Kelp Forests in Deep-Water Habitats at the Gorringe Ridge

João N. Franco¹, David Milla-Figueras², Emma M. Franke³, Mariana S. Coxey⁴, Emanuel J. Gonçalves^5,6, Ester Serrão³

¹ESTM, Polytechnic of Leiria, Portugal - ²University of the Azores, Horta, Portugal - ³Universidade do Algarve, Portugal - ⁴Mardive - Association Science and Education for the Conservation of Marine Biodiversity, Portugal - ⁵Instituto Universitário, Portugal - ⁶Oceano Azul Foundation, Oceanário de Lisboa, Portugal

The Gorringe Bank, also known as the Gorringe Ridge, was first documented in 1875 by Henry Gorringe. This unique underwater mountain rises dramatically from depths of 5000 m to 30 m. Recognized for its ecological importance, it was designated part of the Natura 2000 network in 2015. In September 2024, the Oceano Azul Foundation conducted an expedition to the Gorringe Bank to explore and map its biodiversity. This study focuses on the distribution, abundance, density, and percentage cover of two resident kelp species, Saccorhiza polyschides and Laminaria ochroleuca, at the Gettysburg and Ormonde peaks. Using scientific diving methods, underwater video imaging was collected and analyzed with the standardized SACFOR classification system to quantify abundance. The geographical distribution of both kelp species along the Gorringe Ridge was supplemented by analyzing image frames from other existing video devices. Additionally, individuals were sampled for carbon-to-nitrogen (C:N) content analysis and age determination (L. ochroleuca). Results show L. ochroleuca ecologically dominates both peaks. At Gettysburg, it exhibited an average density of 2,200 individuals per 2,000 m² and 28.7% cover, compared to 150 individuals and 8.5% cover for S. polyschides. Similarly, at Ormonde, L. ochroleuca reached 2,600 individuals per 2,000 m² and 30.5% cover, while S. polyschides was nearly absent (fewer than 50 individuals, 6.7% cover). Populations of L. ochroleuca at Gettysburg showed a broader age range (2–8 years) and variable distribution, whereas those at Ormonde were more localized, with a median age of 4 years. The absence of individuals <2 years at both sites suggests recent recruitment deficits. Variability in L. ochroleuca abundance, particularly at Ormonde, likely reflects hydrodynamic differences and abiotic factors such as temperature and currents. These findings highlight the ecological dominance of L. ochroleuca and provide valuable insights into the structure and dynamics of kelp forests at the Gorringe Bank.