- Title
- Estuarine microalgal bloom dynamics at multiple temporal scales
- Creator
- Lemley, Daniel Alan
- Subject
- Ecological integrity -- South Africa
- Subject
- Phytoplankton populations -- South Africa
- Subject
- Estuarine ecology -- South Africa
- Subject
- Freshwater algae -- Ecology -- South Africa
- Date Issued
- 2018
- Date
- 2018
- Type
- Thesis
- Type
- Doctoral
- Type
- PhD
- Identifier
- http://hdl.handle.net/10948/21044
- Identifier
- vital:29431
- Description
- Microalgal communities sustain the ecological functioning of estuarine ecosystems through the provision of numerous ecosystem services. As such, gaining an understanding of the processes that govern spatio-temporal patterns of phytoplankton communities is key to the effective management of these ecosystems. However, detailed investigations of estuarine phytoplankton dynamics, and particularly harmful algal blooms (HAB), are largely absent in South Africa. Therefore, geared at addressing these knowledge gaps, this study was initiated to elucidate the processes shaping estuarine phytoplankton bloom dynamics at multiple temporal scales. Initially, five permanently open estuaries, each with varying types and levels of anthropogenic pressures, were selected to test the efficacy of an estuarine eutrophic condition index to flow variability (‘Annual’ study). Results from this study highlighted the sensitivity of the proposed index to environmental perturbations (e.g. flood events), whilst also identifying freshwater inflow variability and anthropogenic disturbance as the key processes shaping microalgal responses at an annual scale. Next, the agriculturally-influenced Gamtoos and Sundays estuaries were further investigated to elucidate the role of natural and anthropogenic drivers in defining microalgal responses (‘Seasonal’ study). Despite similarities in nutrient input and temperature patterns, phytoplankton bloom conditions (> 20 μg Chl-a l-1) were episodic and flow-dependent in the Gamtoos Estuary, whilst those in the Sundays Estuary were persistent and seasonal. Related to its reduced hydrodynamic variability – i.e. consistent nutrient-rich baseflows and reduced propensity for flushing events – persistent undesirable disturbances were highlighted for the Sundays Estuary, including: summer bottom-water hypoxia (< 2 mg l-1) and exceptional proliferations (> 550 μg Chl-a l-1) of two HAB species (Heterosigma akashiwo and Heterocapsa rotundata). Finally, fine-scale ecological research was undertaken in the Sundays Estuary to identify the processes – abiotic and biotic – that facilitate HABs (‘Daily’ and ‘Hourly’ studies). Findings from the ‘Daily’ study, identified inorganic nutrient availability (i.e. nitrate and phosphate) and mesohaline conditions (ca. 10) as the key bottom-up controls influencing the magnitude and duration of spring/summer phytoplankton blooms. Additionally, bottom-water hypoxia was explicitly linked to the decay of a single HAB species. During the ‘Hourly’ investigation, four known HAB-forming species were recorded at bloom concentrations. Model results indicated that variability in temperature, salinity profiles and nitrate concentrations were significant in facilitating the occurrence of HAB species. Finally, local biotic interactions (e.g. interspecies competition, diel vertical migration and mixotrophy) were recognized as key mechanisms shaping phytoplankton communities. The persistent occurrence of HABs is a new feature in South African estuaries and continued research is needed to recommend management responses. Ultimately, this research highlights the multitude of processes at work shaping phytoplankton variability in estuaries. From a broad perspective (i.e. seasonal and annual), processes such as freshwater inflow regimes, degree of anthropogenic disturbance, as well as seasonal temperature and nutrient supply patterns are the key processes. At a more refined scale (i.e. hourly and daily), local processes including salinity preferences, nutrient availability, diel light cycles and internal biotic interactions are the key drivers organising phytoplankton dynamics. Given the potentially severe ecological consequences of disrupting natural phytoplankton dynamics (e.g. HABs), an element of ‘unpredictability’ should be restored to the hydrological and chemical makeup of highly-regulated estuaries to prevent the continued exacerbation of eutrophic symptoms.
- Format
- xi, 119 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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