Intra-annual environmental status and phytoplankton community delineation of a karst glacier lake, Gahar, Iran.

Phytoplankton is the main constituent of a healthy high mountain lake, and the turnover in its community composition is directly related to overall diversity and resilience. Concerning the pristine nature of Lake Gahar, this study aimed to assess phytoplankton stability using alpha diversity indices, Simpson dissimilarity index (βSIM), and long-term taxonomic hierarchy. DistLM analysis indicated that environmental heterogeneity was primarily driven by electrical conductivity, Secchi depth, dissolved oxygen, and total alkalinity. CAP results showed that total phosphorus (TP) negatively correlated with competitive success of Cyclotella ocellata, while NO₃^- had a positive influence. A total of 170 species were registered, with Bacillariophyta being the dominant phylum, followed by Chlorophyta and Dinophyta. A low number of species were common across phyla, reflecting high intra-annual variability; specifically, 107 species exhibited turnover rates exceeding 70%, with densities ranging from 0.001 to 1%. Moreover, 50 species had occurrences below 10%, with relative abundances less than 0.001%. Intra-annual variation in dissimilarity/stability was at a high domain, averaging 63.5% (range = 42.5 to 95.8%). Long-term biodiversity indices, Delta + (73.85 to 98.95) and Lambda + (161.4 to 340.9), highlighted high taxonomic distinctness and "even phylogenetic structure" characterized by the replacement of both poor and rich branches in the taxonomic scheme. Despite Lake Gahar's vulnerability due to terrain/geology, it maintains stable environment, reinforced by the fluctuating presence of phylogenetically distanced rare and sporadic species. This baseline study underscores the importance of "monitoring phytoplankton turnover," community heterogeneity, and taxonomic distinctness as vital indicators of overall health and resilience, while also stressing the need for "biodiversity conservation efforts."