Ecological investigations of giant Phaeocystis colonies in Viet Nam: 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 $$ {P}_{\mathrm{max}}^B $$ ) 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 $$ {P}_{\mathrm{max}}^B $$ values may be related to the high cellular chl a of colonial cells and their acclimation to in situ irradiance. Nitrate V max $$ {V}_{\mathrm{max}} $$ and K S $$ {K}_S $$ 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.