Assessing the utility of experimentally derived upper thermal limits to predict vulnerability of marine macrophytes...

Marine foundation species such as large macrophytes (i.e. seagrasses, canopy-forming seaweeds) serve critical ecological functions, yet are increasingly impacted by anthropogenic climate change. Temperature extremes pose significant challenges to these species, and identification of thermal thresholds offer valuable insights into how populations and species may respond to current and future climatologies. The objectives of this study were to compile experimentally-derived upper thermal limits (T_limit) of habitat-forming marine macrophytes into a centralized database, enabling further research. We conducted a Web of Science and Google Scholar search in June-August 2024 with three review participants to avoid bias. Any studies where a T_limit value could be identified for kelps, seagrasses or fucoids globally met the inclusion criteria for the study. Studies where T_limit could not be determined were excluded. After full text extraction, our data was analysed by frequency with regards to study region, target species, methodological approaches and other descriptors. The most extensively studied species were further examined to explore intraspecific associations with these descriptors. A comprehensive database of these values was collated and interrogated for marine macrophytes globally, incorporating 365 T_limit estimates extracted from 63 studies and representing 75 species. For seaweeds (kelps and fucoids), T_limit values ranged from 15 to 36 °C, whereas estimates for seagrasses ranged from 25 to 48 °C. T_limit values increased with decreasing latitude, both within and between species, and were influenced by life history stage, experimental duration and response variable chosen to determine thresholds. The existing database is characterised by several biases, including geographical regions and target species. Variation and uncertainty in reported T_limit values hinder our ability to predict suitable thermal habitats for marine macrophytes. Improved understanding of upper thermal thresholds is needed to improve predictions of current and future impacts of ocean warming, which will inform approaches to management and conservation of wider coastal communities and ecosystems.