https://commons.ru.ac.za/vital/access/manager/Index ${session.getAttribute("locale")} 5 https://commons.ru.ac.za/vital/access/manager/Repository/vital:29431 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.]]> Wed 12 May 2021 23:00:12 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10620 Wed 12 May 2021 19:31:50 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5921 Thu 13 May 2021 05:30:11 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10687 Thu 13 May 2021 01:55:52 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10597 Thu 13 May 2021 00:35:38 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10621 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.]]> Thu 13 May 2021 00:22:20 SAST ]]>