Habitat fragmentation in freshwater ecosystems alters dispersal pathways, creating conditions where stochastic processes can dominate the community assembly. We used a controlled indoor microcosm experiment to investigate how phytoplankton metacommunities assemble when dispersal is reduced under environmentally homogeneous conditions. A single inoculum from a Pampean stream was acclimated and distributed into six microcosms, maintained for 11 weeks under identical abiotic conditions. We tested three hypotheses: (H1) if deterministic processes dominated, communities would converge in taxonomic composition and functional attributes despite isolation; (H2) if stochastic processes dominated, communities would diverge in both taxonomic and functional trajectories under identical conditions; and (H3) the relationship between taxonomic and functional dissimilarity would depend on the degree of functional redundancy, with convergence in functional space when redundancy was high, or tighter taxonomic-functional coupling otherwise. Despite identical starting conditions, communities diverged in both richness and abundance trajectories, with relatively high and sustained beta-diversity throughout the experiment. Functional composition showed lower dispersion than taxonomic composition, but taxonomic and functional dissimilarity were significantly correlated, indicating partial coupling between community structure and function rather than full functional convergence. Our results support the view that fragmentation itself does not directly cause divergence. By reducing dispersal, fragmentation allows stochastic processes such as drift and priority effects to operate independently across fragments. These experimental insights highlight the potential for stochastic dynamics to drive community divergence in fragmented freshwater systems, and stress the importance of integrating taxonomic and trait-based functional dimensions when evaluating ecosystem resilience.
Thermal springs create strong temperature and geochemical gradients that can act as environmental filters for aquatic biota. We assessed seasonal patterns of benthic macroinvertebrates and their relationships with water physicochemistry at four sites influenced by thermal and cold limnocrene springs in the Tuzla region (& Ccedil;anakkale, NW T & uuml;rkiye) during 2023. The system was highly saline (50.97-62.53 ppt), and the thermal pool reached 73.3 degrees C, where no macroinvertebrates were detected. Across the remaining three sites (S1-S3), Diptera dominated and Ephydridae accounted for 61 % of all individuals. Physicochemical conditions showed seasonal varia-tion, with significant differences in dissolved oxygen (F = 5.837, p = 0.0206) and total dissolved solids (F = 45.45, p = 2.27e-05), while salinity showed only weak evidence of variation (p = 0.0923). PERMANOVA indicated no significant overall differences in macroinvertebrate community composition among stations and seasons (R2 = 0.435, F = 0.616, p = 0.935), consistent with the overlap observed in ordination space. Separate regression models showed that Simpson's diversity index declined significantly with increasing total dissolved solids (p = 0.0212). Constrained ordination further suggested positive associations of Chironomidae with dissolved oxygen and pH, and of Ceratopogonidae with water temperature. Overall, the extreme thermal-salinity regime acted as a strong envi-ronmental filter, providing a baseline for biomonitoring and future comparative work in geothermal spring systems.
