Looks can be deceiving: contrasting temperature characteristics of two morphologically similar kelp species co-occurring in the Arctic

Two morphologically similar digitate kelp species, Laminaria digitata and Hedophyllum nigripes, co-occur along a shallow sublittoral depth gradient in the Arctic but, in contrast to L. digitata, very few ecophysiological data exist for H. nigripes. We investigated growth, survival, photosynthetic characteristics and carbon:nitrogen ratios of juvenile sporophytes, and recruitment and survival of gametophytes in genetically verified Arctic isolates of both species along temperature gradients (0-25 degrees C) over 14 days. Laminaria digitata gametophytes survived 23-24 degrees C, while sporophytes survived 21-22 degrees C. Hedophyllum nigripes had lower temperature affinities. Gametophytes survived 19-21 degrees C, while sporophytes survived 18 degrees C. Male gametophytes were more heat-tolerant than female gametophytes in both species. The pronounced cold adaption of H. nigripes compared to L. digitata also became apparent in different sporophyte growth optima (L. digitata: 15 degrees C; H. nigripes: 10 degrees C) and gametogenesis optima (L. digitata: 5-15 degrees C; H. nigripes: 0-10 degrees C). Higher carbon:nitrogen ratios in H. nigripes suggest an adaptation to nutrient poor Arctic conditions. The overall temperature performance of H. nigripes possibly restricts the species to Arctic-Sub-Arctic regions, while Arctic L. digitata behaved similarly to cold-temperate populations. Our data suggest that a future increase in seawater temperatures may hamper the success of H. nigripes and favour L. digitata in Arctic environments.

Automated summary

The study compared two morphologically similar kelp species, Laminaria digitata and Hedophyllum nigripes, in the Arctic region. While L. digitata showed affinities to cold-temperate environments, H. nigripes exhibited greater cold adaptation and higher carbon:nitrogen ratios, suggesting adaptation to nutrient-poor Arctic conditions. The data imply that future increase in seawater temperatures may hinder the success of H. nigripes but favor L. digitata in Arctic environments.