Diatoms as indicators of the effects of river impoundment at multiple spatial scales

Benthic diatoms as indicators of water quality in Sharda (Kali), a transboundary Himalayan River

Bioassessment studies in river systems of India are rather scarce and most of the monitoring programmes still rely on the traditional physical and chemical analysis. We explored the biomonitoring potential of benthic diatoms from the Sharda (Kali) river in the Himalayas, which is due interlinking with the Yamuna River under the National River Linking Programme (NRLP) in India. Seventeen sites along the Sharda were sampled in November 2022 for the analysis of 14 physical and chemical variables and benthic diatoms. Hierarchical agglomerative cluster analysis (HACA) and principal component analysis (PCA) of the physico-chemical data set revealed two major groups of sites; the majorly unpolluted sites at higher elevations of Kumaun Himalayas (KH) and the low or moderately polluted sites of Terai Plains (TP) at lower elevations. Application of Water Quality Index (WQI) assigned a good water quality class (B) to all selected sites. A total of 31 genera including 107 species of diatoms were recorded during the present study. Achnanthes pseudoswazi, Achnanthidium minutissimum, Achnanthidium pusillum, Geissleria decussis, and Reimeria sinuata were the most abundant forms from KH whereas Gomphonema acuminatum, Cymbella excisa, Cocconeis pediculus, Nitzschia linearis, and Navicula angusta were the dominant forms recorded from TP. A decrease in diatom diversity was observed from KH to TP sites due to hydrogeomorphological changes and human interventions. Significant differences (p < 0.05 and p < 0.01) between diatom diversity index scores was observed between KH and TP sites. Diversity indices correlated significantly with important water quality variables. The results of the diatom indices such as Trophic Diatom Index (TDI), Specific Pollution Sensitivity Index (IPS), Generic Diatom Index (IDG), and Louis Leclercq Diatom Index (IDSE) corroborated well with the recorded physicochemical variables and WQI values. IPS diatom index exhibited better resolution than WQI with reference to categorization of sites and subsequent establishment of ecological status. IPS was found to be the most suitable index and could be utilized for a pre-linkage ecological status establishment for the Sharda River. However, weak correlations of diatom indices and water quality variables along with low percentage of taxa included for computation of diatom indices reiterates the importance of establishment of region specific autecological preferences of diatoms and subsequent formulation of a customized diatom index for the Sharda River system.

Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes

Understanding the relationship between a dam's size and its ecological effects is important for prioritization of river restoration efforts based on dam removal. Although much is known about the effects of large storage dams, this information may not be applicable to small dams, which represent the vast majority of dams being considered for removal. To better understand how dam effects vary with size, we conducted a multidisciplinary study of the downstream effect of dams on a range of ecological characteristics including geomorphology, water chemistry, periphyton, riparian vegetation, benthic macroinvertebrates, and fish. We related dam size variables to the downstream-upstream fractional difference in measured ecological characteristics for 16 dams in the mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic residence times (HRTs) ranging from 30 min to 1.5 years. For a range of physical attributes, larger dams had larger effects. For example, the water surface width below dams was greater below large dams. By contrast, there was no effect of dam size on sediment grain size, though the fraction of fine-grained bed material was lower below dams independently of dam size. Larger dams tended to reduce water quality more, with decreased downstream dissolved oxygen and increased temperature. Larger dams decreased inorganic nutrients (N, P, Si), but increased particulate nutrients (N, P) in downstream reaches. Aquatic organisms tended to have greater dissimilarity in species composition below larger dams (for fish and periphyton), lower taxonomic diversity (for macroinvertebrates), and greater pollution tolerance (for periphyton and macroinvertebrates). Plants responded differently below large and small dams, with fewer invasive species below large dams, but more below small dams. Overall, these results demonstrate that larger dams have much greater impact on the ecosystem components we measured, and hence their removal has the greatest potential for restoring river ecosystems.

Understanding the role of natural and anthropogenic forcings in structuring the periphytic algal assemblages in a regulated river ecosystem

Periphytic algal assemblages in the River Sindh of Kashmir Himalaya were studied in relation to environmental factors and anthropogenic alterations like flow regulation for Run-of-River hydropower plants to understand their ecology in a regulated river ecosystem. Sites were sampled from unregulated, regulated, and downstream reaches along the river on a seasonal basis from the year 2017 to 2019. A total of 48 species were identified, spread over 31 genera. Non-metric multidimensional analysis showed clear distinction in periphytic algal assemblage samples based on sites and potentially some more minor distinction based on seasons rendering the sites into two distinct groups (G1 and G2). The ADONIS test showed that the groups (G1 and G2 sites) do not significantly differ in terms of how communities differ from one another, but there is a difference in species compositions based on seasons. However, the betadisper test indicated that groups (G1 and G2 sites) and seasons present homogeneity among group dispersions (compositions vary similarly) while having significantly different compositions. Geo-physical factors (discharge and altitude) accounted for most variations, while the scraper community played a minor role. This study provides scientific insights related to the ecology of a regulated Himalayan river and may provide information relevant to managing the River Sindh sustainably.

Diatom community structure in relation to environmental factors in human influenced rivers and streams in tropical Africa

The contemporaneous effect of natural and anthropogenic factors involved in a watershed contribution to the seasonal and spatial variation of diatom community composition is widely discussed in the scientific literature. Yet, there is a paucity of scientific evidence indicating the effect of these factors on diatoms in tropical African regions characterized by distinct dry and wet seasons and season associated human activities like rainfed agriculture are commonly practiced. We applied multivariate techniques to determine the spatio-temporal drivers of diatom assemblage and diatom species richness in human influenced rivers and streams in Ethiopia. We simultaneously collected water and diatom samples from 24 sampling points during the wet (July) and dry (February) seasons. Both water and diatom samples were processed following standard procedures. We identified 169 species belonging to 45 genera in the studied lotic systems. We found that both season and land use factors were important in defining diatom composition (PERMANOVA, p<0.05) and species richness (ANOVA, p<0.05) patterns. Diatom community composition was driven by conductivity, dissolved oxygen, pH, and turbidity parameters (Monte Carlo permutation test, p<0.05). Besides, diatom species richness was driven by dissolved oxygen, soluble reactive phosphorus, and turbidity (GLMM, p<0.05). The study highlighted physicochemical parameters influenced by seasonal variation and human activity determined the composition of diatoms. This implies that the unique feature of heavy rain during the rainy season in the region followed by extensive flooding aggravated by the steep slope from the highlands to the lowlands plays a major role in shaping the diatom autecology in the region. Therefore, in applying biomonitoring in such regions considering the effect of runoff and dilution is imperative.