Self‐Shading and Meltwater Spreading Control the Transition From Light to Iron Limitation in an Antarctic Coastal Polynya

A G Twelves,D N Goldberg,D C Jones,S F Henley,M R Mazloff

doi:10.1029/2020jc016636

A G Twelves, D N Goldberg + Show 3 more

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https://doi.org/10.1029/2020jc016636

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Abstract

AbstractDotson Ice Shelf (DIS) in West Antarctica is undergoing rapid basal melting driven by intrusions of warm, saline Circumpolar Deep Water (CDW) onto the continental shelf. Meltwater from DIS is thought to influence biology in the adjacent Amundsen Sea Polynya (ASP), which exhibits the highest net primary productivity (NPP) per unit area of any coastal polynya in the Southern Ocean. However, the relative importance of iron and light in colimiting the spring phytoplankton bloom in the ASP remains poorly understood. In this modeling study we first investigate the mechanisms by which ice shelves impact NPP, then map spatio‐temporal patterns in iron‐light colimitation, and finally examine the environmental drivers of iron and light supply. We find that ice shelf melting leads to greater upper ocean iron concentrations, both directly due to release of iron from sediments entrained at the glacier bed, and indirectly via a buoyancy‐driven overturning circulation which pulls iron from CDW to the surface. Both of these mechanisms increase NPP compared to experiments where ice shelf melt is suppressed. We then show that the phytoplankton self‐shading feedback delays the bloom and reduces peak NPP by 80% compared to experiments where light penetration is independent of chlorophyll. Compared to light limitation, iron limitation due to phytoplankton uptake is more important a) later in the season, b) higher in the water column, and c) further from the ice shelf. Finally, sensitivity experiments show that variability in CDW intrusion influences NPP by controlling the horizontal spreading of iron‐rich meltwater.

Highlights

The Southern Ocean exhibits large air-sea carbon fluxes driven by a balance between physical and biogeochemical processes
We find that ice shelf melting leads to greater upper ocean iron concentrations, both directly due to release of iron from sediments entrained at the glacier bed, and indirectly via a buoyancy-driven overturning circulation which pulls iron from Circumpolar Deep water (CDW) to the surface
We show that the phytoplankton self-shading feedback delays the bloom and reduces peak net primary productivity (NPP) by 80% compared to experiments where light penetration is independent of chlorophyll

Summary

Introduction

The Southern Ocean exhibits large air-sea carbon fluxes driven by a balance between physical and biogeochemical processes. Regions where the uptake of carbon by photosynthesizing plankton exceeds physical CO2 outgassing act as carbon sinks, reducing the quantity of carbon dioxide in the atmosphere. On a spatially integrated basis the Southern Ocean acts as an important anthropogenic carbon sink, accounting for as much as 40% of the global transfer of anthropogenic CO2 from the atmosphere to oceans (Caldeira & Duffy, 2000; DeVries, 2014; Mikaloff Fletcher et al, 2006; Orr et al, 2001). A large proportion of this carbon uptake takes place in the highly productive coastal polynyas that fringe the Antarctic continent (Arrigo & Van Dijken, 2003; Sarmiento et al, 2004).

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