Impacts of algal blooms in a temperate Estuarine fish nursery area, South Africa
- Authors: Bornman, Eugin
- Date: 2022-04
- Subjects: Cyanobacterial toxins --Environmental aspects , Estuarine eutrophication --South Africa
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/55899 , vital:54398
- Description: Anthropogenically-derived eutrophication and Harmful Algal Blooms (HABs) are an increasing threat to estuarine ecosystems globally. The rapid population growth in coastal areas and the mismanagement of water resources have led to the rapid increase in eutrophication in estuaries. Consequently, HABs have become frequent phenomena with severe impacts on the ecological functioning of these systems. A paucity exists in understanding the specific mechanisms of how HABs impact estuarine communities globally. This has not been studied in South Africa, despite the observed increase in the occurrence of HABs in the past few decades. Therefore, this study investigated the impact of HABs on the ecological functioning of the anthropogenically altered Sundays Estuary, South Africa, which is affected by predictable and recurrent HABs of the raphidophyte, Heterosigma akashiwo. This study found that a high density (> 1000 cell m L-1 ) of H. akashiwo was present during all but the winter months in the poly- to mesohaline zone, yet it peaked in warmer spring/summer months with a density of ca. 70 000 cell m L-1 corresponding to 800 µg Chl-a L -1 . This species significantly altered the estuary's water quality, causing supersaturated surface- and hypoxic bottom-water during severe HABs or referred to as hypereutrophic bloom phases (> 60 µg Chl-a L -1 ). Despite these changes, the common copepod, Pseudodiaptomus hessei, occurred at similar densities among HAB phases and responded to similar triggers to the HAB. However, a shift in predator-prey interactions within the zooplankton community was evident, with scavenger or opportunist species such as the isopod, Cirolana fluviatilis, and the polychaete, Perinereis falsovariegata, occurring at higher densities during hypereutrophic bloom phases. The overall abundance of juvenile and small adult fishes were high during HABs, but the diversity decreased during HABs. Some species, notably the Mugilidae occurred at lower densities during HABs. Mugilidae also had pronounced histological gill alterations and a lowered proportion of secondary lamellae available for gas exchange during an in situ cage experiment where fishes were isolated inside and outside the HAB. Deformities such as tumorous growths were also found for the first time in a South African Estuary. This indicated a stressed population of Mugilidae in the Sundays Estuary. The ubiquitous mugilid, Mugil cephalus, actively avoided HABs during an acoustic telemetry study, despite having a home range that overlapped the HAB area. Tagged individuals mostly occupied areas directly above the HAB in the upper mesohaline to lower oligohaline zones and only iv briefly transitioned through the HAB area to utilise the lower reaches of the estuary. Since the formation of HABs is reliant on various factors, including specific species responses, environmental variables, nutrient enrichment, and flow characteristics, the impacts of HABs on estuarine communities may vary between estuarine systems or affected species. Therefore, more research that focuses on the impacts of HABs on estuaries is needed to limit the potential loss of ecosystem services. Particularly in light of further unsustainable development, upstream freshwater abstraction and climate change, will cause more frequent and severe HABs, that may result in fish population declines in the long-term. Better management and mitigation strategies are needed to reduce HAB formation such as reducing nutrient enrichment and restoring natural freshwater flow and flooding events to the Sundays Estuary. , Thesis (PhD) -- Faculty of Science, School of Environmental Sciences, 2022
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- Date Issued: 2022-04
An appraisal of warm temperate mangrove estuaries as food patches using zooplankton and RNA: DNA ratios of Gilchristella aestuaria larvae as indicators
- Authors: Bornman, Eugin
- Date: 2018
- Subjects: Mangrove swamps Fishes -- Larvae , Mangrove ecology , Zooplankton
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/17908 , vital:28481
- Description: Mangrove habitats are considered as the ideal fish nursery as they are known to increase the growth and survival of juvenile fishes by providing enhanced food availability and protection. However, most studies have focused on tropical mangroves with a few recent warm temperate studies finding conflicting results. Furthermore, the nursery value of South African mangroves to fishes remain understudied in subtropical areas, while warm temperate mangroves are yet to be evaluated. This study aimed to assess whether mangrove presence leads to any advantage to the larvae of an important estuarine resident fish species, Gilchristella aestuaria, by comparing the food patch quality of South African warm temperate mangrove and non-mangrove estuaries. Results indicate that larvae fed primarily on the dominant prey species, Pseudodiaptomus hessei, Paracrtia longipatella, and Acartiella natalensis. However, postflexion larvae consumed more of the larger species, P. hessei, within the two mangrove estuaries (16.09 %V in Nahoon and 13.79 %V in Xhora) than the two nonmangrove estuaries (12.20 %V in Gonubie and 7.05 %V in Qora), despite other prey species occurring at similar densities. Results indicate that mangrove habitats acted as sediment sinks, slightly reducing the turbidity of these estuaries which resulted in postflexion larvae actively selecting larger, more nutritious prey, which in turn, significantly increased their individual instantaneous growth rates (0.11 ± 0.21 Gi) when compared to postflexion larvae in non-mangrove estuaries (0.09 ± 0.12 Gi). This study found that mangrove presence was significantly related to postflexion larval densities when coupled with abiotic (such as temperature and turbidity) and biotic factors (such as predator-prey interactions). Understanding the spatial and temporal dynamics, predator-prey interactions as well as the growth and survival of G. aestuaria is particularly important as they are key zooplanktivores that are prey to other species in estuarine food webs.
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- Date Issued: 2018