Niclas Einert

As climate change accelerates, increasing ocean temperatures and altered herbivory pressure are reshaping temperate marine ecosystems dominated by macrophyte foundation species. Despite the importance of these processes, it remains unclear how herbivory affects the thermal performance of macrophytes. We simulated herbivory by clipping the leaf length of two temperate seagrass species, Posidonia australis and Heterozostera tasmanica , and grew them at temperatures extending below and above their current thermal range (6°C–32°C). We assessed the effects of reduced leaf size (i.e., simulated herbivory) on thermal performance by quantifying growth, photosynthetic rates, leaf nutrient content and pigment concentrations. Responses to clipping treatment and growing temperature were species‐specific. Highly clipped P. australis showed higher photosynthetic rates and nutrient concentrations at 32°C, relative to low‐clipped and control plants, while growth did not differ between clipping treatments, and survival was high across treatments. In H. tasmanica , clipping lowered optimal growth temperatures, and survival declined sharply at 32°C across treatments. Our results indicate that simulated herbivory can increase the thermal resilience of P. australis but decreases the performance of H. tasmanica under thermal stress. Increased herbivory pressure under climate change may therefore have positive implications for some seagrasses, highlighting the importance of considering species‐specific responses when predicting the future resilience of seagrass ecosystems.

• As climate change accelerates, increasing ocean temperatures and altered herbivory pressure are reshaping temperate marine ecosystems dominated by macrophyte foundation species. Despite the importance of these processes, it remains unclear how reduced above-ground biomass, due to anticipated in-creased herbivory, will affect the thermal performance of macrophytes. • We simulated herbivory by clipping the leaf length of two temperate seagrass species, Posidonia australis and Heterozostera tasmanica, and grew them at temperatures spanning their thermal range (6-32 °C). We assessed the effects of leaf size on thermal performance by quantifying growth, photosynthetic rates, leaf nutrient content and pigment concentrations. • Responses to clipping treatment and growing temperature were species-specific. Highly clipped P. australis showed higher photosynthetic rates and nutrient concentrations at 32 °C, relative to low-clipped and control plants, while growth did not differ between clipping treatments and survival was high across treatments. In H. tasmanica, clipping lowered optimal growth temperatures, and survival declined sharply at 32 °C across treatments. • Our results suggest that smaller leaf size can increase thermal resilience in P. australis, whereas high herbivory of H. tasmanica is likely to increase its sensitivity to thermal stress. Increased herbivory pressure under climate change may therefore have positive implications for some seagrasses, highlighting the importance of considering species-specific responses when predicting the future resilience of seagrass ecosystems.