Heatwaves in summer with freshwater inflows driven by climate change increasingly cause combined thermal-hyposaline stress, threatening intertidal canopy-forming macroalgae on Swedish west shores. Four common species in Skagerrak strait—Fucus spiralis, Fucus vesiculosus, Ascophyllum nodosum, and Fucus serratus—were subjected to a 24-h stress experiment with temperature and salinity gradients (20–32 °C; 2–22 PSU), followed by a three-week recovery, and quantified relative growth and photosynthetic capacity. Growth rates of all species decreased under designed stress, with F. serratus showing the steepest decline; growth ceased universally above 27 °C and below 5 PSU. Linear mixed-effects models revealed significant temperature × salinity interactions for A. nodosum and, to a lesser extent, F. vesiculosus, whereas responses in F. spiralis and F. serratus were additive. Predicted response surfaces based on models confirmed distinct tolerance profiles. Reductions in maximum quantum yield closely tracked growth losses, validating its use as a rapid stress proxy. These results indicate that even brief heat-freshwater pulses can leave continuous damage, with species-specific sensitivities likely to reshape canopy assemblages under projected climate extremes. Geographic variation in tolerance and underlying physiological mechanisms are needed in further investigation.
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