Global meta-analysis reveals the impacts of ocean warming and acidification on kelps

Kelp forests are among the most diverse and productive ecosystems in the world, providing critical habitat for numerous ecologically and economically important species. However, kelps are at risk from climate change, and declining populations worldwide demonstrate the need to characterize and quantify the effects of anthropogenic stressors on kelp physiology. Here, we performed a meta-analysis on true kelps (order Laminariales) in response to ocean warming and acidification based on a global synthesis of 7000 data points from 143 experimental studies. Our results show that ocean warming has a strong negative impact on kelps at all life stages and across various physiological levels, including growth, reproduction, and survival. In contrast, ocean acidification generally has no effect, except for its negative impact on reproduction. In most cases, co-occurring warming and acidification acted synergistically. Response to warming, acidification, and multiple driver scenarios increased as the intensity and duration of exposure increased. In our analyses, the genera Eualaria, Hedophyllum, Lessonia, and Postelsia were among the most vulnerable to warming. Studies conducted in the temperate northern Pacific showed extreme negative effects of warming. We also identify key gaps in our understanding of kelp responses to climate change, such as the impacts on microscopic spores and the combined effects of warming and acidification. This analysis synthesizes trends in a rapidly expanding field of literature and provides a deeper understanding of how kelps will respond to a rapidly changing ocean.