Browsing by Subject "sea ice"

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    Antarctic sea ice phytoplankton growth rates and survival mechanisms
    (2025) Kumadiro, Lisa; Rampai, Tokoloho; Fawcett, Sarah; Fietz, Susanne
    Phytoplankton play an important role in the Southern Ocean food web being the primary producers of food, particularly in winter, and partaking in the uptake of CO2 from the atmosphere via photosynthesis. Despite being photosynthetic organisms, phytoplankton survive at the bottom of sea ice where there is very little irradiance for up to 6 months. Sea ice phytoplankton are understudied. This is mainly because in situ studies on sea ice are not only expensive but logistically difficult. Some researchers have elected to bring sea ice phytoplankton from the Southern Ocean to land-based facilities. This has seen some logistical difficulties as it meant either changing the habitat phytoplankton would have been for transportation, thus changing the species originally found in the Southern Ocean or transporting phytoplankton in ice cores and losing species due to brine drainage or osmotic stress from temperature changes in the core. The objectives of this study were to optimize a previously designed hybrid tank for the purpose of obtaining and preserving phytoplankton species from the Marginal Ice Zone of the Southern Ocean to land-based facilities. The study also included design of an environmental chamber to be used for housing phytoplankton obtained during experimentation. Responses to temperature and irradiance variation on phytoplankton from the Marginal Ice Zone of the Southern Ocean were then evaluated using the designed environmental chamber. The solid-liquid hybrid system known as the hybrid tank was successfully optimized by reducing the size of the tank, adding irradiation to the tank, and making improvements to the sampling protocol. The tank was used to obtain ice cores from the Southern Ocean to the University of Cape Town in winter 2022. Post the winter cruise one hybrid tank sample was melted, and microscopic analysis conducted on the sample. In comparison with transportation of phytoplankton in a solid core and in a liquid melt in the dark, the hybrid tank resulted in an increase in phytoplankton cell concentration. Furthermore, the optimized hybrid tank improved preservation of species transported when compared to the initial tank. A desktop environmental chamber made from Perspex and insulated with polystyrene was successfully designed. The environmental chamber offers temperature and irradiation control by making use of a cold plate attached to a chiller and an LED light. Experiments conducted on the diatom species revealed that all the sea ice species were shade adaptive being photo inhibited at irradiances beyond 42μmolm-2s-1 with the exception of Navicula spp, Cylindrotheca closterium and the unidentified pennates. The diatom species also preferred warmer environments i.e., 8°C to 5°C.
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    Ice - ocean - atmosphere interactions in the Southern Ocean and implications for phytoplankton phenology
    (2021) Hague, Mark; Vichi, Marcello
    The annual advance and retreat of sea ice in the Southern Ocean is recognised as one of the largest seasonal events on Earth. Such considerable physical changes have profound effects on the vertical structure of the water column, and hence controls the availability of both light and nutrients to phytoplankton. This means that in the region seasonally covered by sea ice (the SSIZ), the timing of the growth and decline (phenology) of phytoplankton is determined to a large degree by the dynamic interactions between ice, ocean and atmosphere. However, this region is simultaneously one of the most poorly observed in the global ocean, and one of the most complex. This has led to significant gaps in our understanding of how sea ice modulates the exchanges of heat and momentum between atmosphere and ocean, as well as the implications this has for phytoplankton phenology in the SSIZ. This study seeks to address these gaps by combining both model and observationallybased methods. The lack of observational data are directly tackled through an analysis of BGC-Argo float data sampling under ice. Such data reveal high growth rates in the presence of near full ice cover and deep mixed layers, conditions previously thought to prevent growth. These results suggest a revision of our current understanding of the drivers of under ice phytoplankton phenology, which should take into account the unique character of Antarctic sea ice and its effect on the under ice light environment. In addition, results obtained from several numerical process studies indicates that phytoplankton may have a higher affinity for low light conditions than previously thought. From a modelling perspective, an analysis and intercomparison of 11 Earth System Models (ESMs) and their representation of vertical mixing and phenology is presented. This revealed that misrepresentations in phenology where driven by model biases in sea ice cover and vertical mixing. That is, only models with either too much or too little ice cover were able to simulate phenology close to observations. Furthermore, a strong correlation between the location of the ice edge and the extent of vertical mixing suggested that ESMs overly dampen ocean-atmosphere fluxes as mediated by sea ice. This led to the development of a regional ocean-sea ice model of the Atlantic sector of the Southern Ocean, from which experiments enhancing both heat and momentum fluxes could be conducted. It was found that the model responded more uniformly to enhanced heat flux, generally deepening the mixed layer closer to observations in winter. On the other hand, the effects of enhanced momentum flux (implemented by increased air-ice drag) where more complex and spatially heterogeneous, with contrasting responses depending on the initial vertical density structure of the water column. Overall, the argument is made that the unique features of Antarctic sea ice should be included in models if we are to improve the representation of the SSIZ mixed layer, and hence phenology
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    Modelling of brine transport mechanisms in Antarctic sea ice
    (2021) Cook, Andrea; Skatulla, Sebastian; Machutchon, Keith
    It is evident that the sea ice cycle, from its formation to its melt, is governed by a complex interaction of the ocean, atmosphere and surrounding continents. Once sea water begins to freeze, physical, biological and chemical processes have implications on the evolution of the sea ice morphology [38]. The distinguishing factor between fresh and sea water ice is brine inclusions that get trapped within the ice pores during freezing. Salt inclusions within frozen ice influence the salinity as well as the physical properties of the sea ice [23]. These brine inclusions form part of a dynamic process within the ice characterized by the movement of brine and phase transition which are the foundation of many of its physical properties [23]. Brine removal subsequently begins to occur due to vertical gravity drainage into the underlying ocean water. This study introduces the application of a biphasic model based on the Theory of Porous Media (TPM) which considers a solid phase for the pore structure of the ice matrix as well as a liquid phase for the brine inclusions, respectively. This work explores the use of the TPM framework towards advancing the description and study of the various desalination mechanisms that are significant in aiding the salt flux into the Southern Ocean. This will foster understanding of brine rejection and how it is linked to the porous microstructure of Antarctic sea ice