PHOTOPHYSIOLOGICAL EVIDENCE OF NUTRIENT LIMITATION OF PLATELET ICE ALGAE IN MCMURDO SOUND, ANTARCTICA

Seasonally changing photophysiological and biochemical characteristics of sea ice microalgae are interpreted with respect to light availability and measurements of nutrient concentration made at high vertical resolution (12.5 cm) during a dense bloom in the platelet ice layer of McMurdo Sound during a 6-week study in austral spring of 1989. Platelet ice algae remained highly shade adapted throughout the spring as shown by their low photoadaptive index (E-k, 3.7-8.4 mu mol photons.m(-2).s(-1)), low mean specific absorption coefficient (<0.009 m(2) mg(-1) Chl a), high optical cross-sectional area of photosystem II (sigma(PSII) 3.0-8.2), and high molar ratio of fucoxanthin:chlorophyll a (mean 1.62 +/- 0.25 SD). Between 24 October and 8 November, the algae exhibited a photoacclimative response that was marked by a 30% decrease in photosynthetic efficiency (alpha(B)), a 75% decrease in maximum photosynthetic rate (P-m(B)), and a 60% increase in sigma(PSII). The photochemical conversion efficiency at photosystem II (F-v/F-m = ca. 0.5) and the quantum yield of photosynthesis (empty set(C) = 0.062-0.078 mol C mol(-1) photons) were ca. 80% of their maximal values. After 8 November, changes in algal photophysiology and biochemistry, which were inconsistent with a photoacclimation response, suggest that tice platelet ice alg-ae near the platelet/congelation ice interface became increasingly nutrient limited. The number of pennate diatoms increased threefold to 150 X 10(9) cells m(-3) between 8 and 14 November, then remained unchanged throughout the remainder of the field season. Following the increase in cell number, F-v/F-m, empty set(C), and C:Chla decreased by >40%, sigma(PSII) increased by 70%; and the biochemical ratios C:N and C:Si increased 25%-30%. Nutrient depletion was apparent from the high-resolution vertical profiles, but nutrient concentrations limiting algal growth were not observed. However, nutrient concentrations at the likely site of nutrient limitation near the platelet/congelation ice interface were not measured, indicating that higher resolution sampling is necessary to fully characterize this highly variable habitat.