Assessing vulnerability of habitat-forming marine macrophytes to ocean warming using experimentally-derived upper thermal limitsResistance, Resilience and Phase Shifts

Katie Smith¹, Nathan King¹, Margot Aubin¹, Tayla Leathers¹, Dan Smale¹

¹The Marine Biological Association of the UK, UK

Foundation species, such as marine macrophytes (i.e. seagrasses, canopy-forming seaweeds), serve critical ecological functions but are increasingly impacted by anthropogenic climate change. Temperature extremes pose significant challenges to marine organisms and measures of thermal tipping points offer valuable insights into how species may be impacted by current and future temperature regimes. We collated and interrogated a comprehensive database of experimentally-derived upper thermal limit (Tlimit) values for marine macrophytes globally, incorporating 368 Tlimit estimates extracted from 63 studies and representing 75 species. For seaweeds (kelps and fucoids), Tlimit values ranged from 15 °C to 36 °C, whereas estimates for seagrasses ranged from 25 °C to 48°C. Tlimit values increased with decreasing latitude, both within and between species, and were influenced by life history stage, experimental duration and the measured response variable. The database highlighted several biases, including geographical regions and target species, and we propose research priorities needed to address critical knowledge gaps. A better understanding of thermal tipping points for ecologically-important species is needed to improve predictions of current and future impacts of ocean warming, which in turn is needed to inform approaches to management and conservation of wider coastal communities and ecosystems.