Photo synthesis/irradiance relationships in the Ross Sea, Antarctica, and their control by phytoplankton assemblage composition and environmental factors

The photosynthetic parameters of natural phytoplankton assemblages from the Ross Sea, Antarctica, as well as unialgal cultures of the diatom Pseudonitzschia sp. and the colonial haptophyte Phaeocystis antarctica were investigated to determine if differential responses to irradiance could explain the distribution of phytoplankton in the Ross Sea. Field assemblages had photosynthetic responses that suggested acclimation to low irradiance levels, and the initial rate of photosynthesis per unit chlorophyll (a) and the theoretical maximum rate of production (p(max)(B)) averaged 0,083 mg C (mg chl a)(-1) h(-1) (mumol m(-2) s(-1))(-1) and 2,40 mg C (mg chl a)(-1) h(-1), respectively. An increase in both alpha and P-max(B) were noted as the season progressed, However, no differences existed between the photosynthetic responses of phytoplankton assemblages dominated by diatoms and those dominated by R antarctica. A significant influence of irradiance (reflected in changes in a and the photoadaption index E-k) was observed in the field observations, and this effect was corroborated by laboratory experiments. The carotenoid accessory pigment 19'-hexanoyloxyfucoxanthin in R antarctica also varied with irradiance, but fucoxanthin did not. These results suggest that the spatially distinct distribution of P. antarctica and diatoms that is often observed in the Ross Sea probably does not result simply from different photosynthetic responses, but from a complex series of controls, potentially including trace metal effects, vertical mixing, and other factors.