Changing Biogeochemistry of the Southern Ocean and Its Ecosystem Implications

Sian F Henley,Andrew R Bowie,Raquel Flynn,Robert M Sherrell,Rodrigo Kerr,Sarah E Fawcett,Thiago Monteiro,Irene R Schloss,Emma L Cavan,Tanya Marshall,Shantelle Smith,David K A Barnes,Philip W Boyd

doi:10.3389/fmars.2020.00581

Abstract

The Southern Ocean plays a critical role in regulating global climate as a major sink for atmospheric carbon dioxide (CO2), and in global ocean biogeochemistry by supplying nutrients to the global thermocline, thereby influencing global primary production and carbon export. Biogeochemical processes within the Southern Ocean regulate regional primary production and biological carbon uptake, primarily through iron supply, and support ecosystem functioning over a range of spatial and temporal scales. Here we assimilate existing knowledge and present new data to examine the biogeochemical cycles of iron, carbon and major nutrients, their key drivers and their responses to, and roles in, contemporary climate and environmental change. Projected increases in iron supply, coupled with increases in light availability to phytoplankton through increased near-surface stratification and longer ice-free periods, are very likely to increase primary production and carbon export around Antarctica. Biological carbon uptake is likely to increase for the Southern Ocean as a whole, whilst there is greater uncertainty around projections of primary production in the Sub-Antarctic and basin-wide changes in phytoplankton species composition, as well as their biogeochemical consequences. Phytoplankton, zooplankton, higher trophic level organisms and microbial communities are strongly influenced by Southern Ocean biogeochemistry, in particular through nutrient supply and ocean acidification. In turn, these organisms exert important controls on biogeochemistry through carbon storage and export, nutrient recycling and redistribution, and benthic-pelagic coupling. The key processes described in this paper are summarised in the graphical abstract. Climate-mediated changes in Southern Ocean biogeochemistry over the coming decades are very likely to impact primary production, sea-air CO2 exchange and ecosystem functioning within and beyond this vast and critically important ocean region.

Highlights

Biogeochemistry refers to the cycling of chemical elements through living systems and their environments by physical, chemical, biological and geological processes, and is a fundamental component of the functioning of Planet Earth
Biological carbon uptake is likely to increase for the Southern Ocean as a whole, whilst there is greater uncertainty around projections of primary production in the Sub-Antarctic and basin-wide changes in phytoplankton species composition, as well as their biogeochemical consequences
This assessment of Southern Ocean biogeochemistry has assimilated existing knowledge and presented new data that show the integral role of biogeochemical cycling – of iron, carbon and major nutrients in particular – in supporting marine ecosystem functioning and regulating sea-air CO2 fluxes at regional and global scales

Summary

INTRODUCTION

Biogeochemistry refers to the cycling of chemical elements through living systems and their environments by physical, chemical, biological and geological processes, and is a fundamental component of the functioning of Planet Earth. Suggest that phytoplankton communities in the seasonal ice zone may become less diatom-dominated as climate change proceeds (Montes-Hugo et al, 2008, 2009; Mendes et al, 2012, 2018; Hernando et al, 2015; Schofield et al, 2017) It is well established from a range of in situ and laboratory studies that increased iron supply and ocean temperature have a beneficial effect on diatoms and other bloom-forming species, such that the projected increases in NPP may lead to species shifts to diatoms, and in some regions P. antarctica (Gall et al, 2001; Hutchins and Boyd, 2016; Zhu et al, 2016; Boyd, 2019). The important processes of deep winter mixing and vertical diffusion occur throughout the Southern Ocean, but are not depicted

SUMMARY AND CONCLUSIONS

Findings

DATA AVAILABILITY STATEMENT