Episodes of bleaching in Arctic crustose coralline algae during 8 years of development

Crustose coralline algae (CCA) are important habitat-forming calcifiers that contribute to marine carbonate production, stabilize hard-bottom habitats, and support benthic biodiversity. However, rapid environmental change associated with Arctic Atlantification, including ocean warming, glacier retreat, increased freshwater input, and enhanced sediment delivery, may trigger bleaching, characterized by the loss of photosynthetic pigments, which can impair physiological performance and ultimately lead to tissue necrosis and mortality. We investigated CCA bleaching dynamics in Isfjorden, Svalbard, using an eight-year photographic monitoring dataset (2014-2022) from two stations at 6 and 12 m depth, combined with continuous measurements of seawater temperature and photosynthetically active radiation (PAR). Bleaching increased from <1 cm² during 2015-2016 to peaks of 25.5 cm² (S1-12 m, 2018) and 38.3 cm² (S2-6 m, 2022). Generalized additive models identified year, station, and a nonlinear temperature-PAR interaction as significant predictors of CCA bleaching. However the most extensive bleaching coincided with increased sediment accumulation, suggesting that multiple interacting environmental stressors can cause bleaching in the Arctic CCA.