Seasonal fertility and combined effects of temperature and UV-radiation on Alariaesculenta and Laminaria digitata (Phaeophyceae) from Spitsbergen

Global warming and enhanced UV-radiation due to stratospheric ozone depletion could drastically affect Arctic coastal ecosystems. Previous research revealed that the UV-susceptibility and impact of increased UV-radiation on Arctic kelp zoospores are highly variable, potentially due to seasonal acclimation. Accordingly, for a better understanding of climate change effects on Arctic kelp, we need to determine the fertility period of Arctic kelps and to systematically examine the seasonal differences of increased UV-radiation during the fertility period. We examined the fertility period of Laminaria digitata and Alaria hyperborea , by evaluating sorus maturation, zoospore release and germination. Zoospore germination was studied under photosynthetic active radiation (PAR 400–700 nm) and PAR and increased UVAB-radiation (280–700 nm) from July to September at 2 and 7 °C. Furthermore, we tested whether differences in the zoosporic UV-susceptibility were related to the content of UV-screening phlorotannins. The fertility period of A. esculenta is uniform from July to mid-August and ends in September. Within the fertility period, the UV-susceptibility of A. esculenta zoospores was highest at 2 °C and the beginning of July, whereas it was not affected by seasonality at 7 °C. The fertility period of L. digitata starts in late July and lasts at least until September, and no seasonal differences in the UV-susceptibility were found. However, UV-susceptibility was significantly lower at 7 °C. In both species, the zoosporic phlorotannin content did not affect the UV-susceptibility. We conclude that seasonality strongly influences the UV-susceptibility of A. esculenta but at low water temperatures only. Higher seawater temperatures help both species to cope with increasing UV-radiation.