Light Injury and Inhibition in Antarctic Freshwater Phytoplankton1

Inhibition and injury of the photosynthetic mechanism resulting from high light intensities in the 24‐hr Antarctic summer daylight were studied using C 14 in two small lakes on Cape Evans, Ross Island. The diel rates of carbon fixation were completely out of phase with light intensity, the effect being more pronounced at the surface than at depth. This inhibition could be quantitatively reduced by exposing the plankton to incident light passing a graded series of neutral density filters. A maximum rate was found at 20% of incident photosynthetic light at noon (0.10 langley/min), while photosynthetic efficiency in another experiment had decreased down to 0.06 langley/min close to the previous midnight. The effect of temperature increase upon the photosynthetic uptake in highly inhibited plankton was investigated, and a Q 10 of about 7 determined. Benthic and noninhibited phytoplankton both had an average Q 10 of about 2. A brief sunlight exposure of plankton kept in dim artificial light brought about a reversible depression of photosynthetic rate. The ecological importance of these results to phytoplankton production under the severe Antarctic conditions is discussed.