Ecological investigations of giant Phaeocystis colonies in Viet Nam: II. Photosynthesis—irradiance characteristics and nitrogen uptake

Phaeocystis globosa is a marine phytoplankton species that forms deleterious blooms in temperate and tropical waters. In some locations, “giant” colonies form, although the controls on its size are unknown. During a “giant” colony bloom, measurements were completed to characterize photosynthesis–irradiance relationships, nitrogen uptake kinetics, and nitrogen–irradiance relationships of P. globosa colonies to understand its growth characteristics and their relationship to colony size. The photosynthetic capacity (F_v/F_m) varied from 0.65 to 0.68 among colony sizes ranging from 3.0 to 11.0 mm, indicating that all colonial cells were physiologically robust. The maximum chl a-specific photosynthetic rates ($P_{\max }^B$) ranged from 0.89 to 1.92 μg C · μg⁻¹ chl · h⁻¹, were maximal in the mid-sized colonies (5.5–6.5 mm) and decreased with size. The relatively low $P_{\max }^B$ values may be related to the high cellular chl a of colonial cells and their acclimation to in situ irradiance. Nitrate $V_{\max }$ and $K_S$ values were greater than those of ammonium, although N affinity was greater for ammonium. No differences in light-limited rates in either nitrate or ammonium uptake among colony sizes were observed, and no dark uptake occurred. Both ammonium and nitrate uptake showed a saturation response as a function of irradiance. While the driving forces for the formation of giant colonies remain unknown, their impacts on coastal systems are substantial and a further assessment of their growth is warranted.