Towards an improved understanding of episodic benthic turbidity events (Benthic Nepheloid Layer) on the Eastern Agulhas Bank, South Africa
- Authors: Johnstone, Brett Mordaunt
- Date: 2022-10-14
- Subjects: Nepheloid layer , Turbidity , Loligo reynaudii , Fisheries South Africa , Oceanography , Remote sensing , Altimetry , Climatic changes
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362883 , vital:65371
- Description: The harvest of Loligo reynaudii, or "chokka," represents a critical source of revenue and job creation in the historically impoverished Eastern Cape Province of South Africa. Due to the importance of visual stimuli in the reproductive processes, it has been hypothesized that a primary driver of successful reproduction is the clarity of the water column. The presence of increased particulate matter concentrations within the water column generates turbid conditions near the seafloor (visibility < 1m), that are proposed to restrict spawning activity. This benthic nepheloid layer (BNL) contains both organic and inorganic components, with the BNL intensity a function of bottom turbulence, substratum type, and detritus level. However, the spatial and temporal resolution of BNL intensity on the Eastern Agulhas Bank (EAB) and the environmental drivers thereof remain unknown. Here we show that benthic turbidity events are a common but highly variable occurrence on the EAB. Results from a 17-month time-series of in-situ and remote sensing data between 2002 – 2004 in Algoa Bay, supplemented by experiments in other bays important for spawning, show that turbid conditions existed for ∼ 30 % of the sample period. Exploration of environmental drivers, including the influence of wind, altimeter-derived significant wave height (Hs), sea surface temperature (SST), and chlorophyll-a (Chl-a) concentrations indicate that BNL intensity does not conform to a "one-size-fits-all" approach. Rather, complex local hydrological and physiochemical parameters control the BNL characteristics on the EAB. Global warming is likely to increase the frequency and intensity of extreme westerly-wind and storm events, promoting BNL events on the Eastern Agulhas Bank and possibly causing a shift in the reproductive strategy of chokka squid to the cooler mid shelf region. This is likely to have consequences for both the species in terms of reproductive success and the fishery, which is concentrated on inshore spawning aggregations. Future research needs to quantify and characterize the constituents, source particles and spatial-temporal variability of BNL events in order to build a predictive capacity. Through incorporating the qualitative analysis of the dynamics of nepheloid layers on the EAB into Regional Oceanographic Models (ROMS), General Linear Models (GLM) and particle distribution models such as DELFT-3D, it is possible to move toward predicting the timing and intensity of these events. , Thesis (MSc) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Johnstone, Brett Mordaunt
- Date: 2022-10-14
- Subjects: Nepheloid layer , Turbidity , Loligo reynaudii , Fisheries South Africa , Oceanography , Remote sensing , Altimetry , Climatic changes
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362883 , vital:65371
- Description: The harvest of Loligo reynaudii, or "chokka," represents a critical source of revenue and job creation in the historically impoverished Eastern Cape Province of South Africa. Due to the importance of visual stimuli in the reproductive processes, it has been hypothesized that a primary driver of successful reproduction is the clarity of the water column. The presence of increased particulate matter concentrations within the water column generates turbid conditions near the seafloor (visibility < 1m), that are proposed to restrict spawning activity. This benthic nepheloid layer (BNL) contains both organic and inorganic components, with the BNL intensity a function of bottom turbulence, substratum type, and detritus level. However, the spatial and temporal resolution of BNL intensity on the Eastern Agulhas Bank (EAB) and the environmental drivers thereof remain unknown. Here we show that benthic turbidity events are a common but highly variable occurrence on the EAB. Results from a 17-month time-series of in-situ and remote sensing data between 2002 – 2004 in Algoa Bay, supplemented by experiments in other bays important for spawning, show that turbid conditions existed for ∼ 30 % of the sample period. Exploration of environmental drivers, including the influence of wind, altimeter-derived significant wave height (Hs), sea surface temperature (SST), and chlorophyll-a (Chl-a) concentrations indicate that BNL intensity does not conform to a "one-size-fits-all" approach. Rather, complex local hydrological and physiochemical parameters control the BNL characteristics on the EAB. Global warming is likely to increase the frequency and intensity of extreme westerly-wind and storm events, promoting BNL events on the Eastern Agulhas Bank and possibly causing a shift in the reproductive strategy of chokka squid to the cooler mid shelf region. This is likely to have consequences for both the species in terms of reproductive success and the fishery, which is concentrated on inshore spawning aggregations. Future research needs to quantify and characterize the constituents, source particles and spatial-temporal variability of BNL events in order to build a predictive capacity. Through incorporating the qualitative analysis of the dynamics of nepheloid layers on the EAB into Regional Oceanographic Models (ROMS), General Linear Models (GLM) and particle distribution models such as DELFT-3D, it is possible to move toward predicting the timing and intensity of these events. , Thesis (MSc) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
- Full Text:
- Date Issued: 2022-10-14
Monitoring the impact of deforestation on an aquatic ecosystem using remote sensing: a case study of the Mngazana mangrove forest in the eastern cape province.
- Authors: Madasa, Akhona
- Date: 2020-12
- Subjects: Remote sensing , Mangrove forests , Climatic changes
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/20815 , vital:46598
- Description: Coastal mangrove vegetation at Mngazana continues to be threatened and reduced periodically due to unmonitored harvesting. Covering an area of 148ha, the Mngazana mangrove forest remains unreserved, thus, research on the Mngazana mangroves is essential in order to monitor their state and sustainable management. Since in-situ monitoring of mangrove areas is both challenging and time-consuming, remote sensing technologies have been used to monitor these ecosystems. This study was carried out to monitor the impact of deforestation using ASTER satellite images over ten years: from 2008 - 2018. Validation was carried out by comparing classification results with the ground-referenced data, which yielded satisfactory agreement, with an overall accuracy of 94.64 percent and Kappa coefficient of 0.93 for 2008; and in 2009, the overall accuracy was 88.62 percent and a Kappa coefficient of 0.85. While the overall accuracy of 95.08 percent and a Kappa coefficient of 0.92 for 2016 and 2018 were observed, the overall accuracy of 93.58 percent and a Kappa coefficient of 0.91 was yielded. NDVI and SAVI indices were used as monitoring indicators. The results obtained in the study indicated that the canopy density of the mangrove forest remained unchanged in the years under investigation. However, insignificant changes in canopy density were identified between 2009 and 2016. , Thesis (MSc) (Applied Remote Sensing & GIS) -- University of Fort Hare, 2021
- Full Text:
- Date Issued: 2020-12
- Authors: Madasa, Akhona
- Date: 2020-12
- Subjects: Remote sensing , Mangrove forests , Climatic changes
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/20815 , vital:46598
- Description: Coastal mangrove vegetation at Mngazana continues to be threatened and reduced periodically due to unmonitored harvesting. Covering an area of 148ha, the Mngazana mangrove forest remains unreserved, thus, research on the Mngazana mangroves is essential in order to monitor their state and sustainable management. Since in-situ monitoring of mangrove areas is both challenging and time-consuming, remote sensing technologies have been used to monitor these ecosystems. This study was carried out to monitor the impact of deforestation using ASTER satellite images over ten years: from 2008 - 2018. Validation was carried out by comparing classification results with the ground-referenced data, which yielded satisfactory agreement, with an overall accuracy of 94.64 percent and Kappa coefficient of 0.93 for 2008; and in 2009, the overall accuracy was 88.62 percent and a Kappa coefficient of 0.85. While the overall accuracy of 95.08 percent and a Kappa coefficient of 0.92 for 2016 and 2018 were observed, the overall accuracy of 93.58 percent and a Kappa coefficient of 0.91 was yielded. NDVI and SAVI indices were used as monitoring indicators. The results obtained in the study indicated that the canopy density of the mangrove forest remained unchanged in the years under investigation. However, insignificant changes in canopy density were identified between 2009 and 2016. , Thesis (MSc) (Applied Remote Sensing & GIS) -- University of Fort Hare, 2021
- Full Text:
- Date Issued: 2020-12
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