- Title
- Plankton dynamics in response to algal blooms in a highly eutrophic estuary in South Africa
- Creator
- Smit, Taryn
- Subject
- Algal blooms -- South Africa
- Subject
- Estuaries -- South Africa
- Date Issued
- 2020
- Date
- 2020
- Type
- Thesis
- Type
- Doctoral
- Type
- DPhil
- Identifier
- http://hdl.handle.net/10948/49303
- Identifier
- 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.
- Format
- xiv,107 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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| Thumbnail | File | Description | Size | Format | |||
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