Limnol. Oceanogr., 44(2), 1999, 247–258

Primary production models and pigment algorithms for remote optical systems including satellites, moorings, or drifters depend on an improved understanding of the relationship between spectral light absorption, pigments, and photosynthesis for species of phytoplankton that are widespread and numerically abundant. Cultures of colonial Phaeocystis antarctica, a prymnesiophyte that can dominate the phytoplankton community in the Southern Ocean, were grown under blue light at seven different levels ranging from 14 to 542 mmol quanta m22 s21 at 48C under nutrient-replete conditions. Chlorophyll-specific absorption ( ) at 436 nm increased linearly from 0.03 to 0.11 m2 a* ph mg chlorophyll a21 with increasing light intensities. This variability is attributed to pigment packaging effects and pigmentation. The 2.5-fold range in the values of (676 nm) demonstrates significant pigment packaging effects a* ph due to the intracellular pigment content, single cell diameter within the colony, and thylakoid stacking. Quantum yield for growth (fm) varied 30-fold, ranging from 0.003 to 0.09 mol carbon fixed (mol quanta absorbed)21. Under low light conditions, the relatively high fm and high (l) may enable Phaeocystis to accumulate a seed population a* ph to initiate blooms at the beginning of spring when light levels are low, mixed layers are deep, and sea ice is still significant. These aspects of its photophysiology may contribute to the ecological success of Phaeocystis in polar regions. The genus Phaeocystis is a numerically and functionally important component in polar biogeochemical cycles. Mesoscale blooms of Phaeocystis appear to be frequent and widespread in polar and subpolar seas (Weisse et al. 1994; Stoecker et al. 1996), and single cells can be numerically important during austral winter (Moisan unpubl. data). Phaeocystis blooms in coastal and ice edge regions are at times substantial enough to deplete macronutrients in surface waters (Smith et al. 1991; van Boeckel et al. 1992). In addition to its ecological importance, dimethylsulfide production by Phaeocystis may be a significant contributor to the sulfur cycle (Liss et al. 1994). Dimethylsulfide can be rapidly oxidized to sulfate aerosols in the atmosphere, which can serve as cloud condensation nuclei that may change cloud albedo (Charlson et al. 1987). Phaeocystis exhibits a heteromorphic life history, which alternates between free-living flagellated zoospores and a gelatinous aggregation of nonmotile palmelloid cells arranged in a spherical colony (Rousseau et al. 1994). Alternating life cycle phases between 3-mm single cells and millimeter-size colonies allow Phaeocystis to serve as a food source to a wide size range of predators (Rousseau et al. 1994; Weisse et al. 1994). A significant portion of the primary production may be directed toward bacteria at the end of a bloom by release of dissolved organic carbon through excretion and