In the shallow marine environment, a small number of habitat-forming skeletal organisms provide shelter and nursery grounds for other species. Free-living coralline algae (rhodoliths) build a complex three-dimensional skeleton and in large numbers form maerl beds which are biodiversity hotspots. Experiments suggest that coralline algae will be impacted by climate change. Here we assess phenotypic plasticity within cellular structure, growth rate and elemental composition of four species of coralline algae from the north and south of Britain. We compare contemporary specimens with historical material collected over the last century and determine how potential changes in growth affect structural integrity. Variation in cellular and mineral properties within a specimen and species is large. The cold temperate species L. glaciale and L. erinaceum have rounder, smaller cells compared to the more rectangular, larger cells of the warm temperate species P. calcareum and L. corallioides. Contemporary specimens form weaker skeletons than their historical counterparts, but the difference over time is not as large as the difference between cold adapted and warm adapted species. We predict that warm temperate species will still be structurally weaker as they will form more fragile skeletons compared to their cold temperate counterparts.