Impacts of combined temperature and salinity stress on the endemic Arctic brown seaweed Laminaria solidungula J. Agardh

Macroalgae such as kelp are important ecosystem engineers in the Polar Regions and potentially affected by freshening and ocean warming. The endemic Arctic kelp Laminaria solidungula might be particularly imperiled and become locally extinct from Arctic fjord systems in the future, since temperature increase is most pronounced in the Polar Regions. Additionally, increased temperatures cause glacier and sea ice melting and enhancing terrestrial run-off from snowfields, which eventually can result in hyposaline conditions in fjord systems. We conducted a multiple-stressor experiment at four temperatures (0, 5, 10, 15 degrees C) and two salinities (S-A 25, 35) to investigate the combined effects of increasing temperature and decreasing salinities on the physiological and biochemical status of young L. solidungula sporophytes. Both drivers had significant and interacting impacts, either in an additive or antagonistic way, dependent on the respective response variable. The maximum quantum yield of photosystem II (F-v/F-m) significantly declined with temperature increase and low salinity. Even though the absolute pigment content was not affected, the deepoxydation state of the xanthophyll cycle increased with intensified stress. Higher temperatures affected the C:N ratio significantly, mainly due to reduced nitrogen uptake, while S-A 25 supported the nitrogen uptake, resulting in an attenuation of the effect. The concentration of mannitol decreased at S-A 25. At control S-A 35 mannitol level remained steady between 0 and 10 degrees C but significantly decreased at 15 degrees C. Conclusively, our results show that L. solidungula is very susceptible to both drivers of climate change, especially when they are combined. Implications to species ecology are discussed.