Decadal variation in the diatom community structure of a Himalayan river

Large human habitations exist along the major glacier-fed Himalayan rivers. Over the years, the size of habitations and associated developmental activities has increased exponentially alongside pilgrimage and tourism. All these activities are sources of anthropogenic stress that have the potential to degrade river ecosystems. However, the long-term impacts of these stressors are barely known for the Himalayan rivers. Thus, we propose to examine long-term variation in the structure of the producer community over three decades as an indicator of ecosystem degradation. The Alaknanda sampled at Srikot was deemed suitable for these investigations. The sampling location in 2021-22 was the same as in previous studies conducted in 1991-92 and 2018-19, which made them comparable. The past naphrax mounts available for these periods were re-examined for historical species count to ensure taxonomic consistency in species identification. Species richness and diversity parameters increased over time, though not statistically significant between the years. However, notable variation occurred in the community dominant and their relative abundance, exemplified by Achnanthidium pyrenaicum (26-41%), Achnanthidium minutissimum (35-63%), and Achnanthidium latecephalum (19-50%) for the years 1991-92, 2018-19, and 2021-22, respectively. The assemblages persisted throughout the sampled months in 1991-92 but became inconsistent after three decades. The prevalence of community in each year was influenced by the gradient of ecological preferences. The observed variation in the community is likely driven by deficient discharge in the river stretch below the dam at Srikot, which enhances the impact of the organic load from the surrounding human habitations. This is the first study to examine the long-term variation in diatom community structure of the Himalayan river. Such long-term investigations are hence important for assessing ecosystem degradation and creating reference benchmarks for future ecological assessment. The study also demonstrates the use of historical mounts and diatom community structure, especially the persistence and dominance of sensitive taxa, as a key bioindicator in river health assessment.