Phytoplankton regionalism in natural mesocosm-like wetlands of an island archipelago in the Aegean Sea

Wetlands are globally recognised as biodiversity refugia and critical climate regulators, yet they are increasingly threatened by human pressures. Although phytoplankton plays a foundational role in aquatic food webs, the ecological processes governing their community assembly, particularly the interplay between environmental filtering and dispersal limitation, remain underexplored. Here, we examine how geoenvironmental and anthropogenic drivers and dispersal processes shape phytoplankton communities across 24 small, shallow island wetlands of the Aegean archipelago (Mediterranean Sea), whose intrinsic mesocosm-like characteristics and varying degrees of human influence and biogeographic isolation provide an ideal natural experimental setup. We surveyed phytoplankton species composition and examined a suite of geoenvironmental variables (e.g. salinity, wetland area, nutrients) alongside indicators of human activities (e.g. inclusion of wetlands within protected areas, land cover, human footprint) as drivers of community assembly, using generalized linear and cumulative-link mixed-effects models. We then examined distance–decay relationships of assemblage similarity within and among islands to assess dispersal limitation. Salinity emerged as the dominant environmental filter shaping community composition. Protection status was also significant, with wetlands inside Nominally Protected Areas (NPAs) supporting assemblages distinct from those of unprotected sites. After accounting for these filters, phytoplankton similarity declined sharply with over-land distance among wetlands on the same island, indicating limited within-island dispersal. In contrast, similarity across islands remained uniformly low irrespective of distance, demonstrating that seawater imposes a strong barrier despite the theoretical capacity of phytoplankton for long-distance passive transport. Our findings support a hierarchical “triple insularity” framework, in which phytoplankton assembly is shaped by nested spatial filters: strong among-wetland environmental filtering, island-level dispersal limitation moderated by human accessibility, and cross-island isolation enforced by the sea. We propose that regionalism, typically reserved for macro-organisms, also governs phytoplankton biogeography in fragmented systems. Island wetlands occupy only a tiny fraction of global land area, yet their unique environmental gradients, limited connectivity, and growing anthropogenic pressures make them ideal natural laboratories for testing how environmental filtering, dispersal limitation, and human activity jointly shape community assembly. Environmental filtering – dominated by salinity gradients – plays a key role in structuring phytoplankton communities, while wetlands embedded in more disturbed landscapes or with weaker protection show characteristic shifts in community composition, reflecting human-mediated filtering. Community similarity declines sharply with geographic distance within islands but remains consistently low among islands, indicating strong spatial constraints on phytoplankton dispersal. Our findings provide empirical support for a “triple-insularity” model of microbial regionalism and highlight island wetlands as discrete ecological units of high conservation value.