The large size and rapid growth of kelp is due, at least in part, to their ability to optimize growth under a variety of light conditions through changes in photosynthetic performance. This feature may be most pronounce in one arctic species, Laminaria solidungula, which increases its annual production by more than half over 8-months of continuous ice cover when irradiance levels are lowest. Reported open-water saturation irradiance in this occurs during the summer, a period of minimal growth, when dissolved inorganic nitrogen (DIN) concentrations are lowest. The project will test the hypothesis that photosynthetic sensitivity in L. solidungula increases considerably during the ice-covered period, and that ambient increases in DIN, not low levels of irradiance, are primarily responsible for seasonal changes in the plant's photosynthetic performance. Several photosynthetic parameters will be calculated from photosynthetic irradiance (P-I) curves derived from radioactive carbon uptake and oxygen flux measurements, and the light harvesting characteristics of entire plants incubated in situ in Beaufort Sea. In the lab, photosynthetic parameters of individuals grown under different conditions of light and DIN over a four-month period will be compared to specifically address the role of these factors in regulating winter photosynthesis at low light levels. In addition, the photosynthetic response of L. saccharina, which is common in temperate regions but is relatively rare in the central Alaskan Beaufort Sea, will be compared to that of L. solidungula. Studies of photosynthesis in both species under natural conditions of light and nutrient limitation will enhance our understanding of photoadaptation and will greatly improve our ability to accurately model kelp production from measurements of in situ irradiance.
