Synergistic local and global stressors shape the phenology and physiological limits of a model red macroalga in South Florida

Environmental variability, together with disturbance events, can control the distribution of benthic organisms because it defines the physical boundaries that species can tolerate. To examine how these factors influence the survival and physiological performance of a key benthic group (marine macroalgae), we examined the seasonal population dynamics and tolerances of the red alga, Hypnea spiniformis , to better understand how temperature, salinity and disturbance affect its distribution. We performed field-surveys from February 2022 to April 2023 at Crandon Park in Miami, Florida, USA, to track algal biomass, reproductive phenology and morphological characteristics, while laboratory experiments evaluated growth rates and photophysiology (Fv/Fm, ETR, Ek, α, Y(II), Y(NO), Y(NPQ)) under temperature (24 to 36 °C) and salinity (16 to 40) gradients. The field biomass peaked in June 2022 at 0.32 ± 0.36 g m −2 DW when sea surface temperatures (SSTs) reached 30 °C. Thallus length and tetrasporophytes frequency also peaked at this time. The entire population disappeared between August and December when SSTs exceeded 33 °C. This event also coincided with extensive landings (>85 g m −2 DW) of brown algae, Sargassum spp. In laboratory assays, H. spiniformis achieved its highest growth rate (21.6%·d −1 ), morphological (thallus length and number of differentiated apical ends) and photophysiological performance at 30 °C. All metrics were reduced substantially at 33 °C, with thallus mortality at 36 °C. The optimal salinity range for this species was 28–34 ppt, with decreased performance in both hypo- and hypersaline conditions, with mortality at 16 and 19 ppt. Field and laboratory results indicate that H. spiniformis is well-adapted to prevailing late-spring conditions but is vulnerable to disturbance events that exceed physiological limits. The ecological role of this species may be further affected by climate-driven warming and persistent Sargassum landings in South Florida, reducing survival. This study highlights the value of combining long-term field monitoring with controlled physiological assays to provide a framework for assessing the resilience of species in changing coastal ecosystems. • Year-round monitoring assessed H. spiniformis ecology in Florida. • Optimal growth occurred at 30 °C and salinity 28–34. • Populations collapsed in summer with high SSTs and Sargassum inundations. • Disturbances disrupted natural seasonal patterns of the species. • Local, and global stressors act together to limit resilience. • Hypnea serves as a model for macroalgae under climate change.