Photosynthetic performance of Laminaria solidungula measured in situ in the Alaskan High Arctic

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Photosynthetic performance in the kelp Laminaria solidungula J. Agardh was examined from photosynthesis irradiance (P-I) parameters calculated from in situ 14C uptake experiments, using whole plants in the Stefansson Sound Boulder Patch, Alaskan Beaufort Sea, in August 1986. Rates of carbon fixation were determined from meristematic, basal blade, and second blade tissue in young and adult sporophytes. Differences in saturating irradiance (Ik, measured as photosynthetically active radiation, PAR), photosynthetic capacity (Pmax), and relative quantum efficiency (α) were observed both between young and adult plants and between different tissue types. Ik was lowest in meristematic tissue (20 to 30 μE m−2 s−1) for both young and adult plants, but consistently 8 to 10 μE m−2 s−1 higher in young plants compared to adults in all three tissues. Average Ik for non-meristematic tissue in adult plants was 38 μE m−2 s−1. Under saturating irradiances, young and adult plants exhibited similar rates of carbon fixation on an area basis, but under light limitation, fixation rates were highest in adult plants for all tissues. Pmax was generally highest in the basal blade and lowest in meristematic tissue. Photosynthetic efficiency (α) ranged between 0.016 and 0.027 μmol C cm−2 h−1/μE m−2 s−1, and was highest in meristematic tissue. The relatively lower Ik and higher α exhibited by L. solidungula in comparison to other kelp species are distinct adaptations to the near absence of light during the eight-month ice-covered period and in summer when water turbidity is high. Continuous measurement of in situ quantum irradiance made in summer showed that maximum PAR can be less than 12 μE m−2 s−1 for several days when high wind velocities increase water turbulence and decrease water transparency.