Description: This study highlights the importance of manganese (Mn) next to iron (Fe) for the growth of specific Southern Ocean phytoplankton groups with important implications for carbon export in the Weddell Sea. A Fe-Mn bottle amendment experiment and an aggregation experiment were performed with a natural phytoplankton community during Polarstern expedition PS124 in 2021 in the Weddell Sea. At this location, seawater was pumped (using trace metals clean techniques) from 25 m depth to fill polycarbonate bottles. The Control treatment consist of the sampled seawater without any trace metals addition, while the other three treatments were enriched with either FeCl₃ alone (0.5 nM; +Fe treatment) or MnCl₂ alone (1 nM; +Mn treatment) or both trace metals together (+FeMn treatment). All treatments were done in triplicate 2.5 L PC bottles. After on average 7 days, samples for chlorophyll a content, particulate organic carbon and primary production were taken to detect FeMn co-limitation effect on species composition, primary production and carbon export. Results showed that when Fe and Mn were added together, primary production rates were highest due to an increased abundance of the prymnesiophyte Phaeocystis antarctica. This change within the phytoplankton community led to highly carbon-enriched aggregates and a four-fold increase in the carbon export potential compared to the doubling in the only Fe treatment. Our findings highlight that even small changes in plankton community composition can have significant effects on the carbon export potential of the Southern Ocean, a region of critical importance for anthropogenic carbon dioxide drawdown.

Description: Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was the sampled seawater without any trace metals addition while the other three treatments were enriched with either FeCl3 alone (0.5 nM; +Fe treatment) or MnCl2 alone (1 nM; +Mn treatment) or both trace metals together (+FeMn treatment). All treatments were done in triplicate 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C. To determine the effects of the different treatments on the microplankton composition for the two Fe-Mn enrichment experiments, unfiltered seawater was collected at the start and the end of both experiments for later analysis via light microscopy in the home laboratory. Briefly, samples were fixed with hexamine-buffered formalin solution (2% final concentration) and Lugol's solution (1% final concentration) and stored at 2 °C in the dark until taxonomic analysis. All samples were allowed to settle in Utermöhl sedimentation chambers for at least 24 hours and were analyzed on an inverted light microscope, according to the method of Utermöhl (1958). Species were counted and identified according to taxonomic literature. Each aliquot was examined until at least 400 cells had been counted.