Manganese biogeochemistry in the Southern Ocean

• Main Author: Latour, PMA
• Format: Thesis
• Language: English
• Published: 2022
• Subjects: biogeochemistry; manganese; iron; phytoplankton; co-limitation; Antarctic; Southern Ocean; Antarc*; Antarctica
• Online Access: https://eprints.utas.edu.au/47579/; https://eprints.utas.edu.au/47579/1/Latour_whole_thesis.pdf

Manganese (Mn) is a redox-active metal essential for most life on Earth. In photosynthetic microalgae (phytoplankton), Mn is used in the oxygen evolving complex of photosystem II, and in the superoxide dismutase enzyme to detoxify reactive oxygen species. Thus, phytoplankton have a strict Mn requirement for growth. In the Southern Ocean, the largest High-Nutrient Low-Chlorophyll (HNLC) region, phytoplankton growth is strongly limited by the micronutrient iron (Fe), but recent evidence shows Mn can (co-)limit phytoplankton growth in both coastal and open ocean regions. These results conflict with earlier studies that found Mn levels in Southern Ocean waters were sufficient to support phytoplankton growth. Hence, there is a need to constrain the distribution of Mn in Southern Ocean waters, to describe its sources and sinks and to identify potential regions of limitation. Here, we aim to tackle this problem by firstly describing Mn distribution along a meridional transect between Tasmania and Antarctica. Secondly, we study the zonal distribution of Mn near major Antarctic coastal sources with a focus on the northward supply of fertilised waters into HNLC waters. Finally, we use ship-based bioassays to test Mn limitation of Southern Ocean phytoplankton in subantarctic and polar waters. Manganese concentrations were measured in the Australian sector of the Southern Ocean, following the GEOTRACES-SR3 meridional transect, from Tasmania (Australia) to Antarctica. Manganese distribution was related to two external sources: sediment and hydrothermal inputs. We found both dissolved and particulate Mn concentrations were extremely low along this transect, despite strong inputs from Tasmanian and Antarctic shelf sediments, and hydrothermal vents. The presence of a cold-core eddy induced upward movement of Mn enriched waters. However, this enrichment did not reach surface waters where it could fertilize Mn depleted waters. At the southern end of the SR3 section, we studied the potential export of Mn-enriched shelf waters to ...