Effects of solar radiation on growth, photosynthesis and respiration of marine macroalgae from the Arctic

The effect of artificial ultraviolet (UV) and natural solar radiation on ph otosynthesis, respiration and growth was investigated in 14 red, green and brown macroalgal species on Spitsbergen (Norway) during summer 1998. In Jun e, maximum mean solar radiation at sea level was 120 W m(-2) of visible (37 0 to 695 nm) and 15 W m(-2) of W radiation (300 to 370 nm), and decreased g radually until the end of the summer. In spite of incident irradiance, leve ls were low in comparison with other latitudes, and UV radiation stress on growth of Arctic macroalgae was evident. Transplantation experiments of pla nts from deeper to shallow waters showed, for most algae, an inhibitory eff ect of both UVA and UVB on growth, except in the intertidal species Fucus d istichus. The growth rate of selected macroalgae was directly correlated to the variations in natural solar radiation during the summer. Underwater ex periments both in situ and using UV-transparent incubators revealed a linea r relationship between the depth distribution and the growth rate of the al gae. In almost all species the photosynthetic oxygen production decreased a fter 2 h incubation in the laboratory under 38 mu mol m(-2) s(-1) photosynt hetic active radiation (PAR 400 to 700 nm) supplemented with 8 W m(-2) UVA (320 to 400 nm) and 0.36 W m(-2) WE (280 to 320 nm) compared to only PAR wi thout UV. Like in the growth experiments, the only exception was the brown alga F. distichus, in which photosynthesis was not affected by W. The degre e of inhibition of photosynthesis showed a relation to the depth distributi on, i.e. algae from deeper waters were more inhibited than species from sha llow waters. In general, no inhibitory UV effect on respiratory oxygen cons umption in all macroalgae studied was detected under the artificial radiati on regimes described above, with the exception of the brown alga Desmaresti a aculeata and the green alga Monostroma arcticum, both showing a significa nt stimulation of respiration after 2 h of UV exposure. The ecological rele vance of the seasonal variations in the solar radiation and the optical cha racteristics of the water column with respect to the vertical zonation of t he macroalgae is discussed.