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
between-species variation within communities, for two species of seagrass
(Posidonia oceanica and Cymodocea nodosa) and two species of seaweed
(Padina pavonica and Cystoseira compressa). Experimental populations from
four locations spanning approximately 75% of each species global
distribution and a 6ºC gradient in summer temperatures were exposed to 10
temperature treatments (15ºC to 36ºC), reflecting median, maximum and
future temperatures. Experimental thermal performance displayed the
greatest variability between species, with optimal temperatures differing
by over 10ºC 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.