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:4877 Wed 12 May 2021 20:17:39 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:5904 Wed 12 May 2021 18:35:36 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10697 Wed 12 May 2021 18:32:28 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10614 5 m3.s-1). Salinity gradients from 30.1 PSU at the mouth to 2.2 PSU in the upper reaches were measured in 2006 after a high flow event. However the estuary quickly reverted back to its homogenous condition within weeks after this flood. This study showed that freshwater inflow increased nutrient input to the estuary. Total oxidised nitrogen (TOxN) and soluble reactive phosphorus (SRP) concentrations were higher in August 2006, after the flood, than during the other low flow sampling sessions. TOxN decreased from a mean concentration of 21.6 μM in 2006 to 1.93 μM in February 2009. SRP decreased from 55.3 μM to 0.2 μM respectively. With the increased nutrient availability, the response in the estuary was an increase in phytoplankton biomass. After the 2006 floods the average water column chlorophyll-a was 9.0 μg l-1, while in the low freshwater inflow years it ranged from 2.1 to 4.8 μg l-1. The composition of the phytoplankton community was always dominated by flagellates and then diatoms, with higher cell numbers in the nutrient-enriched 2006 period. Although the water column nutrient data indicated that the estuary was oligotrophic, benthic microalgal biomass (11.9-16.1 μg.g-1) in the intertidal zone was comparable with nutrient rich estuaries. Benthic species indicative of polluted conditions were found (Nitzschia frustulum, Navicula gregaria, Navicula cryptotenelloides). These benthic species were found at the sites where wastewater / sewage seepage had occurred. Benthic diatom species also indicated freshwater inflow. During the high flow period in 2006 the dominant diatoms were fresh to brackish species that were strongly associated with the high concentrations of TOxN and SRP (Tryblionella constricta, Diploneis smithii, Hippodonta cf. gremainii, and Navicula species). During the freshwater limited period of 2008 and 2009 the benthic diatom species shifted to a group responding to the high salinity, ammonium and silicate concentrations. The species in this group were Nitzschia flexa, Navicula tenneloides, Diploneis elliptica, Amphora subacutiuscula and Nitzschia coarctata. Ordination results showed that the epiphytic diatom species responded to different environmental variables in the different years. Most of the species in 2008/2009 were associated with high salinity, temperature, dissolved oxygen, ammonium and silicate concentrations while the response was towards TOxN and SRP in 2006. The dominant species were Cocconeis placentula v euglyphyta in 2006; Nitzschia frustulum in 2008; and Synedra spp in 2009. The average biomass of the epiphytes was significantly lower in May 2008 than in both August 2006 and February 2009; 88.0 + 17.7 mg.m-2, 1.7 + 0.8 mg.m-2, and 61.8 + 14.4 mg.m-2 respectively. GIS mapping of past and present aerial photographs showed that submerged macrophyte (Zostera capensis) cover in 1966 and 1973 was less than that mapped for 2004. Salt marsh also increased its cover over time, from 86.9 ha in 1966 to 126 ha in 2004, colonizing what were bare sandy areas. Long-term monitoring of the health of the Bushmans Estuary should focus on salinity (as an indicator of inflow or deprivation of freshwater), benthic diatom identification and macrophyte distribution and composition (for the detection of pollution input), and bathymetric surveys (for shallowing of the estuary due to sedimentation).]]> Wed 12 May 2021 17:53:52 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5820 Wed 12 May 2021 17:15:36 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10629 Wed 12 May 2021 16:19:53 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5263 20 orders and >35 families. The copepod Pseudodiaptomus hessei dominated (59 %) the zooplankton and occurred in similar densities to those observed in other South African estuaries. Larval fish and zooplankton varied across seasons, peaking simultaneously in summer although zooplankton showed additional density peaks during the closed phase of some estuaries. Both plankton components were more abundant in the oligohaline and mesohaline zones within the estuaries. Freshwater input, estuary type and the biogeography of the area influenced the composition and structure of larval fish and zooplankton assemblages in these estuaries. The findings suggest that the estuaries are functioning as successful breeding areas for the larvae of endemic estuary-resident fish species and that these estuaries have to be managed to ensure an adequate freshwater supply to maintain the biological integrity of the ecosystem, specially the maintenance of the highly productive River-Estuary Interface (REI) regions.]]> Wed 12 May 2021 15:56:36 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:24324 Wed 12 May 2021 15:55:35 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10635 Wed 12 May 2021 15:49:34 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:27029 Thu 13 May 2021 06:18:08 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:15021 Thu 13 May 2021 05:52:19 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:8992 Thu 13 May 2021 01:19:04 SAST ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:10610 Thu 13 May 2021 01:11:09 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 ]]> https://commons.ru.ac.za/vital/access/manager/Repository/vital:5745 Mon 30 Aug 2021 14:30:22 SAST ]]>