Assessment of ecological state of serially impounded Himalayan River using diatoms-based matrices: Identification of critical stretches for management

Abstract

The ecological state of the serially impounded Himalayan river Bhagirathi is assessed using benthic diatom-based matrices. Six stations (S1-S6) located in stretches receiving constant low flow (S2, S4,S5) and regulated flow regimes (S3, S6) along the continuum were sampled for the diatom community at monthly intervals from October 2016 to September 2017. The samples were processed and identified according to standard literature. The van Dam ecologic values revealed identical prevailing condition based on their percent abundance, reflecting the existing state of riverine ecosystem. Compared to S1 (located upstream in natural flow) the percent share of the dominant category tolerant-N-autotroph for nitrogen-uptake; polyoxybiontic (100% O2 saturation) for oxygen requirement; and β-mesosprobe (BOD 2–4 mg l−1) for saprobity valuesshow decline in the modified stretches except for S6 and being serious at S3 followed by S2, S4 and S5. On the contrary, obligatory-N-heterotrophs (taxa needing continuously elevated organically bounded nitrogen), low to moderate oxygen saturation (30–70%), and α-meso-poly saprobous (biological oxygen demand 8–12 mg l−1) appeared in these modified stretches. The eutrophic (high level of nutrients) and hypereutrophic (nutrient rich water characterised by algal blooms) catagories occurred at S2, S3 and S4, reflecting enhanced organic load, which is further demonstrated by water quality indices. Some tolerant taxa (Nitzschia palea, Nitzschia paleacae) contributed a considerable share in the flow modified stretches and hence are considered taxa of concern for future studies as they are rarely reported from Himalayan rivers. The anomalies in ecological state confirm deterioration and hence the sections along the modified stations are called critical stretches, S3 followed by S2 and S4 being the most stressed and recommended for proper management. The study also affirms the applicability of diatom-based matrices for Hydroelectric project (HEP) impact assessment in the Himalayan rivers.