Photosynthesis-irradiance relationships of polar phytoplankton populations during winter conditions

The photosynthesis versus irradiance relationships of polar phytoplankton populations were studied in the Arctic and Antarctic oceans during winter conditions. Comparisons were made of photosynthesis-irradiance responses of populations from various depths within the water column to determine if the populations were the same or distinctly different. Measurements were made using an onboard incubator designed to allow rapid measurements of photosynthesis versus irradiance curves at sea, based on ¹⁴C-carbon assimilation. Average maximum assimilation numbers for Arctic populations during the winter-spring transition were found to be approximately 4.2 mg C mg Ch1^-1 h^-1 and photosynthetic efficiencies ranged from 0.01-0.05 mg C mg Ch1^-1 h^-1 [μEin ,^-2 sec^-1]^-1. The onset varied among populations ranging between 100-300 &,u;Ein m^-2 h^-1. Photosynthesis-irradiance curve parameters were characteristic of shade-adapted populations. Populations from surface waters appear to be different from those of populations from lower depths. Over the 14 day sampling period, primary productivity estimated from P-I responses and incident radiation averaged approximately 65 mg C m^-2 d^-1

Antarctic phytoplankton populations during austral winter were found to have photosynthesis-irradiance curves that seemed not to differ among populations from the surface, 50 meters, and 100 meters. Average maximum assimilation numbers were approximately 1.15 mg C mg Ch1^-1 h^-1 and photosynthetic effects ranged 0.01-0.06 mg C mg Ch1^-1 h^-1 [μEin ,^-2 sec^-1]^-1. Photoinhibition occurred at approximately 300 μEin ,^-2 sec^-1 for both surface populations and populations from 50 meters. Average primary productivity over the 19 day sampling period was estimated at 70.2 mg C m^-2 h^-1