EFFECTS OF NITROGEN SUPPLY AND CONTINUOUS DARKNESS ON GROWTH AND PHOTOSYNTHESIS OF THE ARCTIC KELP LAMINARIA-SOLIDUNGULA

Sporophytes of the arctic kelp Laminaria solidungula were collected fr om the Beaufort Sea and kept for 7 months at 0 +/- 1.5 degrees C in a factorial experiment under two light (continuous darkness or 25 mu mol photons m(-2) s(-1) on a 12: 12 L/D cycle) and two nutrient regimes ( with or without added nitrate). We monitored growth rate and at the en d of the experiment measured the following on both new and old blades: carbon and nitrogen content, chlorophyll and carotenoids, photosynthe tic light-response (P vs. I curves, and photosynthetic unit size (Chl/ P-700). New blade initiation was independent of light and external nit rate, but blades grew minimally in darkness. Both new (B1) and old (B2 ) blades of plants held in darkness (with or without added nitrate) ha d the same quantum yield (phi(m)) and area-normalized photosynthetic c apacity (P-m), but a slightly lower respiration rate (R-d) and compens ation irradiance (I-c less than or equal to 1.5 mu mol photons m(-2) s (-1)) as B2 of plants grown in the light with added nitrate. Nitrogen starvation in the light, but not in darkness, caused reduced pigments, P-m, and phi(m) and increased I-c. Tissue C and N were primarily affe cted by light, whereas pigments were more strongly affected by N and l ight x N interaction. Based on these results, previous results for hel d-collected plants, and continuous in situ light data, we conclude tha t total annual growth in this population is severely light-limited, wi th N having an important but secondary role.