Thermal Performance of Seaweeds and Seagrasses Across a Regional Climate Gradient

Comparative patterns in thermal performance between populations have fundamental implications for a species thermal sensitivity to warming and extreme events. Despite this, within-species variation in thermal performance is seldom measured. Here we compare thermal performance both within-species and between-species, for two species of seagrass ( Posidonia oceanica and Cymodocea nodosa ) and two species of seaweed ( Padina pavonica and Cystoseira compressa ) across the Mediterranean Sea. Experimental populations from four locations representing between 75 and 99% of each species thermal distribution and a 6°C gradient in summer temperatures, were exposed to 10 temperature treatments between 15 and 36°C. Experimental thermal performance displayed the greatest variability between species, with optimal temperatures differing by over 10°C within the same location. Within-species differences in thermal performance were also important for P. oceanica which displayed large thermal safety margins within cool and warm-edge populations and small safety margins within central populations. Our findings suggest patterns of thermal performance in Mediterranean seagrasses and seaweeds retain deep “pre-Mediterranean” evolutionary legacies, suggesting marked differences in sensitivity to warming within and between benthic marine communities.