Abstract:

 Subarctic macrophytes are predicted to expand in the Arctic as a result of on-going global climate change. This will expose them to 24 h of light during the Arctic summer while pCO₂ levels are predicted to rise globally. Here, we tested the photosynthetic activity of two brown macroalgae (Ascophyllum nodosum, Fucus vesiculosus) and one seagrass (Zostera marina) from subarctic Greenland, measuring their relative maximum electron transport rate (rETR_max), photosynthetic efficiency (α) and saturating irradiance (I_k) after 3 days of incubation at different photoperiods (12:12 h, 15:09 h, 18:06 h, 21:03 h and 24:00 h, light:dark) with ambient values of pCO₂ (200 ppm, characteristic of current subarctic surface waters) and increased pCO₂ (400 and 1000 ppm). The photosynthetic parameters rETR_max and I_k increased significantly with longer photoperiods and increased, however insignificantly, with increased pCO₂. Responses differed between species. A. nodosum and Z. marina showed the highest increase of rETR_max and I_k from 12 h to 24 h while the increase of F. vesiculosus was smaller. Our results suggest that as subarctic macrophytes expand in the Arctic in response to retracting sea ice, the long summer days will stimulate the productivity of the species tested here, while the effect of high-CO₂ environment needs further research.  

https://doi.org/10.1016/j.aquabot.2019.06.005