Sensitivity of intertidal and subtidal red algae to UVA and UVB radiation, as monitored by Chlorophyll fluorescence measurements: Influence of collection depth and season, and length of irradiation

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Abstract
Thirteen species from the red algal flora of Helgoland (southern North Sea) were exposed to UVA + UVB radiation for various periods in the laboratory, and dark-adapted variable fluorescence (F v:F m) was measured immediately after the UV treatment and again after various recovery times in white light. With the exception of Porphyra umbilicalis, all species showed a decrease in F v:F m on exposure to UV radiation, followed by recovery towards the initial values during the next 24–48 h in white light. The rate of the initial decrease was greater, and the extent of recovery was less, in deep subtidal species (e.g. Delesseria sanguinea, Plocamium cartilagineum) than in intertidal or shallow subtidal species, although there was no direct correlation between these indicators of sensitivity to UV radiation and the depth range of a species, and only slight differences were detected between populations of a single species collected from different depths. There was also little evidence of a seasonal change in sensitivity to UV radiation in Delesseria or Plocamium. The decrease in variable fluorescence in Delesseria and Plocamium was proportional to the logarithm of the exposure to UV radiation, and reciprocity between irradiance and the length of irradiation appeared to hold for up to 16h. The exposure to UVA that reduced F v:F m by 50% (‘50% exposure’) immediately after the treatment was about 10 kJ m-2 for both Delesseria and Plocamium, but Plocamium appeared to recover more rapidly than Delesseria because the 50% exposure after 24 h recovery was about 37 kJ m-2 for Delesseria compared with 70–120 kJ m-2 for different populations of Plocamium. The removal of UVB from the UV radiation treatments had no detectable effect on the inhibition of F v:F m in either Delesseria or a deep-water population of Plocamium, but reduced the inhibition by 30–50% in a shallow-water population of Plocamium. When only half an intact blade of Delesseria was exposed to UV radiation, variable fluorescence was reduced in the irradiated half but was completely unaffected in the unirradiated half, and steep gradients of F v:F m values were measured which persisted for over 48 h after the irradiation treatment.