Iron and Manganese Availability Drive Primary Production and Carbon Export in the Weddell Sea

Abstract

Since the 90s, the low phytoplankton biomass of the Southern Ocean (SO) has been mainly linked with the availability of iron (Fe). However, recent phytoplankton enrichment experiments have shown that manganese (Mn) also limits/co-limits SO phytoplankton biomass and species composition. Since taxonomic diversity affects cellular aggregation time and sinking rate, the strength and efficiency of the biological carbon pump are, therefore, directly affected by community structure. Yet, the impact of FeMn co-limitation on primary production, community composition, and subsequent export of carbon to the deep waters of the SO has never been investigated. Here, an Fe and Mn addition experiment examined how changes in SO species composition and aggregation capabilities of a natural plankton assemblage in the Weddell Sea are linked. 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 SO, a region of critical importance for anthropogenic carbon dioxide drawdown.

Keywords: Southern Ocean - phytoplankton - diatoms - Phaeocystis - Fe-Mn - co-limitation - phytoplankton taxonomy - aggregation - Particulate organic carbon