Comparative Ecological Responses of Dominant Marine Macroalgal Species to Environmental Gradients Along the Southern Caspian Coast of Azerbaijan

Macroalgae play an important role as photosynthetic eukaryotic organisms widespread in aquatic ecosystems and are reliable bioindicators for assessing the ecological status of coastal ecosystems.The ecological responses of dominant marine macroalgae to environmental gradients along the southern Caspian coast remain poorly understood.This study aimed to assess the spatial and seasonal distribution patterns of three dominant macroalgal species in the Astara, Lankaran, Masalli and Neftchala regions along the southern Caspian coast and to identify the environmental factors (temperature, salinity, pH and TDS) influencing their distribution. It also assessed their bioindicator potential for coastal ecosystem monitoring and management. Seasonal sampling was conducted in 2025 across intertidal and subtidal zones using the random quadrat method (0.04–0.25 m²). A total of 240 samples were collected, with three replicate quadrats per station. Species diversity was assessed using the Shannon-Wiener, Simpson and Pielou indices. The relationships between environmental variables and macroalgae distribution were assessed using Spearman correlation analysis, and differences between groups were assessed using PERMANOVA and SIMPER analysis. The diversity indices indicated relatively stable and evenly distributed macroalgal communities across the study area. Multivariate statistical analyses showed that no statistically significant differences were observed in the spatial and seasonal structure of macroalgae communities on the southern coast of the Caspian Sea in 2025 (p > 0.05). SIMPER analysis showed that the differences between communities were mainly due to changes in the relative abundance of Cladophora glomerata (Linnaeus) Kützing, Ulva linza Linnaeus  and Ceramium secundatum Lyngbye. Spearman correlation analysis revealed a strong positive correlation between pH and temperature (r_s = 0.719, p = 0.035), and a very strong positive correlation between Cladophora glomerata and Ulva linza (r_s = 0.777, p = 0.008). Spearman correlation analysis revealed a significant positive correlation between pH and temperature, indicating that warmer conditions were associated with higher pH values. This relationship suggests that seasonal increases in temperature may enhance photosynthetic activity of macroalgae, resulting in greater CO₂ uptake and a corresponding increase in water pH. These physicochemical changes may influence macroalgal growth and community structure, contributing to seasonal variations in species abundance. The studied macroalgal species exhibited sensitivity to environmental variation and may serve as promising bioindicators for monitoring ecological conditions along the southern Caspian coast.