Susceptibility of zoospores to UV radiation determines upper depth distribution limit of Arctic kelps : evidence through field experiments

Abstract

The UV susceptibility of zoospores of the brown seaweeds Saccorhiza dermato- dea, Alaria esculenta and Laminaria digitata (Laminariales) was determined in field experiments in June 2004 on Spitsbergen (78° 55' N, 11° 56' E).2. Freshly released zoospores were exposed for one or two days at various water depths to ambient solar radiation, ambient solar radiation depleted of UVB radiation (UVBR) and ambient solar radiation depleted of both UVBR and UVAR. Subsequently, germination rates were determined after exposure to favourable light and temperature conditions in the laboratory.3. The radiation regime was monitored at the water surface and in the water adjacent to the exposure platforms for the duration of the field exposure.4. Under ambient solar radiation the tolerance of zoospores to UVR was highest in the shallow water species S. dermatodea, intermediate in the upper to mid sublittoral A. esculenta and lowest in the upper to mid sublittoral L. digitata. There was, however, no difference in the susceptibility of the zoospores to ambient solar radiation or to solar radiation depleted of UVBR.5. The water body was relatively UV transparent especially in the upper water layers. The 1 % UVB depth mostly varied between 5.35 and 6.87 m. On one stormy day the 1 % UVB depth was only 3.57 m indicating resuspension of sediments.6. We propose that as the zoospores are the developmental stages most susceptible to UVR their UVR tolerance is a major if not the most important factor for the determination of the upper depth distribution limit of these species on the shore. The results are discussed with respect to enhanced UVBR due to stratospheric ozone depletion.