Seasonal shifts in spatial and temporal beta diversity of diatoms in a Mediterranean lagoon (El Mellah, Algeria)

Mediterranean lagoons experience pronounced seasonal forcing and sharp environmental gradients that can generate strong spatiotemporal variability in phytoplankton composition. We investigated seasonal changes in diatom community beta diversity in El Mellah Lagoon (Algeria) based on four field campaigns performed in 2016 at 31 stations spanning littoral and pelagic sectors. Spatial beta diversity within each season was estimated using Jaccard dissimilarity and decomposed into turnover and nestedness components to clarify whether among-station differences were primarily driven by species replacement or richness-related patterns. Temporal change between successive seasons was quantified using the Temporal Beta Index (TBI), separating species losses from gains at the station level. The contribution of individual taxa and sites to overall compositional variability was assessed using SCBD and LCBD indices, and spatial dependence was evaluated using Moran’s I and variogram parameters prior to kriging-based mapping. Seasonal typologies were further explored using a Factor Analysis of Mixed Data (FAMD) integrating beta-diversity descriptors with measured environmental variables. Beta-diversity patterns differed strongly among seasons, and temporal transitions showed contrasting loss–gain balances, indicating that seasonal change in composition was not symmetric across the annual cycle. Spatial autocorrelation and variogram ranges varied by season and by beta-diversity component, highlighting shifts between more clustered and more spatially diffuse configurations. A limited set of taxa contributed disproportionately to beta diversity, while several stations emerged as recurrently distinctive assemblages. Overall, these results illustrate how integrating beta-diversity partitioning, contribution metrics and geostatistical analyses can be used to describe seasonal and spatial community change in Mediterranean lagoon systems