Phytoplankton dynamics during an in situ iron enrichment experiment (EisenEx) in the Southern Ocean: a comparative study of field and bottle incubation measurements

Abstract

Composition, physiology and growth response of 5 size classes of phytoplankton, rang- ing from 0.7 to 20 µm, was studied in an in situ iron enrichment experiment (EisenEx) and in bottle incubations in the Southern Ocean during austral spring 2000. In the field, iron enrichment resulted in only minor changes in numerical abundance, cell carbon content, photosynthetic efficiency (Fv:Fm) and the percentage of live cells of Synechococcus spp. and pico-eukaryotes (<2 µm). In these 2 groups, only cellular chlorophyll a (chl a) content increased (by 20 and 100%, respectively). The physiological conditions of the 2 to 20 µm cells improved significantly, but a statistically significant (3- fold higher) biomass was observed only in the largest size fraction (8 to 20 µm). Bottle experiment results were comparable with in situ results, except that the responses occurred earlier. In addition, the total increase in biomass was much larger than that in the field (100- and 25-fold increase for cell carbon and chl a content, respectively). In this respect the control and the iron-enriched bottles showed a comparable trend, suggesting that, next to iron, light was probably a factor of great impor- tance. In the field, phytoplankton cells experience rapidly changing light conditions due to wind- induced turbulence that causes mixing of the water column. In addition to iron-limitation, this results in a generally poor physiological condition of algae in the field. This light-stressor mainly affected the smallest algal size classes (<8 µm). In larger cells, iron enrichment partly compensated for this nega- tive light effect, resulting in a final dominance of larger phytoplankton.