Effect of iron and manganese supply on a natural phytoplankton community of the Weddell Sea and further consequence for carbon export

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.