Cool under heat: humidity moderates the tolerance of <i>Fucus vesiculosus</i> (Phaeophyceae) to changes in air temperature during emersion

Abstract Projections for climate change potentially mean that tidally exposed seaweeds will experience greater air temperature extremes than those that remain immersed. This study investigated the combined effects of temperature and humidity on the relative growth rate of Fucus vesiculosus through two controlled experiments: (1) evaluating responses to various temperature and humidity regimes, and (2) assessing the impact of three consecutive days of heatwave conditions (25 °C). Samples from the upper and lower shore were subjected to 24, 30 and 40 °C at relative humidities of 30–90 % for 4 or 8 h, using an incubator oven to generate six vapour pressure deficit (VPD) levels (0.3–5.3 kPa). A VPD of 5.3 kPa proved detrimental to the seaweeds regardless of duration. In contrast, F. vesiculosus tolerated a VPD of 2.4 kPa for up to 8 h. Little difference was detected between upper and lower shore residing specimens. Heatwave conditions did not negatively affect F. vesiculosus growth irrespective of shore location. Blades could recover from proportional water biomass loss of up to 0.6 but not from a loss of 0.7 regardless of VPD. Nonetheless, greater VPD values tended to coincide with increased water biomass loss and decreased post-exposure growth. These findings suggest that increased humidity may mitigate thermal stress, enhancing resilience under projected climate scenarios.