Seawater flow attenuation by structurally complex kelp forests in a high-latitude fjord

Kelp forests are integral components of coastal marine ecosystems, significantly enhancing biodiversity, productivity and stability. With their large and complex canopy structures, which often occur in dense groups, they are able to physically modify their surroundings (e.g., flow velocity of currents, chemical composition of water), though the extent of this influence is not fully understood. The primary objective of this investigation was to examine the impact of kelp forests on seawater currents in the rapidly changing Arctic coastal ecosystem. The studied kelp forests along Isfjorden (Spitsbergen, 78° N) were predominantly composed of Alaria esculenta , Saccharina latissima and Laminaria digitata , with an average density of 58.9 ± 18.6 ind/m² and maximum length of individuals up to 206 cm ( S. latissima ). Through comprehensive analysis of currents velocity, direction and tidal patterns, significant reductions in flow velocities were observed within kelp forests compared to nearby kelp-free areas. Notably, at a depth of 7 m, mean flow speeds varied from 9.89 cm/s at locations with no kelp forest to 3.03 cm/s at locations with dense kelp forest, indicating distinct differences in flow dynamics. This study quantifies the impact of kelp forest composed of above-mentioned species on seawater currents, showing that aggregations of these morphologies generate microhabitats with distinct flow regimes. This knowledge is essential especially for the Arctic, enabling prediction of coastal ecosystem responses to rapid environmental changes and managing potential impacts on biodiversity and ecosystem functioning.