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The role of iron and light in controlling photosynthate production and allocation in phytoplankton populations of the Atlantic sector of the Southern Ocean was investigated in April–May 1999. The  14 C incorporation into five biochemical pools (glucan, amino acids, proteins, lipids and polysaccharides) was measured during iron/light perturbation experiments. The diurnal Chl  a -specific rates of carbon incorporation into these pools did not change in response to iron addition, yet were decreased at 20 μmol photons m −2  s −1 , an irradiance comparable with the one at 20–45 m  in situ  depth. This suggests that the low phytoplankton biomass encountered (0.1–0.6 μg Chl  a  L −1 ) was mainly caused by light limitation in the deep wind mixed layer (&gt;40 m). Regional differences in Chl a-specific carbon incorporation rates were not found in spite of differences in phytoplankton species composition: at the Antarctic Polar Front, biomass was dominated by a diatom population of  Fragilariopsis kerguelensis , whereas smaller cells, including chrysophytes, were relatively more abundant in the Antarctic Circumpolar Current beyond the influence of frontal systems. Because mixing was often in excess of 100 m in the latter region, diatom cells may have been unable to fulfil their characteristically high Fe demand at low average light conditions, and thus became co-limited by both resources. Using a model that describes the  14 C incorporation, the consistency was shown between the dynamics in the glucan pool in the field experiments and in laboratory experiments with an Antarctic diatom,  Chaetoceros brevis . The glucan respiration rate was almost twice as high during the dark phase as during the light phase, which is consistent with the role of glucan as a reserve supplying energy and carbon skeletons for continued protein synthesis during the night.

Light rather than iron controls photosynthate production and allocation in Southern Ocean phytoplankton populations during austral autumn