Kelp forest diversity and stability at warming and under-explored northern limitsReef-associated Biodiversity

Sidsel Pedersen¹, Albert Pessarrodona^1,2,3, Morten Pedersen⁴, Philippe Archambault⁵, Amanda Savoie⁶, Camille Lavoie⁵, Kjell Magnus Norderhaug⁷, Thomas Wernberg^1,7, Karen Filbee-Dexter^1,7,5

¹University of Western Australia, Australia - ²Conservation International, USA - ³International Blue Carbon Institute, Singapore - ⁴Roskilde University, Denmark - ⁵Université Laval, Canada - ⁶Canadian Museum of Nature, Canada - ⁷Institute of Marine Research, Norway

One third of the world’s coastlines occur in the Arctic, where kelp forests are the dominant biogenic coastal ecosystem. These forests are increasingly recognized as vital blue carbon habitats that support distinct biodiversity. Continued ocean warming is widely expected to lead to kelp forest expansion at some northern range edges. Yet, knowledge of kelp standing stocks, species composition, age and turnover in these locations is very limited. There is also scarce data on how environmental conditions affect northern kelp forests, hindering our ability to predict how these ecosystems will respond into a rapidly changing future. This study aims to address these key knowledge gaps using kelp collections from 65 sites in temperate and Arctic regions in Canada, Greenland, and Norway. Within each of these regions, we assessed kelp species composition, biomass, age structure, morphology, and turnover at locations distributed along a latitudinal gradient (6 sites per location, 6–8 quadrats per site). We then explored how these variables varied across environmental gradients in sea ice cover, light, temperature, wave exposure, substratum, and sea urchin presence. We found that kelp diversity, age structure, and morphology varied significantly across our sites, which ranged from open ocean to over 6 months of sea ice cover, and from 0 to 17°C bottom temperatures. We also found significant relationships between kelp forests environmental factors, which has important implications for biomass production and potential carbon storage, as well as the diversity and stability of these ecosystems into the future. By examining ecological patterns in relation to environmental drivers, we provide insights into the factors shaping the current and future dynamics of Arctic kelp forests. Such insights are crucial for predicting the dynamics of kelp forests in northern temperate and Arctic regions, their contributions to carbon cycling, and their potential stability in a warming ocean.