Freshwater algal diversity of the South-Tajik Depression in a high-mountainous extreme environment, Tajikistan

The research on algal communities' response to altitude and altitude-related climatic conditions is sparse. So far, a full, accessible list of algae from the water bodies of the South-Tajik Depression has not been available. We compiled an algal species list of 1190 taxa that were revealed by us or listed in references in the South-Tajik Depression high-mountain rivers Kafirnigan, Vakhsh, Kyzylsu, Yakhsu, and Tairsu and in the lakes of its basin. The altitude gradient of the studied area is about 2500 m above sea level. An altitude-related algal diversity analysis was done with help of statistics and bioindication. The Willis curve for the distribution of species to genera had a trend line of R-2 = 0.92. A multivariate analysis was used for quantitative estimates of the ecological tolerance ranges of critical species and for detecting cardinal factors and trends at local water bodies to global levels of biodiversity evolution. Our analysis shows that species distribution was strongly affected by altitude and altitude-related climatic variables. Statistical methods revealed that temperatures stimulate algal species diversity, while precipitation and altitude suppress it. One of the floristic complexity criteria is the average species richness of the algal genera or the infraspecies to species ratio, which increased from 1.12 to 1.20 and reflects increases in structural complexity with altitude. Therefore, high altitude stresses algal communities and stimulates species polymorphism as a compensatory mechanism for algal species survival. Our approach is pertinent to the problem of floristic differentiation under climate change and climatic instability.