Seasonal fertility and combined effects of temperature and UV-radiation on Alaria esculenta 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 degrees 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 degrees C and the beginning of July, whereas it was not affected by seasonality at 7 degrees 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 degrees 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.