Browsing by Subject "Applied Ocean Sciences"
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- ItemOpen AccessAssessing techniques to successfully monitor the oceanography of the complex uThukela Marine Protected Area(2022) Ramsarup, Neha; Morris, Tamaryn; Hermes, JulietMarine Protected Areas (MPAs) are increasingly being established to restore and protect coastal and marine environments. The newly established uThukela Marine Protected Area (MPA), located on the central KwaZulu-Natal Bight along the northeast coast of South Africa, has been identified as a key ecological region. The region provides recruitment and is a general nursery area for marine life on the Bight. Knowledge of the oceanographic dynamics in the region is essential for understanding the functioning of the ecosystem and the effectiveness of the MPA. This study analysed changes in ocean temperatures at 20 sites along the Bight over a 41-year period using in situ beach and UTR data, and satellite data. Significant warming of 0.03°C/year occurred at beach sites within and around the MPA. Beach temperatures increased at an average rate of 0.02°C/year across the Bight, during both summer and winter, whereas UTR temperatures warmed by 0.14°C/year during summer and 0.08°C/year during winter. At the event scale, a case study of a persistent decrease in in situ temperatures along the entire Bight during summer 2017/2018 was investigated. The decrease in temperatures was hypothesised to be as a result of the combined effects of a La Niña event and a Natal Pulse, which could have significant consequences for temperature-sensitive species. Temperature trends observed in beach temperatures over the 41 years were not reflected in beach temperatures over the past decade, highlighting the importance of large datasets when investigating climate change. Warm biases of up to 2°C observed in satellite-derived temperature measurements, and its failure to replicate trends seen in the beach data, suggests that in situ temperature measurements in MPAs are better suited for long-term monitoring efforts. These findings can help assess the success of MPAs and to guide monitoring and research activities within the region.
- ItemOpen AccessBenthic metrics as indicators of human disturbance in a marine-dominated lagoon(2018) Jones, Amy M; Pillay, DeenaExpanding anthropogenic developments along with the added stress of climate change, are negatively influencing coastal ecosystems. Because of their many benefits to mankind, it is important to identify key bioindicators that can detect disturbance-induced ecosystem change. Benthic metrics are an excellent example of disturbance indicators in soft sediment based aquatic systems, and are directly applicable to one of South Africa’s most economically and ecologically significant marine-dominated lagoons; Langebaan lagoon. This lagoon is managed through the designation of three zones (A, B and C) with contrasting human presence. Public access, recreation and bait-collecting is permitted in A, but no bait colleting is permitted in B. Human presence is completely restricted in Zone C. This study thus aimed to test the level of impact of human disturbance on two zones of the lagoon (A and C), using benthic metrics as bioindicators. Macrofaunal community metrics (abundance, species richness, Shannon-Weiner diversity, evenness, and community structure), performance of a key ecosystem engineer (sandprawn abundance and condition factor), organic matter content and microphytobenthic biomass were compared between the two sites, comprising multiple subsites. Results showed minimal differences between disturbed and undisturbed sites, with the exception of organic matter content and Shannon-Weiner diversity comparisons, which were greater in undisturbed subsites. There was however, a general trend of increasing dominance by sandprawns (Callichirus kraussi) from undisturbed to disturbed subsites, whilst the undisturbed subsites were numerically dominated by a several codominant polychaetes (Euclymene spp., Notomastus latericeus and Marphysa sanguinea). Interestingly, there were more significant differences at the subsite level, suggesting that localized conditions are more important in shaping macrobenthic communities than disturbance impacts, as supported by previous literature. Despite limitations, this study does provide important baseline data relevant to optimizing sampling designs for detecting human disturbance impacts in Langebaan Lagoon. Key Words: Benthic metrics, bioindicator, macrobenthos, disturbance, trampling, coastal lagoon.
- ItemOpen AccessBuilding A Mean-state Of Oceanographic Properties (Temperature And Salinity) For The Kwazulu-natal Bight Using The Roms Model: A Contribution Towards Marine Protected Areas Analysis(2018) Malange, Mathabo Noxolo; Morris, Tamaryn; Collins, Charine; Lamont, Tarron; Ansorge, IsabelThe KwaZulu-Natal Bight, located along the east coast of South Africa, is an important recruitment and nursery area for various marine species. In an effort to conserve a number of threatened species, two Marine Protected Areas (MPA) have been established in the Bight. The African Coelacanth Ecosystem Programme is conducting MPA analyses along the Bight through a series of biological and oceanographic studies and this study forms part of the oceanographic research component that will assist in the decision-making process of MPAs in the region. This study uses a 30-year, high-resolution, regional ROMS simulation to build a climatology representative of the mean-state of the Bight. The model is also used to investigate the seasonal and annual variability as well as the influence of the Agulhas Current on the shelf. The Bight was cooler and less saline than the surrounding waters and seasonal variation was limited to the upper 50 m of the water column. The depth of the Bight ranges from 50 m in the inner shelf to 100 m at the shelf edge in the central region of the Bight. In the northern and southern region of the Bight, the depth of the water column extends down to about 150 m at the shelf edge. In summer, surface temperatures were on average 4.8°C and 4.3°C warmer than in winter over the uThukela Banks and Aliwal Shoal respectively. Bottom temperatures at both MPAs had a mean seasonal variation of about 3°C. Salinity, a more conservative variable, showed little variability over the year throughout the water column except for at 50 m where lower salinities were observed in the winter months. Wavelet analysis showed that a strong annual (12 month) signal was dominant at the surface (10 m). Bottom temperatures displayed a weaker annual signal than the surface in addition to a slight semi-annual cycle. Further investigations indicated that the Agulhas Current influenced the Aliwal Shoal MPA more than the uThukela Banks MPA as they shared similar temperature values (at the surface and bottom) throughout the 30-year period. In contrast, the uThukela was cooler than the Agulhas Current by 0.5 to 1.5°C at the surface and 1 to 2.5°C at the bottom. These time series also enabled us to identify anomalous features such as the Natal Pulse that could have important implications for temperature-sensitive species in the area.
- ItemOpen AccessEstimates of length-at-50% maturity of two South African demersal species: Monkfish, Lophius vomerinus, and Kingklip, Genypterus capensis(2019) Guerra, Alessandro; Attwood, Colin; Reed, Cecile C; Singh, LarvikaLong-lived and slow-growing fish species are especially vulnerable to overfishing. Deep-sea species are elusive and difficult to study, making their management challenging. South African monkfish, Lophius vomerinus, and kingklip, Genypterus capensis, are two demersal species living off the west and south coast of South Africa. These species are exploited as bycatch in the Hake fisheries, and represent an important resource for the fishery. However, reproductive information is scarce, and key life-history parameters have not been updated in recent years. Length-at-50% maturity (L50) is a fundamental biological parameter to monitor stock-health, spawning potential and improve fishery management. This research was focused on computing L50 estimates for both species, separated by sex and coast. As the stock structure for both species is still under debate, reproductive information and L50 estimates were compared between west and south coast. In addition, L50 was compared with previous studies to detect significant differences or changes over time. Demersal survey samples were used. Individuals were classed a 5 stage maturity scale and consequently referred to as mature or immature. For L. vomerinus, a significant difference in L50 between sexes was found, in contrast with previous findings in literature. However, no significant difference between the two areas was reported, corroborating the one-stock hypothesis. Furthermore, I speculate that L. vomerinus aggregate for spawning activities. The analysis of G. capensis revealed differences between coasts, reinforcing the theory of multiple South African stocks. Most importantly, the results showed a critical reduction in females L50 from previous studies. This reduction is believed to be a consequence of excessive fishing pressure exerted particularly during the latter half of the 20th century, which overexploited immature females on the west coast and main female spawners on the south coast. Further research needs to be done to verify these findings and the stock structure of the populations.
- ItemOpen AccessHuman disturbance impact on shorebirds at Muizenberg Beach, South Africa, during and prior to the COVID-19 lockdown(2021) Lewis, Jemma Elizabeth; Pillay, DeenaThe impact of humans on the natural environment has been so extensive that the scientific community has defined a new geological era: the Anthropocene. Coastal ecosystems rank amongst the most threatened of global habitats due to high urban human population density and demand for socio-ecological services provided by the coast. Sandy beach ecosystems are prominent constituents of coastal regions and sought after for their recreational value, but these systems are understudied relative to more charismatic marine habitats such as coral reefs and kelp forests. Human disturbance has been identified as one of the most serious threats to sandy beach ecosystems, but several knowledge gaps prevail, including its effects on shorebirds. The 2020 global COVID-19 pandemic resulted in a period of abnormally limited human mobility that has been coined the ‘anthropause'. In South Africa, a national lockdown commenced on 27 March 2020 and consisted of five levels to combat the pandemic. Specifically, lockdown levels 5, 4, and 3 did not permit human presence in public areas, including beaches. The aim of this study was to understand the impact of human disturbance during and prior to the COVID-19 lockdown on shorebird abundance and community structure on Muizenberg Beach, South Africa (34°06.3′S 18°28.3′E), using two approaches. The first approach used photographic data to compare shorebird numbers across five lockdown levels in 2020 with equivalent periods in 2019 (prior to the pandemic) to determine the effects of contrasting levels of human abundance on shorebirds. Results from this component indicated that shorebird abundance was negatively related to human abundance and year, but positively associated with the lockdown levels. Most importantly, greatest shorebird abundance across the study were recorded under lockdown levels 5 and 4, when there were virtually no humans on the beach. Similarly, the lowest bird abundance was recorded under lockdown level 1, when human abundance was greatest for 2020. The second approach implemented in this study was based on in situ counts of shorebirds on Muizenberg Beach between lockdown levels 3 and 1 in 2020, to understand how intermediate to moderate levels of lockdown influenced shorebird community structure. PERMANOVA analysis indicated that while a significant difference in the shorebird community structure among lockdown levels was detected, it was secondary relative to zonal variation. SIMPER analysis showed Larus dominicanus (Kelp Gull) to be the dominant shorebird species, contributing the most to community structure within all three lockdown levels. Significant differences in community indices and L. dominicanus abundance were detected between zones but not lockdown levels, apart from the Shannon-Wiener diversity metric. In general, the minor effects of lockdown levels 3 to 1 on the shorebird community were attributed to high human numbers on the beach and low species variation across the respective lockdown levels. Overall, results indicate a strong negative relationship between shorebird and human abundance on Muizenberg Beach over a two-year period prior to, and including, periods of enforced lockdown. At intermediate to moderate levels, however, lockdown had minimal effects on community structure. In a global context, findings provide valuable information on human impacts on sandy beach shorebirds and suggest that closure for two months may positively impact bird communities within urban beach ecosystems.
- ItemOpen AccessImpact of the Agulhas Current on storm development(University of Cape Town, 2020) O'Connor, Jason; Rouault, MathieuA high-resolution atmospheric model (WRF) is used to investigate the impact of the Agulhas Current on synoptic storm development. A sensitivity experiment is conducted to analyse the influence of the Agulhas Current's sea surface temperature (SST) on rain producing, synoptic scale weather features. Two model configurations: Control (CTL) and Smooth (SMTH) are analysed to understand the effect of the Agulhas Current's SST and high latent heat fluxes on storms that develop or track over the Current. The two configurations are identical except that the SMTH simulation has the SST signature of the Agulhas reduced by smoothing out the strong SST gradients associated with the Current. This results in the Agulhas Current core having SSTs reduced by roughly 1.5°C in the SMTH configuration. Consequently, lower (100 - 150 W.m¯²) latent heat fluxes are also simulated at the Current core's location in the SMTH run. Using daily South Africa Weather Service synoptic charts from 2001 - 2005, when the model output is available, two hundred (200) synoptic scale storms are found to track over the Current. Using the TRMM 3B42 3-hourly 0.25 x 0.25° precipitation rain rate product, 70 (of the 200) are found to have produced rainfall. Five model variables are used as proxies for the storm intensity of these 70 storms. Ten storms are found to show storm intensification when passing over the Current. In the CTL simulation, of these ten storms, ten show lower 850mb geopotential heights (m), nine show higher surface wind speeds (m.sˉ¹), seven show higher rain rates (mm.hrˉ¹), eight show higher Eddy Kinetic Energy (EKE) (m².sˉ²) and nine show greater upward moisture flux at the surface (g.mˉ².sˉ¹) compared to the SMTH run once each storm has propagated over the Current. Model output analysis shows sustained or dissipating storm intensity of the other 60 storms while passing over the Current. Nonetheless, these results provide a strong case for the influence of the Agulhas Current on the intensification of synoptic scale, rain producing events.
- ItemOpen AccessImpact of Wind Driven Variability on Sea Surface Temperature and Ocean Colour in False Bay(2019) Seymour, Sian; Krug, Marjolaine; Smith, Marie; Mouche, Alexis; Rouault, MathieuFalse Bay is the largest true bay in South Africa and is an important area for conservation, the local fishing industry and marine based recreational activities. A large amount of studies, both recent and historical, have been carried out on the biology of the bay, but studies on the physics of the bay are very few in comparison. In this study high resolution satellite imagery is used to investigate wind variability and its impact on sea surface temperature (SST) and chlorophyll concentration (Chl-a) variability within False Bay and the Cape Peninsula region. High resolution (1 km) coastal winds derived from the Sentinel-1 satellite Synthetic Aperture Radar (SAR) show that winds are strongly influenced by topography under the predominantly south-easterly wind regime. The Hottentots Holland mountain range and Cape Peninsula mountain range create wind shadows as well as areas of increased wind speed within False Bay and west of the Cape Peninsula. Our observations also show that global atmospheric models, such as ECMWF, are not able to capture the spatial variability in the wind fields driven by the orography. Analyses of the SST and ocean colour imagery show that wind shadows are generally associated with warmer surface waters and higher Chl-a. In contrast, regions of enhanced wind speeds show colder surface waters and decreased chlorophyll concentration. Our results suggest that spatial variation in the horizontal wind fields have direct and significant impact on the water properties within False Bay. This study highlights the need for high resolution wind observations and simulations to force regional oceanic models of False Bay and the Cape Peninsula region.
- ItemOpen AccessRevisiting the links between the Southern Annular Mode and rainfall over the Western Cape region of South Africa(2018) Mahlalela, Precious; Reason, Christopher; Blamey; RossThe winter rainfall region of South Africa displays considerable interannual variability and prevalence to prolonged dry periods. Although not completely understood, a wide range of factors have been highlighted to contribute to this interannual variability. The relatively poor understanding of rainfall variability in this region is of concern considering the low rainfall received in 2015-2017, resulting in the City of Cape Town enforcing severe water restrictions due to dam levels falling dangerously low. The focus of this thesis is on the influence of the Southern Annular Mode (SAM) on rainfall over the region, the possible influence of El Niño Southern Oscillation (ENSO) is also considered. To achieve this, a correlation analysis was conducted using the Marshal (2003) SAM index and station rainfall anomalies over the region for the period 1979 to 2016. The results show that five (three) of the six driest (wettest) years were associated with a positive (negative) SAM phase. However, the relationship is found to be statistically insignificant at a 95% significance level. The relationship is also found to show spatial variability, with strong negative correlations over the West Coast, while a weak positive correlation is observed over the South Coast. Furthermore, a decadal analysis in the relationship found it to be statistically insignificant (at the 95 th significance level) for most of the study period, with an exception of the early winter over the West Coast which shows a strong negative correlation after 2015. A composite analysis showed that dry (wet) winters tend to be associated with a positive (negative) SAM pattern superimposed with a wave number 3 anomaly. In addition, there are La Niña (El ivNiño) – like SST anomalies in the tropical Pacific. These circulation and SST patterns are more or less observed during the generally dry 2015-2017 winters except that winter 2015 shows an El Niño SST anomaly.
- ItemOpen AccessSeasonal and long-term change in the abundance, accumulation and distribution of beach litter within Table Bay, Cape Town, South Africa(2021) Opie, Brandon; Ryan, Peter; Moloney, ColeenThere is growing global concern with regard to the pollution of the world's ocean, particularly by marine debris and plastics. The daily accumulation rates of stranded beach litter were measured at two sites within Table Bay, repeating similar studies from 1994/95 and 2012. Milnerton is a popular recreational beach near the city, while Koeberg is a seldom visited beach in a nature reserve 39 km from the city. Daily sampling was conducted for ten days in winter (August), spring (October) and summer (November-December) 2019. Of the 39 602 items (116.6 kg) sampled in 2019, plastics (including expanded polystyrene) dominated at both sites in terms of numbers (Milnerton: 97.8 %; Koeberg: 98.7%) and mass (Milnerton: 45.2%; Koeberg: 58.9%). The accumulation rates were generally an order of magnitude greater at Milnerton than Koeberg. Plastics were dominated by single-use items (eg: expanded polystyrene clam shells, food wrapping and straws) and Milnerton's composition showed that there was a strong urban influence on the debris. Statistical analyses indicated there were large seasonal differences in accumulation rates at both sites. Milnerton's accumulation rate was ~8 times greater in winter (801.8 items·100 m-1 ·day-1 ) than in spring (97.4 items·100 m-1 ·day-1 ) and summer (86.4 items·100 m-1 ·day-1 ) in 2019. The winter peak was attributed to increased rainfall, which flushed the rivers, and to the reduced cleaning efforts in the catchments in the winter. The marine debris at Koeberg consisted of proportionally more buoyant items than Milnerton, items which can be transported vast distances, and debris at both sites was predominantly of local land-based origin. Across most sample years (1994/95, 2012 and 2019) and seasons (winter, and summer) Milnerton had significantly greater accumulation rates (min winter 1994/95: 286.7 items·100 m-1 ·day-1 to max winter 2019: 801.8 items·100 m-1 ·day-1 ; min summer 2019: 86.4 items·100 m-1 ·day-1 to max summer 2012: 1698.0 items·100 m-1 ·day-1 ) than Koeberg (min winter 2019: 55.9 items·100 m-1 ·day-1 to max winter 1994/95: 129.3 items·100 m-1 ·day-1 ; min summer 2019: 45.7 items·100 m-1 ·day-1 to max summer 2012: 151.4 items·100 m-1 ·day-1 ), attributed to many more sources of debris. Across all sample years, both sites had significantly greater winter accumulation rates than summer. A large decrease was seen in summer at both sites from 2012 to 2019, with a 95% (Milnerton) and 70% (Koeberg) reduction in total accumulation rates. The commencement of municipal cleaning efforts in the catchment areas and along the adjacent beach areas in the spring, which continued into summer, was likely a contributing factor to the decreases. Plastics (including expanded polystyrene) dominated the marine debris composition at both sites across all years and seasons and their proportions at both beaches have increased since 1994/95 from approximately 80 % to 95 %. It is evident that plastics are still prevalent in the environment. Improving waste management facilities and implementing effective cleaning measures throughout the year seem to be effective ways to reduce the marine debris problem. There is a need to shift away from single-use plastic items (such as straws, earbuds and food packaging) and to find more sustainable alternatives.
- ItemOpen AccessThe variability of retention in St Helena Bay(University of Cape Town, 2020) Manyakanyaka, Anathi; Jackson-Veitch, Jennifer; Rouault, MathieuThe circulation in St Helena Bay and the variability of the retention of the Bay are investigated using seasonal climatologies of the Regional Ocean Modelling System (ROMS). While retention has been studied biologically, the seasonality of the hydrodynamics contributing to the retention have received less attention. In this study we explore how the sea temperature, atmospheric forcing and currents contribute to the seasonal recirculation dynamics in St Helena Bay. Ichthyop, a lagrangian particle tracking method is used to study the spatial variations of local retention rates, with the particles released from the Bay. The circulation on the shelf of the west coast is dominated by upwelling dynamics with the equatorward boundary current, the Benguela Current located just off the shelf. St Helena Bay is protected from the direct impact of the Benguela current by coastal geographical features. A cyclonic circulation pattern is observed in the bay especially in autumn and winter. However, the results suggest that the recirculation patterns are prominent in summer and spring due to the intensification of the Benguela Jet and the nearshore southward current flows along the coast. Similar cyclonic features are observed at 100 m depth in the water column. An analysis of the particle tracking reveals that more drifters are retained in winter than in summer, supported by what is observed in the circulation patterns. Moreover, more drifters are retained in the surface waters than the deep waters.