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Kelps are a dominant macrophyte group and primary producer in Arctic nearshore waters that provide significant services to the coastal ecosystem. The quantification of these services in the Arctic is constrained, however, by limited estimates of kelp depth extent, which creates uncertainties in the area covered by kelp. Here, we test the environmental drivers of the depth extent of Arctic kelp. We used Southampton Island (SI), Nunavut, Canada, as an example region after an initial survey found deep Arctic kelp (at depths to at least 50 m) with relatively low grazing pressure within diverse hydrographic conditions. We found abundant rocky substrata, but no influence of substratum type on kelp cover. The kelp cover increased with depth until 20 m and then decreased (the median maximum depth for all stations was 37 m). The best predictor of kelp depth extent was the number of annual open (ice-free) water days with light (r2 = 44–52%); combining depth extent data from SI with published data from Greenland strengthened this relationship (r2 = 58–71%). Using these relationships we estimated the maximum kelp-covered area around SI to be 27,000–28,000 km2, yielding potential primary production between 0.6 and 1.9 Tg Cyr−1. Water transparency was a key determinant of the underwater light environment and was essential for explaining cross-regional differences in kelp depth extent in SI and Greenland. Around SI the minimum underwater light required by kelp was 49 mol photons m−2 yr−1, or 1.4% of annual integrated incident irradiance. Future consideration of seasonal variation in water transparency can improve these underwater light estimations, while future research seeking to understand the kelp depth extent relationship with nutrients and ocean dynamics can further advance estimates of their vertical distribution. Improving our understanding of the drivers of kelp depth extent can reduce uncertainties around the role of kelp in Arctic marine ecosystems.

The coastal zone of the Canadian Arctic represents 10% of the world’s coastline and is one of the most rapidly changing marine regions on the planet. To predict the consequences of these environmental changes, a better understanding of how environmental gradients shape coastal habitat structure in this area is required. We quantified the abundance and diversity of canopy forming seaweeds throughout the nearshore zone (5–15 m) of the Eastern Canadian Arctic using diving surveys and benthic collections at 55 sites distributed over 3,000 km of coastline. Kelp forests were found throughout, covering on average 40.4% (±29.9 SD) of the seafloor across all sites and depths, despite thick sea ice and scarce hard substrata in some areas. Total standing macroalgal biomass ranged from 0 to 32 kg m–2 wet weight and averaged 3.7 kg m–2 (±0.6 SD) across all sites and depths. Kelps were less abundant at depths of 5 m compared to 10 or 15 m and distinct regional assemblages were related to sea ice cover, substratum type, and nutrient availability. The most common community configuration was a mixed assemblage of four species: Agarum clathratum (14.9% benthic cover ± 12.0 SD), Saccharina latissima (13% ± 14.7 SD), Alaria esculenta (5.4% ± 1.2 SD), and Laminaria solidungula (3.7% ± 4.9 SD). A. clathratum dominated northernmost regions and S. latissima and L. solidungula occurred at high abundance in regions with more open water days. In southeastern areas along the coast of northern Labrador, the coastal zone was mainly sea urchin barrens, with little vegetation. We found positive relationships between open water days (days without sea ice) and kelp biomass and seaweed diversity, suggesting kelp biomass could increase, and the species composition of kelp forests could shift, as sea ice diminishes in some areas of the Eastern Canadian Arctic. Our findings demonstrate the high potential productivity of this extensive coastal zone and highlight the need to better understand the ecology of this system and the services it provides.