Browsing by Author "Waldron, Howard"

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    A Comparison of Gas Exchange Models in the Estimation of CO2 Fluxes in the South Atlantic South of Africa for the Summer Season of 2008/2009
    (2014) Rainier, stephanie Megan; Monteiro, Pedro; Waldron, Howard
    There is a problem in the determination of air-sea CO2 fluxes because of the number of different relationships used in calculating gas transfer velocities. There is also a problem with the CO2 sink in the Southern Ocean being greatly underestimated. Data were collected underway using an autonomous pCO2 system during three separate relief cruises over the course of austral spring 2008 to austral autumn 2009 onboard the RV SA Agulhas in the South Atlantic Ocean. The wind speed product was extracted from QuikSCAT. Using the data we investigated the sensitivity of the five gas transfer velocity parameterisations to the uncertainty in the wind speed product of 2m.s-1. We found that the Stagnant Film Model was unresponsive. Liss and Merlivat's (1986) linear model for three wind regimes showed a gradual increase in sensitivity with wind speed. The quadratic relationship developed by Nightingale et al., (2000) also showed a steady increase in sensitivity with an increase in wind speed. Wanninkhof's (1992) quadratic relationship showed the greatest response at low wind speeds and then a continuing increase in response through medium to high wind regimes. The cubic relationship from Wanninkhof and McGillis (1999) showed the smallest response at low wind speeds but had the greatest response to the uncertainty in the wind speed product in medium and high wind regimes. We also calculated regional and seasonal averages of the CO2 flux with the five gas transfer velocities based on the different relationships between gas transfer velocity and wind speed. We found that there was a CO2 flux into the ocean ranging from 4mmol.m-2.day-1 to 12mmol.m-2.day-1 between 33.5 and 68°S, except during autumn between 45-50°S where there is a flux out of the ocean of 2mmol.m2 .day-1. Between 68-70°S the flux into the ocean strengthens to between 28mmol.m2 .day-1 and 52mmol.m-2.day-1. Gas transfer velocity is not dependant on wind speed alone, but currently it is the only variable that it measureable on a global scale. Further investigations are in place to measure gas transfer velocity in situ. The Southern Ocean is greatly undersampled spatially and temporally which leads to a lack of understanding 13 about possibly one of the most important CO2 sinks. New methods are being taken on to increase the sampling resolution.
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    An ADCP study of subtidal scale density-driven exchange in Saldanha Bay, South Africa
    (2003) Stewart, Helen Frances; Monteiro, Pedro M S; Waldron, Howard; Brundrit, Geoff
    An ADCP and water-column temperature study was conducted to determine the circulation aspect of subtidal-scale, density-driven exchange in Saldanha Bay, South Africa. Density-driven exchange conditions develop in response to synoptic-scale wind events in the southern Benguela region, even under light (<5m s-') wind conditions. During a density-driven exchange event, directionally opposing bi-level flow, similar to an estuarine system, develops in response to remote upwelling-favourable winds. The bi-level flow component occurs in two distinct bands, bayward at 0-9m height from bottom and seaward 15-20m height off bottom, and is very sensitive to changes in wind forcing. Observations of current behaviour are added to the four-phase conceptual model of density-driven exchange developed by Monteiro and Largier (1999). In addition, estimates of bay flushing based on ADCP current velocities and the four-phase conceptual model are calculated and implications of shelf water influx into Saldanha Bay are discussed.
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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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    A comparative study of nitrogen uptake and nitrification rates in sub-tropical, polar and upwelling waters
    (2015) Philibert, Marie Catherine Raïssa; Waldron, Howard; Clark, Darren
    Nitrification is the oxidation of ammonium to nitrate through a two step biological process. Nitrification in the euphotic zone has, in the past, been considered negligible even though quantifying this process correctly is important when linking carbon export to nitrate uptake by phytoplankton. However, studies of both nitrogen uptake and nitrification rates in surface waters are rare. This thesis presents such data for the Southern Ocean and St-Helena Bay, located in the Southern Benguela upwelling system. Using 15N tracers, Nitrogen uptake and regeneration rates were measured in the Southern Ocean (during a winter cruise in July 2012 and a summer cruise in February-March 2013) and St-Helena Bay (during three studies in November 2011, March 2012 and March 2013). In St-Helena Bay, the upwelling (bloom) cycle was one of the main drivers of the nitrogen cycle. As the bloom cycle started, nitrate uptake rates (5:47 ± 670:48nmol∙L⁻¹∙h⁻¹) and nitrite regeneration (4:36 - 1:28 ± 22:83 - 1:63nmol∙L⁻¹∙h⁻¹) were high but the contribution of nitrification to the nitrate demand was low. Nitrite regeneration at this time could have been driven by phytoplankton excretion. In contrast, at the end of the bloom cycle, nitrate uptake rates were low and was exceeded by nitrate regeneration rates (25:34 - 6:16 ± 82:74 - 34:41nmol∙L⁻¹∙h⁻¹). Nitrite regeneration decreased and was most likely due to ammonium oxidation at this stage of the upwelling cycle. Nitrification in the Southern Ocean was more variable than in St-Helena Bay. It was only detected at five stations out of fifteen and the accuracy of the high nitrite oxidation rates (37:21 - 9:13 ± 217 - 88nmol∙L⁻¹∙h⁻¹) observed can only be assessed with repeat measurements. Nitrate uptake rates ranged from 0.07 to 57:00nmol∙L⁻¹∙h⁻¹ while ammonium uptake rates ranged from 0.81 - 160:94nmol∙L⁻¹∙h⁻¹. The nitrogen uptake rates were similar for both seasons. Using multivariate statistical approach, it was found that during winter, in the Southern Ocean, light and ammonium availability were the most important factors regulating nitrogen uptake while in the late summer, changes in the mixed layer depth had a larger effect. This study provides new observational data for two undersampled regions and contributes to further the mechanistic understanding of the factors regulating nitrogen uptake and nitrification in the Southern Ocean and St-Helena Bay.
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    A comparison of gas exchange models in the estimation of CO2 fluxes in the South Atlantic South of Africa for the summer season of 2008/2009
    (2011) Ranier, Stephanie Megan; Monteiro, Pedro; Waldron, Howard
    There is a problem in the determination of air-sea CO2 fluxes because of the number of different relationships used in calculating gas transfer velocities. There is also a problem with the CO2 sink in the Southern Ocean being greatly underestimated. Data were collected underway using an autonomous pCO2 system during three separate relief cruises over the course of austral spring 2008 to austral autumn 2009 onboard the RV SA Agulhas in the South Atlantic Ocean. The wind speed product was extracted from QuikSCAT. Using the data we investigated the sensitivity of the five gas transfer velocity parameterisations to the uncertainty in the wind speed product of 2m.s-1. We found that the Stagnant Film Model was unresponsive. Liss and Merlivat’s (1986) linear model for three wind regimes showed a gradual increase in sensitivity with wind speed. The quadratic relationship developed by Nightingale et al., (2000) also showed a steady increase in sensitivity with an increase in wind speed. Wanninkhof’s (1992) quadratic relationship showed the greatest response at low wind speeds and then a continuing increase in response through medium to high wind regimes. The cubic relationship from Wanninkhof and McGillis (1999) showed the smallest response at low wind speeds but had the greatest response to the uncertainty in the wind speed product in medium and high wind regimes. We also calculated regional and seasonal averages of the CO2 flux with the five gas transfer velocities based on the different relationships between gas transfer velocity and wind speed. We found that there was a CO2 flux into the ocean ranging from 4mmol.m-2.day-1 to 12mmol.m-2.day-1 between 33.5 and 68°S, except during autumn between 45-50°S where there is a flux out of the ocean of 2mmol.m- 2.day-1. Between 68-70°S the flux into the ocean strengthens to between 28mmol.m- 2.day-1 and 52mmol.m-2.day-1. Gas transfer velocity is not dependant on wind speed alone, but currently it is the only variable that it measureable on a global scale. Further investigations are in place to measure gas transfer velocity in situ.
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    The effect of nutrient limitation and oxidative stress on bromoform production from axenic cultures of marine diatoms
    (2012) Nguvava, Mariam; Bucciarelli, Eva; Waldron, Howard; Palmer, Carl J
    Bromoform is mainly produced naturally by marine organisms. The production by microalgae however is not well quantified, and the mechanism by which phytoplankton produce bromoform is not fully elucidated. The production of this compound at the cellular level seems to be linked to oxidative stress via the use of the antioxidant enzyme bromoperoxidase (Manley and Barbero, 2001). However, no experiment has been conducted under bacteria-free conditions, which may bias the results because bacteria produce bromoform. Diatoms, which are wide spread in the open ocean, could be amongst the most important producers of bromoform. The purpose of this study was to quantify the effect of nutrient limitation and oxidative stress on bromoform production from axenic cultures of marine diatoms.
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    The effect of quick freezing and fast thawing on stored seawater nutrient concentrations of dissolved inorganic phosphate, nitrate and ammonium
    (2004) Wedeinge, Josef; Brundrit, Geoff; Waldron, Howard
    In an attempt to improve the quality of preserving seawater nutrients for delayed analyses, a sample of unfiltered and aged nutrient-depleted seawater was obtained from Marine and Coastal Management (M&CM) and spiked with small different volumes of standard nutrient solutions. These were prepared into 100 mL glass bottles to constitute the lowest, intermediate and highest levels of dissolved inorganic phosphate (0.5, 1.5, 3.0 μM), nitrate (5, 15 and 30 μM) and ammonium (0.5, 1.5, 2.5 μM) that are found in the southern Benguela region. After spiking, the samples were analysed to determine their "true" initial concentrations and immediately quick frozen using liquid nitrogen. A time series analysis (up to seven weeks) was performed by first fast-thawing the samples using a microwave oven and fixing up the nutrients within 20 minutes. Variations in nutrient concentrations did not exceed 7% for phosphate, 4.0% for nitrate at all levels and 8% for ammonium at the intermediate and highest levels, but varied by 19.2% at the lowest level. Statistical tests for significance of the means were made at a = 0.05 level and no difference was found at any level for the nitrate, and the lowest level phosphate. The higher levels of phosphate, and all ammonium were found to be significantly different. Hence, the experimental conditions employed in this paper would not be recommended for the all ammonium and the higher levels of phosphate concentrations. These variations were however not large or regular in pattern but erratic, which suggested that they were not due to microorganisms and optimizing those conditions may well to yield improved results.
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    The effect of the Cape Flats Aquifer on the water quality of False Bay
    (2002) Giljam, Rhys; Waldron, Howard
    The aim of this project was to investigate the effect that the Cape Flats Aquifer, situated on the north shore of False Bay, is having on the water quality of the bay. Following increased development on the Cape Flats, the risk of the aquifer becoming contaminated has increased as it is situated below the informal settlement of Khayelitsha (where there is poor sanitation) and the Philippi agricultural area (where fertilisers are widely used).
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    Evaluation of storage strategies and techniques for the hydrochemical analysis of nutrients in Atlantic Ocean coastal water samples
    (2014-08-15) Harding-Goodman,Amy; Waldron, Howard
    Filtered and unfiltered samples of coastal seawater from the Atlantic Ocean (Muizenberg and Llandudno beaches, Cape Town) representing a non upwelling region and an upwelling region respectively were stored at -20 deg C. Deviations in the concentrations of silicate, inorganic phosphate, nitrite, nitrate and urea from the initial concentrations determined on collection of the seawater samples were monitored at intervals over a twelve week storage period. Chemical analysis of filtered and non-filtered water samples on the day of collection demonstrated that filtration affected the concentration of nutrients in the samples. In three of the eight filtered seawater samples the filtrate contained a lower concentration of nutrients than the corresponding non-filtered seawater samples. Sample t-tests indicated that these differences were not significant. The study shows that no single storage strategy for the determination of all nutrients in seawater can be recommended and that the filtering/storage strategy used should be adapted depending on the nature of the nutrient to be analysed. The mean inorganic phosphate concentrations present on sampling at Llandudno and Muizenberg beach ranged from approximately 0.4 μmol to 0.7 μmol L -1. Filtered frozen samples for inorganic phosphate determinations stored well for twelve weeks with minor deviations in concentration from the initial concentration obtained at week 0 being recorded. This strategy showed less variability over the twelve week trial than shown with non-filtered samples and is the recommended strategy for the preservation of inorganic phosphate. Ideally samples should be analysed within the first 2 weeks of storage. The mean dissolved silicate concentrations present on sampling at Llandudno and Muizenberg beach ranged from approximately 9 μ mol to 14 μmol L -1. Neither of the two storage strategies explored during this study proved ideal as fluctuations in the level of dissolved silicate were noted using both storage regimes. Where immediate analysis is not possible the strategy which provided the least variability in this study i.e. storage of non-filtered samples at -20°C is recommended. The mean nitrite concentrations present on sampling at Llandudno and Muizenberg beach ranged from approximately 0.4 μmol to 0.7 μmol L -1. Ideally samples (filtered and non-filtered) should be analysed within the first 2 weeks of storage as further delays in analysis resulted in fluctuations in the nitrite concentration. The mean dissolved urea concentrations present in non-filtered water on sampling at Llandudno was 0.2 μmol L -1 with 1.5 μmol L-1 present in the sample taken at Muizenberg beach. The major feature of the preservation study on variations in the concentration of urea in stored water over time is the relative stability of samples stored frozen for periods for periods of up to 8 weeks. A second study on ten Atlantic Ocean coastal water samples collected at sites on the Cape Peninsula compared analytical techniques used to determine silicate and dissolved nitrate concentrations in seawater. For inorganic silicate analysis a modification of the manual method of Grasshoff (1976) and an automated flow injection analysis (FIA) method were used. The FIA method yielded concentrations that were on average two times higher in seven out of the ten analyses. For the determination of dissolved nitrate concentrations two manual methods (differing in the method of reduction of nitrate to nitrite) were used: shaking the sample with cadmium pellets and passing the sample through a cadmium column. In both cases the effect of using Tris and ammonium chloride (NH4 Cl) buffers was determined. In addition nitrate concentrations were determined using FIA. Dissolved nitrate concentrations determined by the FIA and the cadmium column method (using both Trisand NH4 Cl buffers) differed significantly whereas those obtained using the shaking method were frequently low or negative. The shaking method used to determine nitrate concentrations is not recommended.
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    Exploring optimising strategies for sampling air-sea carbon dioxide flux in the southern ocean
    (2014) Pringle, Nicholas; Monteiro, Pedro M S; Waldron, Howard
    A model study was undertaken to investigate the optimization of sampling strategies for returning low-uncertainty sea-air CO₂ flux measurements in the Southern Ocean. Replicating Lenton et al. (2006) using the ORCA2/PISCES ocean biological model shows that sampling 4 times a year, every 2⁰ in latitude and every 40⁰ in longitude reduces the uncertainty of estimating annual CO₂ flux estimates such that sampling at a higher frequency does not reduce the total uncertainty in proportion to the increase in sampling effort.
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    Lateral carbon export from the southern Benguela upwelling system
    (2008) Swart, Neil C; Waldron, Howard
    The objective of this study was to quantify the lateral export of organic carbon from the continental shelf of the southern Benguela upwelling system to the open ocean. The flux is potentially important because the Benguela is one of the most productive and biogeochemically active ecosystems in the global ocean. Furthermore, a significant fraction of oceanic carbon storage is modulated through the biological pump mechanism, and on millennia 1 timescales the global ocean regulates atmospheric concentrations of carbon dioxide. The current study builds on previous work, and examines both the physical and biogeochemical aspects of the lateral carbon flux from the southern Benguela. Multiple physical mechanisms capable of inducing cross-shelf advection were examined, including dynamic interaction with Agulhas Rings and upwelling front instability, however the bottom boundary layer (BBL) was the focus.
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    Lateral carbon export from the southern Benguela upwelling system
    (2008) Swart, Neil C; Waldron, Howard
    The objective of this study was to quantify the lateral export of organic carbon from the continental shelf of the southern Benguela upwelling system to the open ocean. The flux is potentially important because the Benguela is one of the most productive and biogeochemically active ecosystems in the global ocean. Furthermore, a significant fraction of oceanic carbon storage is modulated through the biological pump mechanism, and on millennia 1 timescales the global ocean regulates atmospheric concentrations of carbon dioxide. The current study builds on previous work, and examines both the physical and biogeochemical aspects of the lateral carbon flux from the southern Benguela. Multiple physical mechanisms capable of inducing cross-shelf advection were examined, including dynamic interaction with Agulhas Rings and upwelling front instability, however the bottom boundary layer (BBL) was the focus.
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    Primary productivity and its variability in the Atlantic Southern ocean
    (2014) Joubert, Warren Ryan; Monteiro, Pedro M S; Waldron, Howard
    The two principal bottom-up drivers of the High Nutrient Low Chlorophyll (HNLC) characteristics of the Southern Ocean are light and nutrient (mainly dissolved iron) limitation ( Boyd , 2002; Mitchell et al., 1991), which have varying limiting roles over the growing season ( Boyd, 2002; Swart et al., 2014). This research commenced with an investigation of the meridional characteristics of primary productivity in the Atlantic Southern Ocean during austral summer 2008.
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    The role of water column and benthic communities in the spatial and temporal production and uptake of nutrients in controlling the trophic status of the Knysna River Estuary, South Africa
    (2003) Switzer, Todd Brent; Allanson, Brian; Waldron, Howard
    The Knysna Estuary is the largest estuarine ecosystem in the warm temperate coastal region of South Africa. The estuary remains oligotrophic (chlorophyll-a < 5 mg l-1) despite increases in anthropogenic input of nitrogen and phosphate associated with the rapidly increasing urbanization of its littoral and catchment. This dissertation relies on original data, which includes the measuring of concentrations of ammonium, nitrates (nitrate and nitrite), dissolved inorganic phosphate and urea in the water column quarterly for one year at 21 stations. It estimates the contribution to these nutrients from rivers, storm drains and rainfall entering the estuary during this year by monitoring concentration and flow in these source waters. Seasonal, tidal and post-flood patterns of nutrient concentration in the water column are assessed with respect to nutrient loading to determine if variations in concentration are the result of loading or processes in the estuary.
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    Sea water quality and processes in Simon's Bay
    (2000) Wainman, Carl Kevin; Waldron, Howard
    The continued sustainable beneficial use of the sea, and in particular Simon's Bay, as an important industrial Naval Harbour, beach and recreational retreat, tourist destination, penguin haven, trek-net fishery, maritime archaeological area and research zone has been highlighted. If these benefits are to be maintained or in the context of expanded future use, then the quality of seawater in the region is seen as a key maritime health indicator of this interdependent system. Information about near-shore interaction with False Bay and beyond is important for good management. This work is primarily concerned with the marine coastal region defined by the port limits of Simon's Bay. Simon's Bay cannot, however, be studied, without consideration of the external influences of the greater False Bay and, still further afield, from the South Atlantic Ocean and the Agulhas bank The primary area of interest includes the port limits of Simon's Bay as defined by the South African Navy, depicted on the SAN 1017, 1998 Simon's Bay hydrographic chart. This work aims to provide an initial understanding of the spatial and temporal environmental dynamics of Simon's Bay and establish baseline values.
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    Seasonal and interannual variability of the marine carbonate system at the ice shelf in the eastern Weddell Gyre and its sensitivity to future ocean acidification
    (2012) Weeber, Amy; Monteiro, Pedro M S; Waldron, Howard
    Ocean Acidification through the uptake of anthropogenic CO₂ is resulting in a decrease in surface water carbonate ion concentration, a critical compound for marine calcifying organisms (Fabry et al., 2008; Orr et al., 2005). Natural seasonal variability is predicted to hasten the effects of Ocean Acidification in the Southern Ocean, resulting in possible surface water wintertime aragonite (the more soluble form of calcium carbonate) undersaturation (Ωarag< 1) south of the Antarctic Polar Front by the year 2030 (McNeil and Matear, 2008). An Ocean Acidification study was conducted to determine the seasonal and interannual variability in aragonite saturation state (Ωarag) at the Antarctic ice shelf between 4°E and 14°W and in the Eastern Weddell Gyre, during the Austral summers of 2008/2009, 2009/2010, 2010/2011 and 2011/2012. This study shows that at the Antarctic ice shelf andin the Eastern Weddell Gyre (EWG), seasonal summertime phytoplankton blooms were a critical factor in the observed decrease in summer surface water CO 2 and the subsequent increase in summer surface water Ωarag.
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    Seasonality of the marine carbonate system in the southern Benguela nutrient stoichiometry, alkalinity production, and CO flux
    (2012) Gregor, Luke; Monteiro, Pedro M S; Waldron, Howard
    An observational study was undertaken to determine the seasonality of the marine carbonate system of the southern Benguela focusing on three key points: the processes driving bulk stoichiometry, alkalinity production on the continental shelf, and the air-sea flux of CO2. Monthly samples were taken along the St. Helena Bay Monitoring Line in the southern Benguela for ten of the months in 2010. Samples were analysed for dissolved inorganic carbon (DIC) and total alkalinity (TA). Temperature, salinity, oxygen and nutrients were also measured.
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    Shelf biogeochemical interactions and feedback processes in the Benguela upwelling system
    (2017) Mashifane, Thulwaneng; Vichi, Marcello; Waldron, Howard
    Two coupled physical-biogeochemical models namely, (Regional Ocean Modelling System and Biogeochemical of Eastern Boundary Upwelling Systems) ROMS-BioEBUS (3D) and (Nucleus for European Modelling of the Ocean and Biogeochemical Flux Model) NEMO-BFM (1D) are applied in the Benguela upwelling system to understand biogeochemical interactions and their related feedback processes. The models are formulated differently but achieve similar objectives with respect to the physics and biogeochemistry. The BioEBUS model is used to simulate nitrogen processes under oxic and suboxic conditions in upwelling systems with no option for other cycles. Intermediary nitrogen processes, nitrous oxide production and nitrogen loss mechanisms are studied using this model. Physical and advection processes that drive the oceanic nitrogen cycle in the region are also studied with BioEBUS. The BFM is used to understand the implications of the nitrogen loss and suboxic-anoxic conditions on related biogeochemical cycles. The 1D model was selected for its low computational costs and flexibility for addition of new code. BFM includes the carbon, nitrogen, phosphorus, silicate, iron cycles and hydrogen sulphide production, which is a known occurrence in the Namibian shelf waters. New variables, nitrite and nitrous oxide production, are added in BFM to complete the nitrogen cycle. The nitrification process in BFM is also formulated in two stages as in BioEBUS to obtain comparative results in both models. Both models are compared and validated with data from the Maria S. Merian (MSM) 19/1b cruise and available products respectively. Simulated results from BioEBUS show primary and secondary nitrite maxima in the Benguela shelf waters. The primary nitrite maxima are attributed to nitrification and nitrate assimilation. Secondary nitrite maxima accumulate in the Angola-Benguela Front (ABF) oxygen minimum zone (OMZ) and are attributed to denitrification. Off Walvis Bay, these secondary nitrite maxima and ammonium are thought to be consumed by high rates of anaerobic ammonium oxidation (anammox). The nitrite maxima are restricted to the shelf off Walvis Bay and advected offshore in the ABF region. Interchanges between the poleward South Atlantic Central Water (SACW) and the equatorward, well-oxygenated Eastern South Atlantic Central Water (ESACW) drive the seasonality of nitrogen processes in the Benguela. Nitrous oxide concentrations are high in the ABF as a result of nitrification and accelerated production under suboxic conditions. Off Walvis Bay, nitrous oxide production is low when compared to the ABF. Nitrous oxide production in the ABF occurs in thermocline, intermediate and deeper water masses. Off Walvis Bay, nitrous oxide production in deeper water masses is missing because of the shallow coast. High fixed nitrogen fluxes in the Benguela are attributed to nitrification rather than anammox and denitrification. Simulated results show denitrification to be the dominant nitrogen loss mechanism in the Benguela shelf waters. Simulated results from BFM show higher nitrogen uptake rates than phosphate in shelf and offshore stations. The uptake rates are high on the shallow shelf due to luxury nutrient uptake. High N:P ratios occur at the stations at 21ᵒS than off Walvis Bay and are attributed to the presence of nutrient-rich, oxygen depleted SACW and denitrification respectively. Increased fixed nitrogen deficits (N*) occur in surface and subsurface waters at shallow stations as opposed to offshore. The positive N* anomalies off Walvis Bay are attributed to organic matter remineralization in deep, offshore stations. In contrast, increased phosphate (P*) concentrations occur in surface and subsurface waters. Phosphate is regenerated in subsurface waters and released under suboxic-anoxic conditions increasing P* concentrations. Nitrogen loss coupled with hydrogen sulphide production accelerate phosphate release in suboxic-anoxic bottom waters. The N:P stoichiometry, uptake rates, N* and P* concentrations appear to have considerable implications on potential estimated nitrogen fixation in the Benguela. BFM results suggest that the Benguela is a system characterized by excess nitrate in relation to silicate. This has been drawn from the low Si:N ratios observed at the simulated stations. A secondary Si:N peak is shown on the shallow coast due to high denitrification rates in suboxic waters. Note that high silicate concentrations occur in suboxic conditions and can be attributed to organic matter remineralization. The high silicate concentrations in the well-oxygenated offshore station are linked to sinking particles in deep waters. Increased silicate dissolution occurs in warm, surface waters and the particles that pass through the water column undissolved settle at the bottom where dissolution continues. From these results, it can be assumed that increased warming in the Benguela coastal waters should result in silicate being a limiting nutrient. This could affect carbon export as it has been shown that increased POC export is high in coastal waters due to ballasting of diatom biomass. The models used in this study simulated biogeochemical interactions in the Benguela fairly-well and can be applied in other regions.
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    The circulation and thermal structure of False Bay: a process-oriented numerical modelling and observational study
    (2012) Nicholson, Sarah-Anne; Shillington, Frank; Waldron, Howard; Vietch, Jennifer
    False Bay, the largest bay in South Africa, lies between two of the world's most in-fluential current systems: the Agulhas and Benguela Current Systems. False Bay is situated at a biogeographic break (Pitcher et al., 2010), belonging to both the South-ern Benguela upwelling system and the Western Agulhas Bank. The oceanography of Southern Africa, particularly this region, is some of the most complex and highly variable in the world (Roberts, 2005). Owing to the surrounding steep orography and its position, the meteorology of the False Bay region is also complex, having wind con-ditions that are unique when compared to other regions of South Africa (Jury, 1991). It is under these special conditions that the circulation of False Bay is regarded as highly complex.
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    The Threat and Cascade Method of estuarine health assessment: a logical sequence from human impact to biological degradation via system physics and chemistry
    (2005) Stevens, Victoria; Waldron, Howard
    A methodology for the comparative assessment of estuarine health over a range of systems is presented. It is based on the assumption that anthropogenic impact is the causative variable when considering negative impacts on estuarine health. The methodology follows a logical cascade of estuarine health assessment protocols. The first step in the Threat and Cascade Method (TaCM) incorporates socio-economic to produce a scaled indicator used to identify estuarine systems that are potentially threatened by anthropogenic inputs. The socio-economic algorithm incorporates the following variables: land cover, population density, per capita wealth, state of the estuarine mouth, abstracted mean annual runoff, encroachment of development, estuary use, and sewerage input. If the Socio-Economic Threat Index identifies the estuary as being threatened, then the second stage of the TaCM is initiated. This is an assessment of the system's physics and is accomplished by considering the following variables: residence time, estuary number (freshwater inflow/ tidal prism), coastal exchange, and the proportion of the time the estuary mouth is closed to the ocean. The Threat and Cascade Method assumes that an anthropogenically threatened system with a short residence time is less likely to be impacted on than a threatened system with a long residence time. If the Physical Threat Index identifies the estuary as being threatened, then the third stage of the TaCM is initiated. This involves assessing the chemistry and then the biology of the threatened estuarine system. The TaCM was tested using both local (South African) and international case studies. The results showed that the TaCM has the potential to become a universal methodology. The results also showed that the TaCM allows estuarine researchers and managers to rapidly assess the 'health' of a number of systems, as it mainly concentrates on estuaries that are likely to be impacted upon. The TaCM assessment identifies 'what' is causing the estuary's health to deteriorate, therefore identifying the problem areas that need to be addressed in order to mitigate the impacts on the estuary. This will allow managers to assess the success of remedial action on the estuary. The results also revealed that the TaCM could be used to predict what impact 'change' in the estuary catchment would have on an estuary's health.