Photosynthetic community restructuring along a desalination brine-discharge gradient in northern Chile

Rapid expansion of seawater desalination in arid coastal regions has increased the need for sensitive biological tools to assess brine-discharge effects. This study evaluated marine photosynthetic communities along a discharge gradient in Flamenco Bay (northern Chile) using physicochemical profiling, pigment measurements, 18S rRNA metabarcoding, and diversity metrics. The discharge point showed a strong near-field salinity anomaly (41.75±3.36 versus 33.71 ± 0.44 PSU at the reference station), whereas differences in the photosynthetic assemblage extended over a broader area. Genus richness was stable across stations (9-11 genera), but Shannon diversity was reduced at impacted stations (1.06-1.09) relative to the reference (1.87). Near-field stations were dominated by Myzozoa (especially Karlodinium), whereas the reference supported a more balanced assemblage of Ochrophyta, Myzozoa, Chlorophyta, Cryptophyta, and Haptophyta. Bray-Curtis dissimilarity showed that differences among stations were driven mainly by shifts in relative sequence abundance rather than taxonomic replacement. PERMANOVA confirmed significant differences in community composition among stations (pseudo-F = 1.99, p = 0.018), and ordination (PCoA) showed discharge-influenced stations clustering separately from the reference site. The Integrated Response Index identified the discharge point as showing the strongest deviation and biological differences at 500 m despite low physicochemical deviation. Community-level differences may remain detectable where salinity and related variables approach reference conditions, so physicochemical monitoring alone may not fully capture spatial variation in community structure around desalination outfalls. Future studies integrating high-resolution hydrodynamics with replicated temporal sampling would distinguish brine-driven effects from natural spatial variability, strengthening the bioindicator value of these community-level metrics.