sustainable and sound management strategies for river ecosystems.