Approaches to detecting and assessing patterns, processes and responses to change in South African estuaries
- Authors: Van Niekerk, Lara
- Date: 2018
- Subjects: Estuaries -- South Africa , Climatic changes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PHD
- Identifier: http://hdl.handle.net/10948/45491 , vital:38621
- Description: The research presented indicates deep knowledge of estuary pattern-process-function across a broad range of estuary types. This study successfully developed and applied approaches to detect change at varying temporal and spatial scales in South African estuaries. This assessment was based on the physical characteristics of estuaries and predicted or measured change in the drivers of estuary health. This is the first study in the country to review and synthesise the vulnerability of South African estuaries to Climate Change on a regional scale. Through sound scientific approaches this research has advanced our knowledge on complex estuarine systems and made a significant contribution globally to approaches for the conservation and management of estuaries.
- Full Text:
- Date Issued: 2018
- Authors: Van Niekerk, Lara
- Date: 2018
- Subjects: Estuaries -- South Africa , Climatic changes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PHD
- Identifier: http://hdl.handle.net/10948/45491 , vital:38621
- Description: The research presented indicates deep knowledge of estuary pattern-process-function across a broad range of estuary types. This study successfully developed and applied approaches to detect change at varying temporal and spatial scales in South African estuaries. This assessment was based on the physical characteristics of estuaries and predicted or measured change in the drivers of estuary health. This is the first study in the country to review and synthesise the vulnerability of South African estuaries to Climate Change on a regional scale. Through sound scientific approaches this research has advanced our knowledge on complex estuarine systems and made a significant contribution globally to approaches for the conservation and management of estuaries.
- Full Text:
- Date Issued: 2018
Phytoplankton chlorophyll a concentration and community structure in two temporarily open/closed estuaries in the Eastern Cape, South Africa
- Authors: Gama, Phumelele Thuthuka
- Date: 2008
- Subjects: Estuarine ecology -- South Africa , Chlorophyll -- South Africa , Estuaries -- South Africa , Phytoplankton populations -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10621 , http://hdl.handle.net/10948/707 , Estuarine ecology -- South Africa , Chlorophyll -- South Africa , Estuaries -- South Africa , Phytoplankton populations -- South Africa
- Description: River flow is important in controlling phytoplankton distribution in estuaries. Data on the effect of river inflow on phytoplankton distribution patterns in temporarily open/closed estuaries is lacking. This study investigated the influence of river inflow on size-fractionated phytoplankton biomass (Chl a), community composition and environmental parameters measured monthly over three years in two temporarily open/closed estuaries in the Eastern Cape, South Africa. A once-off primary production study over an annual cycle was completed in the Van Stadens and Maitland estuaries. The study monitored physical, chemical and biological characteristics in both estuaries to examine the effects of changes in environmental factors and river inflow. Daily sampling of physico-chemical and biological variables from river to sea was carried out in the Van Stadens to investigate short-time scale effects of changes in environmental factors and river inflow on the phytoplankton biomass. Five and three stations in the main channel of the Van Stadens and Maitland estuaries respectively were sampled at 0.5 m below the water surface and 0.5 m above the sediment surface for biological and chemical variables and at the surface, 0.25 m and every 0.5 m thereafter for physical parameters. Five stations adjacent to the main channel along the estuary were monitored for groundwater macronutrient concentrations and five additional sites located within the upper catchment of the Van Stadens River were sampled on a quarterly basis over two years. Both estuaries were characterised by distinct hydrological conditions, an overwash, an open, a closed and a semi-closed mouth phase. Flooding in the Maitland and Van Stadens estuaries in 2001 and 2002 caused sediment scour, altered channel morphology and brought about breaching of the mouth. Flood driven mouth-breaching events occurred three and four times in each of the estuaries during the study. The mouth stayed open 20 – 25 percent and was closed 60 – 65 percent of the time. In the Van Stadens the closed overwash mouth condition occurred approximately 10 – 20 percent of the time while in the Maitland it occurred less with the semi-closed mouth condition occurring 10 – 20 percent of the time. Incidents related to mouth opening not associated with strong river floods occurred approximately 10 – 15 percent of the time, although in the Maitland a semi-closed mouth state persisted more frequently than in the Van Stadens Estuary. During flooding events salinity dropped to low levels (< 5 psu) but soon recovered to brackish conditions when river flow was reduced and marine water penetrated deep upstream. Reduction in river flow combined with marine sediment deposition resulted in the closure of the mouth. During closed mouth conditions strong onshore storm surges and spring high tides introduced marine water through overwash that kept salinity high. In both estuaries salinity showed a negative correlation with rainfall (R2 = 0.12), indicative of the strong influence of marine overwash that kept salinity high thus masking the influence of freshwater. High rainfall in the Van Stadens Estuary caused high levels of turbidity that reduced light penetration at depth. Light attenuation was positively correlated with the high rainfall (R 2 = 0.26) suggesting that increased turbidity was linked to rainfall induced discharge. In contrast, in the Maitland Estuary light attenuation did not show any correlation with increased rainfall possibly because of the reduced water depth and increased euphotic zone following the floods in 2002. High river inflow introduced macronutrients in both estuaries such that dissolved inorganic phosphates (DIP) and dissolved inorganic nitrogen (DIN) concentrations in the Van Stadens Estuary were strongly correlated with rainfall (R2 = 0.78 and 0.57 respectively). In the Maitland Estuary DIP and DIN concentrations remained significantly higher (p < 0.05) compared to that in the Van Stadens suggesting that the Maitland catchment contributed greater nutrient input into the estuary and may be associated with farming activities. Phytoplankton chlorophyll a (Chl a) ranged from 0.8 – 13.9 μg L-1 in the Van Stadens and in the Maitland Estuary from 5.3 – 138 μg L-1 during the 3-year study. During the open mouth condition Chl a biomass and primary production ranged from 5.4 – 52.9 μg Chl a L-1 and 1.2 – 11.7 mg C m-2 d-1 in the Maitland and in the Van Stadens from 1.6 – 9.8 μg Chl a L-1 and 1.2 - 14 mg C m-2 d-1 respectively. Maximum annual primary production in the Maitland and Van Stadens estuaries was 8.8 and 5.1 g C m-2 y-1 respectively. When the mouth was open in the Van Stadens Estuary the microphytoplankton (> 20 μm) accounted for > 65 percent of the Chl a, whereas during closed mouth conditions they accounted for about 55 percent of the Chl a biomass. Chlorophytes became the dominant taxon in the dry summer months but were replaced by cryptophytes and dinoflagellates during the wet season. When nutrient concentrations were low during low flow conditions in the Van Stadens Estuary mixotrophic microphytoplankton became an important fraction of the water column together with phototrophic dinoflagellates and cryptophytes. In the Maitland large sized chlorophytes were the dominant taxa in late spring and summer seasons and made up more than 80 percent of the cell numbers. In the Maitland before the floods in 2002 cyanophytes were the dominant group in late spring contributing more than 75 percent in cell abundance. Data from the short-term study in the Van Stadens Estuary showed similarities and differences in the Chl a response to increased river inflow. High river inflow initially reduced Chl a biomass followed by a recovery period of a couple of days compared to a 8 – 10 week recovery period in studies monitored over seasonal and annual temporal scales. The responses may be dissimilar but help to illustrate that there are similar response patterns to environmental forcing necessary to support phytoplankton biomass at different temporal scales. This study has demonstrated that flooding events caused by strong river flow cause breaching of the mouth, a reduction in salinity and marked nutrient input. Although the causes of flooding can be similar in both estuaries the resultant effects are varied and can alter the ability of the estuary to retain water. This study was able to demonstrate that the supply of macronutrients from the catchment was strongly correlated with rainfall (R2 = 0.67) and that phytoplankton growth mainly depended on an allochthonous source of macronutrients although internal supplies could be critical at times in controlling microalgal biomass.
- Full Text:
- Date Issued: 2008
- Authors: Gama, Phumelele Thuthuka
- Date: 2008
- Subjects: Estuarine ecology -- South Africa , Chlorophyll -- South Africa , Estuaries -- South Africa , Phytoplankton populations -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10621 , http://hdl.handle.net/10948/707 , Estuarine ecology -- South Africa , Chlorophyll -- South Africa , Estuaries -- South Africa , Phytoplankton populations -- South Africa
- Description: River flow is important in controlling phytoplankton distribution in estuaries. Data on the effect of river inflow on phytoplankton distribution patterns in temporarily open/closed estuaries is lacking. This study investigated the influence of river inflow on size-fractionated phytoplankton biomass (Chl a), community composition and environmental parameters measured monthly over three years in two temporarily open/closed estuaries in the Eastern Cape, South Africa. A once-off primary production study over an annual cycle was completed in the Van Stadens and Maitland estuaries. The study monitored physical, chemical and biological characteristics in both estuaries to examine the effects of changes in environmental factors and river inflow. Daily sampling of physico-chemical and biological variables from river to sea was carried out in the Van Stadens to investigate short-time scale effects of changes in environmental factors and river inflow on the phytoplankton biomass. Five and three stations in the main channel of the Van Stadens and Maitland estuaries respectively were sampled at 0.5 m below the water surface and 0.5 m above the sediment surface for biological and chemical variables and at the surface, 0.25 m and every 0.5 m thereafter for physical parameters. Five stations adjacent to the main channel along the estuary were monitored for groundwater macronutrient concentrations and five additional sites located within the upper catchment of the Van Stadens River were sampled on a quarterly basis over two years. Both estuaries were characterised by distinct hydrological conditions, an overwash, an open, a closed and a semi-closed mouth phase. Flooding in the Maitland and Van Stadens estuaries in 2001 and 2002 caused sediment scour, altered channel morphology and brought about breaching of the mouth. Flood driven mouth-breaching events occurred three and four times in each of the estuaries during the study. The mouth stayed open 20 – 25 percent and was closed 60 – 65 percent of the time. In the Van Stadens the closed overwash mouth condition occurred approximately 10 – 20 percent of the time while in the Maitland it occurred less with the semi-closed mouth condition occurring 10 – 20 percent of the time. Incidents related to mouth opening not associated with strong river floods occurred approximately 10 – 15 percent of the time, although in the Maitland a semi-closed mouth state persisted more frequently than in the Van Stadens Estuary. During flooding events salinity dropped to low levels (< 5 psu) but soon recovered to brackish conditions when river flow was reduced and marine water penetrated deep upstream. Reduction in river flow combined with marine sediment deposition resulted in the closure of the mouth. During closed mouth conditions strong onshore storm surges and spring high tides introduced marine water through overwash that kept salinity high. In both estuaries salinity showed a negative correlation with rainfall (R2 = 0.12), indicative of the strong influence of marine overwash that kept salinity high thus masking the influence of freshwater. High rainfall in the Van Stadens Estuary caused high levels of turbidity that reduced light penetration at depth. Light attenuation was positively correlated with the high rainfall (R 2 = 0.26) suggesting that increased turbidity was linked to rainfall induced discharge. In contrast, in the Maitland Estuary light attenuation did not show any correlation with increased rainfall possibly because of the reduced water depth and increased euphotic zone following the floods in 2002. High river inflow introduced macronutrients in both estuaries such that dissolved inorganic phosphates (DIP) and dissolved inorganic nitrogen (DIN) concentrations in the Van Stadens Estuary were strongly correlated with rainfall (R2 = 0.78 and 0.57 respectively). In the Maitland Estuary DIP and DIN concentrations remained significantly higher (p < 0.05) compared to that in the Van Stadens suggesting that the Maitland catchment contributed greater nutrient input into the estuary and may be associated with farming activities. Phytoplankton chlorophyll a (Chl a) ranged from 0.8 – 13.9 μg L-1 in the Van Stadens and in the Maitland Estuary from 5.3 – 138 μg L-1 during the 3-year study. During the open mouth condition Chl a biomass and primary production ranged from 5.4 – 52.9 μg Chl a L-1 and 1.2 – 11.7 mg C m-2 d-1 in the Maitland and in the Van Stadens from 1.6 – 9.8 μg Chl a L-1 and 1.2 - 14 mg C m-2 d-1 respectively. Maximum annual primary production in the Maitland and Van Stadens estuaries was 8.8 and 5.1 g C m-2 y-1 respectively. When the mouth was open in the Van Stadens Estuary the microphytoplankton (> 20 μm) accounted for > 65 percent of the Chl a, whereas during closed mouth conditions they accounted for about 55 percent of the Chl a biomass. Chlorophytes became the dominant taxon in the dry summer months but were replaced by cryptophytes and dinoflagellates during the wet season. When nutrient concentrations were low during low flow conditions in the Van Stadens Estuary mixotrophic microphytoplankton became an important fraction of the water column together with phototrophic dinoflagellates and cryptophytes. In the Maitland large sized chlorophytes were the dominant taxa in late spring and summer seasons and made up more than 80 percent of the cell numbers. In the Maitland before the floods in 2002 cyanophytes were the dominant group in late spring contributing more than 75 percent in cell abundance. Data from the short-term study in the Van Stadens Estuary showed similarities and differences in the Chl a response to increased river inflow. High river inflow initially reduced Chl a biomass followed by a recovery period of a couple of days compared to a 8 – 10 week recovery period in studies monitored over seasonal and annual temporal scales. The responses may be dissimilar but help to illustrate that there are similar response patterns to environmental forcing necessary to support phytoplankton biomass at different temporal scales. This study has demonstrated that flooding events caused by strong river flow cause breaching of the mouth, a reduction in salinity and marked nutrient input. Although the causes of flooding can be similar in both estuaries the resultant effects are varied and can alter the ability of the estuary to retain water. This study was able to demonstrate that the supply of macronutrients from the catchment was strongly correlated with rainfall (R2 = 0.67) and that phytoplankton growth mainly depended on an allochthonous source of macronutrients although internal supplies could be critical at times in controlling microalgal biomass.
- Full Text:
- Date Issued: 2008
Plankton dynamics in response to algal blooms in a highly eutrophic estuary in South Africa
- Authors: Smit, Taryn
- Date: 2020
- Subjects: Algal blooms -- South Africa , Estuaries -- South Africa
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/49303 , vital:41619
- Description: The global rise in the incidence of harmful algal blooms in estuaries is a major concern given their importance as a nursery area for a variety of invertebrates and fishes. On the warm-temperate, south-east coast of South Africa, the Sundays Estuary experiences persistent, seasonal HABs due to stable eutrophic conditions. Spring blooms of the dinoflagellate, Heterosigma akashiwo, were recorded for several years in the estuary. This Raphidophyte is known for its consequences in fish and shellfish aquaculture and exhibits harmful properties such as mucilage and reactive oxygen species production. Subsequently, the dissertation investigated larval fish assemblage dynamics, larval prey and predator dynamics and body condition in association with H. akashiwo blooms. HABs mainly altered the water quality within the mesohaline zone in terms of microalgal biomass and dissolved oxygen. Hypereutrophic blooms of H. akashiwo reached 24 996 cells∙mL-1 during the study period. Supersaturated dissolved oxygen concentrations (up to ~13 mg∙L-1 at night and ~21 mg∙L-1 during the day) were recorded during hypereutrophic blooms (> 80 μg∙L-1, ≥ 2781 H. akashiwo cells∙mL-1). Dissolved oxygen differed significantly by bloom state, with the highest dissolved oxygen concentrations. Larval fish density, diversity and community structure variability were lower during hypereutrophic blooms compared to standard blooms and bloom absence, albeit statistically insignificant. Estuarine resident larval density was negatively correlated with dissolved oxygen concentration (P < 0.001). Dominant prey copepod of G. aestuaria larvae, P. hessei, was less abundant and variable during hypereutrophic blooms (mean: 664, range: 4-4301 individuals per m3) compared to standard blooms (mean: 950, range: 2-5237 individuals per m3) and bloom absence (mean: 1182, range: 23-7259 individuals per m3). Although HABs did not appear to drive a mismatch between prey and G. aestuaria larvae, early juvenile larvae made only a small contribution to the assemblage (0.2-1.8%). In terms of predator effects, yolk sac larvae were negatively related to R. terranatalis (P < 0.05). A significant positive relationship was found between prey, P. hessei and G. aestuaria preflexion (P < 0.001), flexion (P < 0.001) and postflexion (P < 0.05) larvae. In a world first, the effects of HABs were investigated on a larval fish species, Gilchristella aestuaria, using biochemical body condition analyses. During severe bloom conditions, G. aestuaria larvae were smaller and the assemblage consisted of mainly early developmental stages (stats). Body condition (RNA per dry weight) was poorest in the presence of hypereutrophic blooms (P < 0.001). A bell-curved relationship between RNA/DW and dissolved oxygen concentration was also produced in a 3-day time-lagged GAM (P < 0.001). In conclusion, hypereutrophic bloom conditions appeared to trigger adult G. aestuaria spawning in the upper mesohaline zone of the estuary in response to heightened primary productivity which occurs during the early bloom stage. However, reductions in copepod abundance (P. hessei) and unfavourable oxygen concentrations interfered with grow-out of G. aestuaria larvae. This research showed that HABs can alter nursery properties to the detriment of G. aestuaria larvae can have serious knock-on effects up the food chain. The loss in abundance of adult, juvenile and larval G. aestuaria, a key forage fish in South African estuaries, has major implications for estuarine food webs, especially in reduced prey availability to juvenile fishes and birds. There is still much to learn about the effects of HABs on the physiological and behavioural aspects on plankton communities worldwide.
- Full Text:
- Date Issued: 2020
- Authors: Smit, Taryn
- Date: 2020
- Subjects: Algal blooms -- South Africa , Estuaries -- South Africa
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/49303 , vital:41619
- Description: The global rise in the incidence of harmful algal blooms in estuaries is a major concern given their importance as a nursery area for a variety of invertebrates and fishes. On the warm-temperate, south-east coast of South Africa, the Sundays Estuary experiences persistent, seasonal HABs due to stable eutrophic conditions. Spring blooms of the dinoflagellate, Heterosigma akashiwo, were recorded for several years in the estuary. This Raphidophyte is known for its consequences in fish and shellfish aquaculture and exhibits harmful properties such as mucilage and reactive oxygen species production. Subsequently, the dissertation investigated larval fish assemblage dynamics, larval prey and predator dynamics and body condition in association with H. akashiwo blooms. HABs mainly altered the water quality within the mesohaline zone in terms of microalgal biomass and dissolved oxygen. Hypereutrophic blooms of H. akashiwo reached 24 996 cells∙mL-1 during the study period. Supersaturated dissolved oxygen concentrations (up to ~13 mg∙L-1 at night and ~21 mg∙L-1 during the day) were recorded during hypereutrophic blooms (> 80 μg∙L-1, ≥ 2781 H. akashiwo cells∙mL-1). Dissolved oxygen differed significantly by bloom state, with the highest dissolved oxygen concentrations. Larval fish density, diversity and community structure variability were lower during hypereutrophic blooms compared to standard blooms and bloom absence, albeit statistically insignificant. Estuarine resident larval density was negatively correlated with dissolved oxygen concentration (P < 0.001). Dominant prey copepod of G. aestuaria larvae, P. hessei, was less abundant and variable during hypereutrophic blooms (mean: 664, range: 4-4301 individuals per m3) compared to standard blooms (mean: 950, range: 2-5237 individuals per m3) and bloom absence (mean: 1182, range: 23-7259 individuals per m3). Although HABs did not appear to drive a mismatch between prey and G. aestuaria larvae, early juvenile larvae made only a small contribution to the assemblage (0.2-1.8%). In terms of predator effects, yolk sac larvae were negatively related to R. terranatalis (P < 0.05). A significant positive relationship was found between prey, P. hessei and G. aestuaria preflexion (P < 0.001), flexion (P < 0.001) and postflexion (P < 0.05) larvae. In a world first, the effects of HABs were investigated on a larval fish species, Gilchristella aestuaria, using biochemical body condition analyses. During severe bloom conditions, G. aestuaria larvae were smaller and the assemblage consisted of mainly early developmental stages (stats). Body condition (RNA per dry weight) was poorest in the presence of hypereutrophic blooms (P < 0.001). A bell-curved relationship between RNA/DW and dissolved oxygen concentration was also produced in a 3-day time-lagged GAM (P < 0.001). In conclusion, hypereutrophic bloom conditions appeared to trigger adult G. aestuaria spawning in the upper mesohaline zone of the estuary in response to heightened primary productivity which occurs during the early bloom stage. However, reductions in copepod abundance (P. hessei) and unfavourable oxygen concentrations interfered with grow-out of G. aestuaria larvae. This research showed that HABs can alter nursery properties to the detriment of G. aestuaria larvae can have serious knock-on effects up the food chain. The loss in abundance of adult, juvenile and larval G. aestuaria, a key forage fish in South African estuaries, has major implications for estuarine food webs, especially in reduced prey availability to juvenile fishes and birds. There is still much to learn about the effects of HABs on the physiological and behavioural aspects on plankton communities worldwide.
- Full Text:
- Date Issued: 2020
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