Interactions between ice shelves and phytoplankton blooms in the Amundsen Sea, and their relevance to the Southern Ocean

Abstract

The coastal Southern Ocean is both highly sensitive to climate change and disproportionately important as a regulator of global carbon and nutrient fluxes. In particular, the polynyas which occur at many locations along the coastline host annual phytoplankton blooms which act as sinks of nutrients and carbon, whilst fueling the growth of higher trophic levels. Spring phytoplankton blooms in the Amundsen Sea are amongst the most intensely productive in the Southern Ocean and occur near some of the fastest melting ice shelves around Antarctica. In recent years both observations and modelling have been used to investigate the possible role of ice shelf melting in fuelling phytoplankton growth, with ice shelves implicated as sources of the dissolved iron whose scarcity would otherwise severely limit primary production. However there remains debate in the literature as to the importance of this iron limitation in comparison to light limitation, and as to the precise mechanism by which ice shelf melting enhances local iron concentrations. Meanwhile, expanded observations and advances in data assimilation have revealed pronounced zonal variation in both physical and biogeochemical aspects of oceanography near the Antarctic margin.

In this thesis, idealised and realistic models of the Amundsen Sea are combined to examine the role that ice shelves play in driving Net Primary Production (NPP) in ice-free polynyas on Antarctic continental shelves, and how this role is affected by climate-driven processes. Addtionally, the impact of phytoplankton blooms themselves on the physical components of the Antarctic margin – particularly ice shelf melting and sea ice cover – are investigated. The modelling is undertaken using the MIT General Circulation Model (MITgcm) to represent ocean physics and the Biology Light Iron Nutrients and Gases (BLING) to represent biogeochemistry. This is the same combination used for the Biological Southern Ocean State Estimate (BSOSE), here modified and repurposed to study coastal processes with an emphasis on how these processes impact the wider Southern Ocean.

In the first results chapter of this thesis, modelling using an idealised domain covering the Amundsen Sea Polynya (ASP) reveals transitions between light and iron limitation, with iron limitation shown to be more important a) later in the season, b) higher in the water column and c) further from the ice shelf, as compared to light limitation. Thus the categorization of the ASP as either iron or light limited is shown to be overly simplistic. Meanwhile, a significant driver of light limitation is shown to be the self-shading feedback by which surface phytoplankton reduce the light available to deeper phytoplankton. This result emphasizes the importance of including the self-shading feedback in BSOSE and in other ocean state estimates. In addition, iii the use of a simple one-dimensional Lagrangian particle-tracking model demonstrates the importance of mixing and photo-adaptation timescales in modifying the light limitation of phytoplankton blooms such as that in the ASP, motivating further studies of how different phytoplankton species adapt to fluctuating light environments.

In the second results chapter, a different set of experiments on the same model setup is used to confirm that ice-shelf melting leads to greater upper ocean iron concentrations, both directly due to release of glacial iron, and indirectly via a buoyancy-driven overturning circulation which pulls iron from Circumpolar Deep Water to the surface. Both of these mechanisms drive increased NPP in front of Dotson Ice Shelf, and sensitivity experiments reveal further complexity in the coupled physical-biological system. Varying the level of shortwave radiation incident on the ocean leads to a moderate, linear response in NPP. However, varying the level of the thermocline gives a non-monotonic and counter-intuitive response. Despite increasing the amount of iron in the upper ocean, a warmer ocean does not necessarily lead to an increase in NPP, since the increased melt rate modifies coastal currents, potentially steering the iron away from where it is most limiting to phytoplankton growth.

In the third results chapter, the same basic setup of MITgcm-BLING is employed in a larger, more realistic domain covering the entire Amundsen Sea sector. Notably, the phytoplankton bloom in Pine Island Polynya is shown to modify basal melting underneath the adjacent Pine Island Glacier Ice Shelf. This surprising result follows from the increased shortwave attenuation arising from high chlorophyll concentrations in the euphotic zone, which warms the sea surface whilst cooling the sub-surface. Model outputs also show that high NPP reverses the sign of the annual air-sea carbon flux on the Amundsen Sea continental shelf, with regional variability in the annual flux driven by meltwater distributions, phytoplankton blooms, and the coupling between the two.

The results presented in this thesis have wider implications for studies of the Southern Ocean, including due to their novel implementation of self-shading and biophysical feedbacks within an ice-ocean model. In the final part of the thesis, a list of scenarios summarising possible future changes in the Amundsen Sea are described, encompassing changes to ice shelves, sea ice and cloud cover, as well as to the timing, magnitude and species composition of the phytoplankton bloom. Each of these scenarios has potential impacts for processes at lower latitudes, and each motivates further study of ice-ocean-biological interactions in other coastal polynyas around Antarctica.