Long-term changes in coastal seaweed communities in response to contemporary climate change across a wave exposure gradientResistance, Resilience and Phase Shifts

Emil Kotta¹, Jonne Kotta¹, Helen Orav-Kotta¹, Ilmar Kotta¹, Mihkel Kotta¹, Ants Kaasik¹, Kristiina Nurkse¹, Merli Rätsep¹, Francisco Barboza¹

¹Estonian Marine Institute, University of Tartu, Estonia

This study investigates long-term, climate-driven changes in benthic seaweed communities along a wave exposure gradient in the Baltic Sea from 2006 to 2023. Using the Hierarchical Modelling of Species Communities (HMSC) framework, we assessed how shifts temperatures, wave intensity, and nutrient regimes are associated with the cover of seaweed and invertebrate species. Over the study period, winter and summer temperatures rose by nearly 5 °C, alongside changes in wave dynamics and nutrient availability. Despite overall stability in total seaweed cover, the key habitat-forming species Fucus vesiculosus experienced a significant decline in cover from 50% to 10% between 2003 and 2013. This decline was driven by increased wave-induced losses during winter and low summer productivity, which constrained the recovery capacity. Since 2014, this trend has reversed despite similar winter losses, with higher temperatures enhancing summer productivity, driving recovery, and offsetting earlier declines. The filamentous algae Cladophora glomerata and Vertebrata fucoides expanded in response to higher summer temperatures, whereas the habitat-former Chorda filum nearly disappeared. The predatory round goby reduced Mytilus edulis populations, allowing barnacles to recolonize substrate. HMSC analyses revealed limited interspecies associations, indicating that changes in abiotic environment primarily drive community changes. While current warming levels remain within the tolerance limits of key habitat-forming species, continued warming at this pace may lead to significant declines in habitat-formers and an expansion of turf and filamentous algae. These findings emphasize the value of long-term time series in disentangling ecological processes and highlight the need to account for both abiotic conditions and novel species introductions when projecting Baltic Sea benthic community structure under accelerating climate change.