Browsing by Author "Thomalla, Sandy"
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- ItemOpen AccessA bio-optical approach to phytoplankton community sturcture, physiology and primary production on the Weddell Gyre(2014) Smith, Ceinwen; Waldron, Howard; Thomalla, Sandy; Lucas, MikeThe Southern Ocean is an important “sink” for anthropogenic CO2, but it requires a detailed understanding of the sensitivity of the biological carbon pump to variability in physical forcing mechanisms in order to predict its continuing role. However, due to the remote and tempestuous nature of this region, in situ measurements of phytoplankton variability are scarce. Consequently, satellites, autonomous floats and gliders are increasingly being utilized as platforms for observing biogeochemical variability over broad spatial and temporal scales, through satellite ocean colour radiometry linked to inherent optical properties (IOPs) of the upper water column. In this study, the variability of in situ IOPs was investigated together with phytoplankton biomass, cell size, species composition and chlorophyll to carbon (Chl:C) ratios to isolate and understand the relationships between IOPs and biogeochemistry.
- ItemOpen AccessEstimates of Phytoplankton carbon from high resolution optical sensors in the Southern Ocean(2015) Ogunkoya, Ayodele Gilbert; Vichi, Marcello; Thomalla, SandyPhytoplankton is an important component of the oceanic carbon cycle, and deriving a good estimate of its carbon biomass (Cphyto) at ocean scale is difficult due to the lack of automatic sampling procedures. This is particularly difficult in the Southern Ocean, where winter conditions limit the sampling. This study explored the opportunity of using a high resolution data from the glider tracks in the Sub-Antarctic Zone of the Southern Ocean. The data consisted of particulate backscattering and chlorophyll and four different methods of estimating phytoplankton carbon were used, three of them based on backscattering (named 30%POC, B05 and M13) and one on chlorophyll (S09). The methods are different in their empirical formulations and source of original data. Three methods showed similar results despite the fact that one of them makes use of chlorophyll to derive Cphyto. Method M13 doubles that of the 3 other methods (~80mg C m-³ vs 40-50 mg C m-³). It was observed that discrepancy between M13 and the other 3 methods decreases with depth and when biomass was low (~0.25 mg Chl-a m-³) e.g., at depth 80 m. Investigating the drivers of variability in chl-a:C phyto ratios with depth and MLD shows little response and highlighted the need for more research in this region. Although M13 has a very low chl-a:Cphyto ratios, the range of variability was similar to that of the 30%POC and B05 methods and likely driven by variability in light and Fe limitation and changes in community structure. Despite a similar magnitude, the S09 method show a tight constrain in chl-a:Cphyto ratios that were methodologically driven and thus less sensitive to physiological adjustments in cellular chl-a:Cphyto ratios. The analysis also confirms that each oceanic region has factors that drive their variability and care needs to be taken when applying a method that was derived from one oceanic region to another.
- ItemOpen AccessInvestigating on-board FE Fertilization experiments using fast repetition rate Fluorometry in the Southern Ocean(2013) Preston-Whyte Fiona Kate; Lucas, Mike; Thomalla, Sandy
- ItemOpen AccessOptical characterisation of Southern Ocean phytoplankton(2023) Morrison, Frieda; Vichi, Marcello; Lisl, Robertson Lain; Thomalla, SandyRetrieving the optical properties of Southern Ocean (SO) phytoplankton with high confidence is critical to understanding the role of the Biological Carbon Pump (BCP). Satellite-based ocean colour remote sensing radiometry is the only observational capability that can provide synoptic views of upper ocean phytoplankton characteristics, at high spatial and temporal resolution of approximately 1 km globally, and a daily temporal resolution over a period of years to decades, as is required for climate studies. In many cases, these are the primary systematic observations available for chronically undersampled marine systems such as the SO. Inversion algorithms are applied to satellite radiometry with the goal of characterising the optical properties of the in-water constituents, primarily phytoplankton. If the relationship between Inherent Optical Properties (IOPs) and biophysical phytoplankton assemblage characteristics, in terms of abundance, cell size and pigment composition, is well understood, biogeochemical information can be inferred from satellite-derived phytoplankton IOPs. This approach has greatly augmented global comprehension of climate change and the carbon cycle. The SO is typified by unique phytoplankton optical properties, distinct from those elsewhere in the world. Most notably among these is displaying characteristically “depressed” phytoplankton absorption spectra. It is understood that there are two main drivers behind this: unusually large cell sizes, and elevated pigment density resulting from physiological changes in response to the often low light environment (photoacclimation). The primary aim of this study is to investigate the observed seasonal variability in measured in situ SO IOPs, in conjunction with the UCT-CSIR1 Equivalent Algal Populations (EAP) model of phytoplankton optical properties, to better understand the causal drivers of the optical differentiation of SO phytoplankton absorption. The EAP model is used to illustrate the biophysical source of the observed unique absorption characteristics, showing that the flattening of the spectra is driven primarily by photophysiological changes occurring during photoacclimation, namely increased density of intracellular chlorophyll a. The satellite-derived OC-CCI2 phytoplankton absorption product, observed to reproduce elevated spectra more typical of other oceans, is investigated to determine this. The model is then used to simulate biophysically consistent phytoplankton backscatter, in order to investigate whether photoacclimation, resulting in the distinctive absorption properties of the SO, has any impact on phytoplankton backscatter and the magnitude of its contribution to the bulk water-leaving signal. A reduction in phytoplankton backscatter signal was demonstrated by the model. Discussion was made in the context of opportunities for the detection of constituent IOP retrievals and biogeochemical parameters from satellites in space.
- ItemOpen AccessPhytoplankton community structure, productivity and nitrogen metabolism as a function of light availability in the Atlantic sector of the southern Ocean by Erika Anne Kean.(2012) Kean, Erika Anne; Lucas, Mike; Thomalla, SandyThis study covered six oceanic regions within the south Atlantic sector of the Southern Ocean during austral summer 2008/2009. Four transects from Cape Town to Antarctica, Antarctica to South Georgia Island and the return reciprocal legs surveyed the following regions, the Subtropical zone (STZ) north of the Subtropical Front (STF), the Northern Antarctic Circumpolar Current zone (N-ACC) from the STF to the Antarctic Polar Front (APF), the Antarctic zone (AAZ) from the APF to the Southern Boundary (SBdy) of the ACC, the Weddell Gyre zone south of the SBdy to 68°S, the Subantarctic Islands and Shallow Bathymetry zone (SAISB) and finally the Antarctic Continental Shelf zone (ACS), each providing a natural laboratory to test lightdependent uptake of both oxidised and reduced N species.
- ItemOpen AccessQuantifying spatial and temporal scales of phytoplankton variability in the Sub-Antarctic Ocean using a high-resolution glider dataset(2016) Little, Hazel Jean; Thomalla, Sandy; Swart, Sebastiaan; Vichi, MarcelloPhytoplankton in the Sub-Antarctic Southern Ocean have a distinct seasonal cycle, which is highly variable in both space and time. The seasonal and spatial distribution of chlorophyll can be attributed to the complex nature of the physical and biogeochemical factors controlling phytoplankton production. Studies show that high-resolution sampling is required to understand variability in phytoplankton distribution and primary production. In this study, high-resolution glider data sampled in the Atlantic Sub-Antarctic Zone are used to characterise the scales of phytoplankton variability. Continuous glider data provide a novel way to assess phytoplankton variability at small time and space scales (meso- to submesoscale), especially in an area that has a lack of continuous measurements, which are necessary for addressing climate related questions. Temporal variability of phytoplankton was investigated using Empirical Mode Decomposition of surface chlorophyll-a concentrations collected from a Seaglider over a period of 5.5 months (25 September 2012 to 15 February 2013). This study found that during spring, chlorophyll-a concentrations were dominated by small scale daily fluctuations as well as by the rising seasonal ramp due to seasonal stratification. The removal of these signals revealed that the chlorophylla variability was dominated by submesoscales. In spring, phytoplankton blooms occurred as a result of features that shoaled the mixed layer depth when the wind stress weakened, elevating light conditions for short periods and allowing increased growth. In summer phytoplankton blooms were found to occur at submesoscales periods as well. This variability was found to be driven by synoptic storms varying the strength of the wind stress and consequently the mixed layer depth (that alters the nutrient and light environment). Additionally, through reconstructing the time series through subsampling at the dominant signals, this study found that in order to accurately resolve and characterise the multiseasonal variability of phytoplankton, chlorophyll needs to be sampled at high frequencies (<10 days). Spatial variability was investigated using daily MODIS ocean colour and sea surface temperature images coincident with the glider track. Spatial variability was characterised by the variance calculated at different length scales. Spatial analysis found that phytoplankton were patchier in both spring and summer when compared to sea surface temperature, at all length scales and that a greater variance was contained at small scales. There was also a greater variance in summer chlorophyll-a compared to spring due to higher maximum biomass. Further spatial analysis compared satellite spatial variance with glider measurements at the same length scale (70 km). This study found that a third of the variability found by the glider was caused by spatial patchiness, while the remainder could be contributed by local growth. These dominant meso- and submesoscale changes in chlorophyll-a at both temporal and spatial scales, highlights the need to resolve for both meso- to submesoscales in order to accurately reflect phytoplankton seasonal variability and ultimately to understand the impact of phytoplankton variability on carbon flux.
- ItemOpen AccessSeasonal characteristics of phytoplankton bloom phenology in the northern Benguela Upwelling System(2019) Matlakala, Mmakabele Lebogang; Thomalla, Sandy; Smith, Marie; Vichi, Marcello; Louw, DeonSeasonal phytoplankton blooms in the Benguela Upwelling System (BUS) play a crucial role in ecosystem services and driving climate change through air-sea gas exchanges. Upwelling systems are particularly are sensitive to effects of climate change including the transport of nutrients, which influence the composition of phytoplankton communities. This is important because species composition affects a number of key processes that have significant climate feedbacks. This study uses historical long-term in situ data (at 10 and 70 NM stations) as well as OC-CCI satellite ocean colour data to investigate seasonal phytoplankton bloom phenology and community structure of diatoms, dinoflagellates and coccolithophores in the northern BUS. The seasonal cycle of satellite chlorophyll was used to determine the timing of bloom initiation at inshore and offshore boxes that overlapped the in situ stations. An ocean colour algorithm to detect coccolithophore presence and absence was used to determine the inshore-offshore seasonal cycle of coccolithophores. Results indicate a gradual decrease in chlorophyll concentration further offshore as well as high intra-seasonal, inter-annual and spatial variability. Offshore blooms initiate later and last longer than inshore blooms which have a higher magnitude. Diatoms are dominant over dinoflagellates and coccolithophores throughout the northern BUS, with higher concentrations observed at the inshore station (10 NM) for all three phytoplankton groups. However, satellite results show a higher presence of coccolithophores in the offshore region during spring and summer that is associated with periods of strong stratification. This study provides a better understanding of the characteristics of the phytoplankton seasonal cycle in the northern BUS which is useful for detecting trends and possible change associated with climate change forcing in response to global warming.
- ItemOpen AccessWintertime nitrate isotope dynamics in the Atlantic sector of the Southern Ocean(2014) Smart, Sandi; Sigman, Daniel; Fawcett, Sarah; Thomalla, Sandy; Reason, ChrisWe provide the first data on wintertime patterns of the nitrogen (N) and oxygen (O) isotopes of seawater nitrate for the region south of Africa. Water column profile and underway surface samples collected in July 2012 span a range of latitudes from the subtropics to 57.8°S, just beyond the Antarctic winter sea-ice edge (56.7°S). The data are used in the context of simple models of nitrate consumption (including the Rayleigh model) to estimate the isotope effect (the degree of isotope discrimination) associated with the assimilation of nitrate by phytoplankton. We focus on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile N isotope data yields considerably lower isotope effect estimates (1.6-3.3‰) than commonly observed in the summertime Antarctic