Manganese co-limitation of phytoplankton growth and major nutrient drawdown in the Southern Ocean

Thomas J Browning,Edward Mawji,Anja Engel,Eric P Achterberg

doi:10.1038/s41467-021-21122-6

Thomas J Browning, Edward Mawji + Show 2 more

Open Access

https://doi.org/10.1038/s41467-021-21122-6

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Abstract

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. Variability in utilization alters oceanic CO2 sequestration at glacial-interglacial timescales. The role for insufficient iron has been examined in detail, but manganese also has an essential function in photosynthesis and dissolved concentrations in the Southern Ocean can be strongly depleted. However, clear evidence for or against manganese limitation in this system is lacking. Here we present results from ten experiments distributed across Drake Passage. We found manganese (co-)limited phytoplankton growth and macronutrient consumption in central Drake Passage, whilst iron limitation was widespread nearer the South American and Antarctic continental shelves. Spatial patterns were reconciled with the different rates and timescales for removal of each element from seawater. Our results suggest an important role for manganese in modelling Southern Ocean productivity and understanding major nutrient drawdown in glacial periods.

Highlights

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients
The Southern Ocean contrasts with much of the global ocean in a combination of four ways pertinent to the availability of surface dissolved Mn: (i) dust inputs are very low[9]; (ii) surface waters are fed by strong upwelling of low Mn waters from the oceans interior[4,10]; (iii) low surface light levels prevail for a large part of the year[11], which, potentially in combination with lower dissolved organic matter reducing agents[6,12], restrict Mn photoreduction[6,7]; and, (iv) concentrations of major nutrients are perennially high and do not limit biological micronutrient (e.g. Fe, Mn) drawdown[2]
We found that Mn was limiting to phytoplankton growth and major nutrient drawdown in central Drake Passage, whereas Fe was limiting nearer the South American and Antarctic continental shelves

Summary

Introduction

Residual macronutrients in the surface Southern Ocean result from restricted biological utilization, caused by low wintertime irradiance, cold temperatures, and insufficient micronutrients. We initiated simulations with subsurface DFe-DMn concentrations pairs from a published observational data set from around the Antarctic Peninsula[15,47], which reflect waters in close contact with shelf micronutrient sources (circles in Fig. 3a; Methods).

Results

Conclusion