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

Abstract

The effect of artificial ultraviolet (UV) and natural solar radiation on photosynthesis, respiration and growth was investigated in 14 red, green and brown macroalgal species on Spitsbergen (Norway) during summer 1998. In June, maximum mean solar radiation at sea level was 120 W m-2 of visible (370 to 695 nm) and 15 W m-2 of UV radiation (300 to 370 nm), and decreased gradually until the end of the summer. In spite of incident irradiance, levels were low in comparison with other latitudes, and UV radiation stress on growth of Arctic macroalgae was evident. Transplantation experiments of plants from deeper to shallow waters showed, for most algae, an inhibitory effect of both UVA and UVB on growth, except in the intertidal species Fucus distichus. The growth rate of selected macroalgae was directly correlated to the variations in natural solar radiation during the summer. Underwater experiments both in situ and using UV-transparent incubators revealed a linear relationship between the depth distribution and the growth rate of the algae. In almost all species the photosynthetic oxygen production decreased after 2 h incubation in the laboratory under 38 μmol m-2 s-1 photosynthetic active radiation (PAR 400 to 700 nm) supplemented with 8 W m-2 UVA (320 to 400 nm) and 0.36 W m-2 UVB (280 to 320 nm) compared to only PAR without UV. Like in the growth experiments, the only exception was the brown alga F. distichus, in which photosynthesis was not affected by UV. The degree of inhibition of photosynthesis showed a relation to the depth distribution, i.e. algae from deeper waters were more inhibited than species from shallow waters. In general, no inhibitory UV effect on respiratory oxygen consumption in all macroalgae studied was detected under the artificial radiation regimes described above, with the exception of the brown alga Desmarestia aculeata and the green alga Monostroma arcticum, both showing a significant stimulation of respiration after 2 h of UV exposure. The ecological relevance of the seasonal variations in the solar radiation and the optical characteristics of the water column with respect to the vertical zonation of the macroalgae is discussed.