Sea Ice and Substratum Shape Extensive Kelp Forests in the Canadian Arctic

Abstract

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.

Increasing depth distribution of Arctic kelp with increasing number of open water days with light

“…We assumed that the observable substratum in any given image frame was representative of the entire image, even on images where less than 10% of the seafloor was visible due to high kelp cover. This assumption is consistent with diver observations of the substrata under the canopy at nearby sites (Filbee-Dexter et al, 2022).…”

“…Kelps are distributed as a function of substrata, nutrients, grazing pressure, temperature, and the underwater light environment (e.g., Mohr et al, 1957;Lu ¨ning, 1991;Wiencke and Amsler, 2012;Krause-Jensen et al, 2020;Goldsmit et al, 2021;Filbee-Dexter et al, 2022). Among these drivers, the underwater light environment is probably the most significant variable influencing kelp growth in sea-ice-associated waters (Chapman and Lindley, 1980;Dunton, 1990;Bonsell and Dunton, 2018;Filbee-Dexter et al, 2019;Mora-Soto et al, 2021).…”

“…In northern Hudson Bay, Nunavut, Canada, a recent survey documented the abundance of kelp around Southampton Island (SI) (Filbee-Dexter et al, 2022). This survey presented a unique opportunity to investigate drivers of kelp depth extent in a physically diverse region with highly variable sea ice cover and without a strong influence of latitudinal gradient or grazing pressure.…”

“…By identifying the drivers of the depth extent of Arctic kelp in this region, we aim to help improve the prediction of the total kelp forest area. This prediction is essential to improve estimates of the contribution of kelp to blue carbon and benthic primary production (Dunton et al, 2013;Filbee-Dexter et al, 2022). Furthermore, we combined our data on kelp depth extent with existing data from Greenland (Krause-Jensen et al, 2012;Krause-Jensen et al, 2019) to re-assess the relationship between kelp depth extent and OWL across a larger spatial scale.…”

Kelp carbon sink potential decreases with warming due to accelerating decomposition

“…This would mean a loss of potential carbon sequestration within the current distribution of kelp forests (e.g., [ 90 ]) under future warming. For example, if we apply this to kelp forests in Norway and the Canadian Arctic where maps of extent and total NPP exist (total NPP = 1.09 to 4.3 Tg C y −1 [ 91 ] and 2.2 to 6.4 Tg C y −1 and 10.4 to 30.6 Tg C y −1 [ 92 ], respectively), a 6.7% reduction of kelp export (long-term RCP8.5 scenario) is equivalent to a loss of 73 to 288 Gg C y −1 in Norway and 0.3 to 1.8 Tg C y −1 in the Eastern Canadian Arctic. Faster decomposition would also alter the nature of kelp as a resource subsidy, which will have ramifications for detrital food webs within kelp forests and in adjacent habitats that rely on this source of production [ 25 ].…”