Temperate seagrass and kelp ecosystems will decline with the increased frequency of heatwaves and sea temperature extremes as well as through the impact of invasive subtropical species (high confidence).’ SROCC (Collins et al., 2019a) ‘Marine heatwaves (MHWs), periods of extremely high ocean temperatures, have negatively impacted marine organisms and ecosystems in all ocean basins over the last two decades, including critical foundation species such as corals, seagrasses and kelps (very high confidence).’‘Marine heatwaves are projected to further increase in frequency, duration, spatial extent and intensity (maximum temperature)