Hyposaline conditions impact the early life‐stages of commercially important high‐latitude kelp species

AbstractThis study examines how hyposaline stress impacts the early life‐stages of commercial kelp species from Alaska. Kelp are important species both ecologically and commercially and are likely to experience significant impacts due to ongoing climate change. Climate‐driven glacial melt and changing rainfall patterns globally will release large amounts of freshwater into coastal systems in the coming decades. Both bull kelp (Nereocystis luetkeana) and ribbon kelp (Alaria marginata) are high‐latitude species of commercial and ecological importance. These species inhabit very different environments: While bull kelp is a subtidal, canopy‐forming species, ribbon kelp is an intertidal subcanopy species. In this study, fertile specimens of both were collected from various locations in Alaska and induced to release spores. These were cultivated for 30 days in four salinity treatments: 32, 25, 20, and 13. Both species grew and produced gametophytes in salinities down to a salinity of 20, although A. marginata seems to be better adapted to hyposaline conditions. Below a salinity of 20, we observed several impacts on progression between life stages. The response of gametophyte growth and the production of eggs and sporophytes to different salinities varied both by species and by population. Gametophytes of N. luetkeana grew fastest at a salinity of 32, while those of A. marginata grew fastest between 20 and 25 (Juneau) or 25 and 32 (Kodiak). In terms of egg production, A. marginata displayed significant population‐level variation. Juneau individuals produced the same number of eggs regardless of salinity. Kodiak individuals produced fewer eggs in hyposaline conditions. The production of sporophytes from eggs for both species from all locations was unaffected by salinities above 20; however, no sporophytes at all were produced at 13. All of this has implications for commercial production in the hatchery phase, as hyposaline stress may induce N. luetkeana to produce sporophytes faster than in full oceanic salinity. In terms of wild populations, the observed population‐level and species‐level differences in adaptation to hyposaline conditions suggest that decreased salinities in coastal areas are likely to impact the distribution of these two species over the coming decades.