Light-Dependence of Phytoplankton Photosynthesis in the Antarctic Ocean: Implications for Regulating Productivity

Summary

The relationships of vertical profiles of phytoplankton photosynthesis to under-water light were studied at 23 stations in the South Scotia Sea and Bransfield Strait. On 3 occasions diurnal photosynthetic patterns were monitored. Chlorophyll-a concentrations varied by a factor of 16.5. Eighty per cent of the variations in light extinction could be explained by variations in chl-a concentration. Accordingly, the euphotic zone (1% surface light level) varied from 15 to 70 m. Photosynthetic profiles were studied in order to assess the production potential of Antarctic phytoplankton. The photosynthetic capacity (photosynthesis per chl-a at optimum light) and maximum quantum yield of photosynthesis (moles carbon dioxide assimilated per mole light quanta absorbed) on the average were smaller by a factor of 7 and 4, respectively, than in phytoplankton at lower latitudes. Diminished low-light photosynthesis suggests that in Antarctic waters temperature-controlled processes take over as rate-limiting steps in otherwise light-limited situations. The utilization efficiency of incident irradiance by phytoplankton at any level of chlorophyll concentration is diminished by both reductions in the photosynthetic capacity and lower light-limited quantum yields. By the combination of both effects, phytoplankton in Antarctic waters can utilize incident light only inefficiently even in situations where biomass accumulation is high.