The rapid expansion of seawater reverse osmosis (SWRO) desalination has raised concern about the effects of brine discharges on coastal marine ecosystems. This study evaluated stress biomarkers in two benthic species with contrasting physiological strategies, the red seaweed Rhodymenia corallina and the sea anemone Anthothoe chilensis, following field transplantation near a desalination brine discharge in northern Chile. Biomarkers of photosynthetic performance, oxidative stress, antioxidant defense, and osmoregulation were assessed after 24 h, 48 h, and 5 d of exposure. R. corallina showed limited photosynthetic changes, moderate oxidative damage, and reduced antioxidant reserves, whereas A. chilensis showed reduced oxidative damage and maintained antioxidant and osmotic homeostasis. The Biomarker Response Index (BRI) revealed moderate alteration at 24 h followed by lower scores at later times. These distinct response patterns reveal marked species-specific sensitivity to brine exposure and support the value of integrating multiple biomarkers to characterize physiological responses. Given their ecological relevance and suitability for field transplantation in desalination-impacted areas, these findings support the evaluation of R. corallina and A. chilensis as candidate biomonitors for desalination environmental monitoring.