Spatial and temporal variability of mayfly communities (Insecta: Ephemeroptera) in mountainous and semimountainous rivers of the western Aegean Basin

Mayflies are among the most abundant benthic invertebrates inhabiting natural lotic freshwater systems. Their assemblages vary with environmental, geographical, and hydrological conditions along altitudinal gradients and under different types of watershed disturbance. The main objective of this study was to analyze the composition and β-diversity of mayflies across three river watersheds and to assess how these assemblages differ among them. We collected biological and environmental data from 38 sampling sites distributed across the three river watersheds. We hypothesized that mayfly assemblages would vary with environmental gradients on both spatial and temporal scales; specifically, we expected pristine streams to exhibit higher taxonomic richness and a greater proportion of rare species. In total, 59 morphologically distinct mayfly taxa belonging to 18 genera and eight families were identified. In this study, four endemic species—Epeorus yougoslavicus, Ecdyonurus (Helvetoraeticus) epeorides, Rhithrogena braaschi, and Rh. thracica—were recorded. The communities at mountainous sites were dominated by Baetis alpinus, which was gradually replaced by B. rhodani or Alainites muticus in lower reaches. We found little variation in the number of taxa, total and relative abundance, and species richness between seasons and river types, mainly because site seasonality was influenced not by low water levels in autumn but by fine-scale microhabitat diversity. T he Struma River watershed exhibited the highest richness (>20 mayfly taxa), observed at several sampling sites. Along the elevational gradient, the number of taxa increased toward lower-altitude sites, except at human-impacted locations. Significant positive (general degradation score) and negative (sites shading) correlations were found with total mayfly abundance, suggesting a potential loss of stenothermal and endemic species. Therefore, the diverse mayfly assemblages—including rare and endemic species—underscore the importance of protecting these rivers from future degradation and pollution across the western Aegean basins and the broader Balkan Peninsula.