Browsing by Author "Lucas, Mike"

Now showing 1 - 9 of 9
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    Acid-base regulation in the sea urchin Parechinus angulosus during CO₂-induced seawater acidification
    (2013) Dyer, Alexei; Lucas, Mike
    Ocean acidification is predicted to have adverse effects on the physiologies of marine organisms, particularly those that produce calcified structures. Extracellular homeostasis is considered to be critical to mediating the effects of ocean acidification. Due to their low metabolic rates and weak ability to regulate ion exchange, sea urchins are thought to be particularly weak acid-base regulators. Recent findings showing species-specific capacities for extracellular pH regulation however suggest that species currently exposed to natural CO₂ elevations, such as upwelling events, may have a higher capacity tolerate elevated CO₂. The sea urchin Parechinus angulosus currently experiences natural CO₂ variations within the Benguela upwelling system and is therefore predicted to possess the capacity to compensate moderate acid-base disturbances. Urchins were submitted to control (8.0), intermediate (7.7) and low (7.4) seawater pH treatments for 14 days to investigate the capacity to regulate extracellular acid-base status. Extracellular pH changes induced by exposure to intermediate (pH 7.7) seawater acidification were fully compensated through the accumulation of approximately 2.0 mmol l-1 of bicarbonate. The bicarbonate accumulation was only sufficient to partially compensate extracellular acid-base status during exposure to low (7.4) seawater pH. Results from acute (24 hour) exposure to low (7.4) seawater pH reveal that bicarbonate accumulation, despite being evident within 24 hours, is not sufficient to compensate extracellular pH. This study provides further support that sea urchins exposed to natural CO₂ variability possess a limited capacity to regulate extracellular acid-base disturbances. P.angulosus may therefore already be adapted to deal with a moderate reduction in seawater pH to 7.7, but lacks the iono-regulatory capacity to accumulate sufficient bicarbonate to deal with a reduction of seawater pH to 7.3. Long-term studies are needed to assess the role of acid-base regulation as a mediator of broader physiological tolerance to ocean acidification, and its consequences at the level of the whole organism.
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    A bio-optical approach to phytoplankton community sturcture, physiology and primary production on the Weddell Gyre
    (2014) Smith, Ceinwen; Waldron, Howard; Thomalla, Sandy; Lucas, Mike
    The 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.
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    Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
    (CrossMark, 2017-06) Ansorge, Isabelle J; Skelton, Paul; Bekker, Annie; de Bruyn, P J Nico; Butterworth, Doug S; Cilliers, Pierre; Cooper, John; Cowan, Don A; Dorrington, Rosemary; Fawcett, Sarah; Fietz, Susanne; Findlay, Ken P; Froneman, P William; Grantham, Geoff H; Greve, Michelle; Hedding, David; Hofmeyr, G J Greg; Kosch, Michael; le Roux, Peter; Lucas, Mike; MacHutcho, Keith; Meiklejohn, Ian; Nel, Werner; Pistorius, Pierre; Ryan, Peter; Stander, Johan; Swart, Sebastiaan; Treasure, Anne; Vichi, Marcello; Jansen van Vuuren, Bettine
    Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.
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    Mineralised phytoplankton community composition in the Scotia and Weddell seas (Southern Ocean), with emphasis on diatoms and coccolithophores Amy Harington.
    (2012) Harington, Amy; Lucas, Mike; Poulton, Alex
    Phytoplankton community composition in the Southern Ocean (SO) determines levels of primary production, which support marine ecosystems and export of material to the deep sea. Nanoplankton (cell diameters 2-20 μm) are poorly resolved by traditional microscopy, and it is becoming apparent that unknown diversity and ecosystem functionality may be contained in this size class: for example, small diatoms (< 10 μm) appear widespread in the SO, and may limit our understanding of the response of phytoplankton communities to climate change.
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    Particulate organic carbon and mineral export from the North and South Atlantic gyres : the 234Th/238U disequilibrium approach.
    (2007) Thomalla, S J; Lucas, Mike; Turnewitsch, Robert
    Subtropical ocean gyres are typically characterised by low carbon export into the deep ocean. However, due to their large area, even relatively small average carbon export may be globally significant. Strong correlations observed between deep-sea organic carbon, calcite and opal suggest that mineral phases may enhance the export and survival of organic matter as it sinks (the 'ballast effect'). However, the processes underlying these correlations are not well understood and remain key uncertainties in models that predict global carbon cycling. To better constrain carbon and mineral export from the surface ocean of subtropical gyres, radioactive disequilibria between 234Th and 238U were used to estimate fluxes of particulate organic carbon (POe), calcite and opal in the North and South Atlantic subtropical gyres.
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    Phytoplankton community structure determined through remote sensing and in situ optical measurements
    (2014) Evers-King, Hayley Louise; Bernard, Stewart; Henson, Stephanie; Shillington, Frank; Lucas, Mike
    Linking variability in optical signals with phytoplankton community characteristics is important to extend the use of the vast resource that is the satellite ocean colour archive. Detection of species, functional types or size classes has been addressed through a spectrum of empirical to analytical approaches. A key step in developing these techniques is quantifying the sensitivity in reflectance, which can be attributed to phytoplankton characteristics (e.g cell size) under different optical regimes. Ultimately, highly spatially and temporally resolved information on phytoplankton characteristics can help the global scientific community to answer important questions relating to primary ecosystem variability. In the southern Benguela, Harmful Algal Blooms threaten public health and the economic viability of fishery and aquaculture industries in the region. Concurrently, the dominance of phytoplankton biomass amongst optically significant constituents in the southern Benguela makes the region ideal for assessing the extent to which phytoplankton characteristics beyond biomass can influence the ocean colour signal. A forward and inverse approach is presented. Phytoplankton absorption and back scattering are generated from a phytoplankton particle population model coupled to two radiative transfer approaches: a reflectance approximation and the radiative transfer model, EcoLight-S. Non-linear optimisation inversion schemes are then implemented. A simulated dataset is created to investigate how much variability in reflectance can be associated with changes in phytoplankton cell size in different bio-optical water types. This dataset is inverted to investigate the errors inherent in the inversion process as a result of ambiguity. Comparison of the two radiative transfer techniques allows for consideration of the suitability of approximations for bidirection-ality and subsurface propagation. The inversion algorithm is then applied to hyperspectral in situ radiometric data to provide validation and further assessment of errors from all sources. Results indicate that size related sensitivity in reflectance is highly dependent on phytoplank-ton biomass, as determined by the relative phytoplankton contribution to the Inherent Optical Property budget. The algorithm is finally applied to ten years of MERIS data covering the southern Benguela. A time series of biomass and cell size is presented and metrics developed to demonstrate the utility of this approach for identifying previously unobserved interannual variability in Harmful Algal Blooms.
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    Phytoplankton 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, Sandy
    This 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.
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    The interaction of acidification and warming on the South African abalone, Haliotis midae, and the potential for mitigation in aquaculture
    (2021) Lester, Nina Catherine; Bolton, John; Lucas, Mike; Auerswald, Lutz
    The South African abalone, Haliotis midae, is an economically important species. H. midae is the largest of the five endemic abalone species in South Africa and is especially valuable in Asian markets. Over-fishing, increased predation (due a geographical shift in lobster populations), and prolific poaching of this commercially valuable species have depleted natural populations. Commercial abalone aquaculture began as a means to meet the market demand for H. midae and currently accounts for 77 % of South Africa's aquaculture revenue. Despite marked growth in this industry over the past decade, further increases will be challenged by the predicted threat of climate change, particularly ocean acidification. Calcifying organisms, such as abalone, are particularly susceptible to the impacts of ocean acidification and its resultant alteration in seawater carbonate chemistry. Most calcifying organisms display reduced calcification and growth in response to ocean acidification, with possible alterations to acid-base regulation, development, gonadal maturation and behaviour, as well as reduced larval and gamete survival. A further potential challenge to abalone aquaculture is global warming. The effects of temperature change depend on the organism's level of sensitivity, taxonomy, distribution and life history. The distribution of H. midae ranges from Saldanha Bay (cool-temperate) to Riet Point (warm-temperate), with the majority of commercial aquaculture production occurring in Hermanus (cool-temperate). Abalone aquaculture facilities will need to adapt to these environmental changes and assess potential mitigation strategies. This study investigated the long-term (12 months) impact that ocean acidification and warming will have on the South African abalone, Haliotis midae, by incorporating the natural variability of seawater pH and temperature in Hermanus. Ambient seawater retained natural pH and temperature variability and acidified seawater was offset to natural pH variability using CO2/O2 diffusion and a data-logger-relay system to incorporate local-scale variability of seawater in the abalone farm, where this experiment was based. A multi-parameter approach was used to investigate the effects of reduced pH (- 0.4 from ambient) and warming (+ 1.5 °C from ambient) on abalone growth, spawning patterns, acid-base regulation, shell growth, morphology, shell strength and mineralogy over 12 months. This study also investigated the potential use of Ulva (Chlorophyta) as a mitigational tool to ameliorate acidified seawater, by photosynthetic carbon dioxide uptake, in a flow-through aquaculture system on a South African abalone farm. This study assessed the effects of seaweedtreated seawater on abalone growth, spawning patterns, acid-base regulation, shell growth, morphology, shell strength and mineralogy over 12 months in comparison to ambient and acidified seawater. Ocean acidification conditions resulted in a decrease in H. midae haemolymph pH and an increase in pCO2 (indicative of uncompensated respiratory acidosis), which resulted in reduced growth (whole-, muscle-, and shell-mass) and an alteration in spawning patterns. Acidification conditions also altered shell shape (smaller area with a wider shape) and significantly reduced shell strength. Warming conditions were within the thermal optimum ranges for H. midae and did not significantly affect abalone growth; however warming did bring about significant changes in Condition Factor, shell shape, and strength over time and shifted acid-base regulation towards a more stable status. The combined impact of warming and acidification were similar to the effects of reduced pH alone, with the exception of effects on acid-base regulation (severe uncompensated respiratory acidosis) and shell shape (lengths and widths were moderately increased). Ocean acidification and warming conditions, singularly and in combination, had no significant impact on shell mineralogy (percentage weight of aragonite, and aragonite and calcite crystal diameter). Bio-mitigation of acidified seawater by Ulva increased abalone wet weight, GBI, shell length, shell width and shell area in comparison to acidified conditions. Warming, caused by Ulva cultivation, resulted in similar effects on abalone shell growth and acid-base regulation as those exposed to warmed conditions alone. However, ambient and acidified seaweed-treated seawater caused a significant reduction in abalone muscle mass during summer months in comparison to abalone grown in ambient seawater. This decrease in muscle mass occurred concurrently with a decline in Ulva yield (due to photoinhibition), suggesting an interactive effect of Ulva (under stressor conditions) and abalone which needs to be studied further. The findings of this thesis are of particular concern for the South African abalone industry as ocean acidification conditions are likely to result in slower abalone growth, increased cultivation time to reach market-size, and reduction in quality of abalone (as the shells are more easily damaged). This study highlights the importance of incorporating local-scale, natural variability into ocean acidification and warming studies to guide management practices for cultivation and protection of this valuable species. The incorporation of natural seawater variability highlights an overexaggerated effect of warming on abalone exposed to constant-temperature experiments. Although predicted increases in seawater temperature (+ 1.5 °C) are within the optimal thermal ranges for H. midae in Hermanus, warming could pose a risk for aquaculture sites in warm-temperate areas of the South African coast. This thesis provides feedback on a potential mitigation strategy for abalone farms, with options for improvements in design as well as further mitigational options in the face of climate change. This is the first study to assess the effects of long-term elevated CO2 and warming on H. midae, and the first to incorporate long-term, natural variability into climate change research for any species outside of a laboratory.