Estuarine microalgal bloom dynamics at multiple temporal scales
- Authors: Lemley, Daniel Alan
- Date: 2018
- Subjects: Ecological integrity -- South Africa , Phytoplankton populations -- South Africa , Estuarine ecology -- South Africa , Freshwater algae -- Ecology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/21044 , 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.
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- Date Issued: 2018
Assessing symptoms of eutrophication in estuaries
- Authors: Lemley, Daniel Alan
- Date: 2015
- Subjects: Estuarine eutrophication -- South Africa , Water quality -- South Africa , Ecological integrity -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/3427 , vital:20429
- Description: Water quality and ecological integrity of estuaries reflect activities within the entire upstream catchment. Much emphasis has been placed on the response of estuaries to anthropogenic stressors through the use of monitoring programmes. Key to the success of these programmes is the use of indicators as they transform data into useful information. The ultimate aim of this study was to identify eutrophic symptoms in selected estuaries in the Gouritz Water Management Area (WMA) using a multi-metric classification method. The study sites included the permanently open Duiwenhoks, Goukou, Gouritz, and Kaaimans estuaries, as well as the temporarily open/closed Hartenbos, Klein Brak, Great Brak, Gwaing, and Goukamma estuaries. Initially, the “pressure” associated with all the estuaries in the Gouritz WMA was determined (e.g. total daily nutrient loads and changes in river inflow affecting flushing time) using long-term flow and water quality monitoring data provided by the Department of Water Affairs (DWA). Subsequently, the “state” of the selected estuaries was assessed using a variety of indicators, including: inorganic nutrients (~ N and P), phytoplankton, epiphytes and microphytobenthos. Estuaries with longer flushing time suggested greater vulnerability to eutrophication. For example, the Gwaing (281.11 kg DIN d-1; 78.85 kg DIP d-1) and Hartenbos (38.33 kg DIN d-1; 21.51 kg DIP d-1) estuaries generally received the highest daily inorganic nutrient loads. However, at the time of sampling, the Hartenbos Estuary had a longer flushing time (i.e. weeks) compared to the Gwaing Estuary (i.e. couple of days). Field data confirmed the greater vulnerability to eutrophication associated with longer flushing times, as the indicators measured in the Hartenbos Estuary exceeded all the proposed ‘thresholds’ of a eutrophic system (~ overall ‘Poor’ condition). The Great Brak Estuary (~ overall ‘Fair’ condition) provided an interesting example where eutrophic symptoms were only detected when assessing the microalgal primary producers (i.e. high biomass and low diversity). Furthermore, the need to distinguish between naturally and anthropogenically induced symptoms was highlighted. One such scenario was noted in the Kaaimans Estuary, where its natural hydro-morphological characteristics (i.e. steep banks, low river inflow, and deep/narrow channel) led to the possible exaggeration of its overall eutrophic condition, i.e. received a ‘Fair’ rather than ‘Good’ rating, arising from ‘Poor’ ratings for dissolved oxygen and benthic diatom diversity. Overall, this study demonstrated the importance of adopting a holistic approach when assessing the condition and trophic status of estuaries. Studies such as these allow for the detection of vulnerable and degraded systems, which can provide important information with regards to the identification of management priorities.
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- Date Issued: 2015