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A two-dimensional hydrodynamic model for the St Lucia Estuary mouth.Jaaback, Kathryn Margaret. January 1993 (has links)
The reduced fresh water input into the St Lucia Estuary combined with the increase
of sediment in the St Lucia Lake System has necessitated the implementation of a
dredging programme. To ensure the effectiveness of the dredging programme, the
behaviour of the sediment under various flow and tidal conditions needs to be
determined.
To establish how sediment will move, it is necessary to understand the hydrodynamics
of the estuary. To achieve this, a hydrodynamic model which can be linked to a
sediment transport model needs to be developed. Various existing types of
hydrodynamic and sediment transport models are reviewed, to determine their
suitability for the above purpose. Results of the analysis indicate that a two-dimensional
hydrodynamic model is required.
The two-dimensional hydrodynamic model developed is based on the momentum and
continuity equations for an unsteady, non-uniform, free-surface flow for an
incompressible fluid. The two dimensions are in the horizontal plane and flow is
averaged over the depth. The equations are non-linear and are not decoupled, thus
a numerical technique was needed to solve them. An Alternating Direction Implicit
technique has been used. Boundary conditions in the modelled region were specified
as flow velocity at the upstream boundary, and water levels, relative to the Mean
Lake Level, at the downstream boundary.
Two short simulations using hypothetical data were run on a 80826 IBM compatible.
Results of the simulation indicate two areas where irregularities in the model output
are a consequence of the use of hypothetical data in defining the boundary conditions.
Recommendations for the collection of data in order to improve and calibrate the
model are discussed. / Thesis (M.Sc.)-University of Natal, 1993.
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The dynamics of nano- and microplankton in the St. Lucia estuarine lake system, KwaZulu-Natal.Fru Azinwi, Nche-Fambo. 10 September 2014 (has links)
St. Lucia estuarine lake system has a history of episodic droughts and floods leading to a high variability
in physico-chemical characteristics which may influence the community structure of nano- and
microplankton that are the main primary energy synthesisers in aquatic systems. Originally, the St. Lucia
estuary and the Mfolozi River shared the same mouth with the Mfolozi River acting as the main source of
fresh water to the system and as stabiliser of the common inlet. Due to prolonged mouth closure from
2001-2012 and high evaporation rates, severe extended droughts and high salinities were experienced in
2001-2012.This project aimed to characterise community composition and biomass of nano-and
microplankton (autotrophic and heterotrophic) under the varying and extreme physico-chemical
conditions in order to characterize the energy basis of the planktonic food web in St. Lucia. Its main
objectives were to compare and add onto Johnson’s (1977) list of phytoplankton taxa in the system, to
estimate the biomass (carbon) of nano- and microplankton from cell counts and biovolume measurements,
and lastly to understand and establish trends in the change in community structure of these organisms
with the varying physico-chemical characteristics.
Nano- and microplankton samples were collected monthly from October 2010 to September 2011 at three
different sites: Lister’s Point, Charters Creek and the mouth representing the lakes and estuary Channel.
Chlorophyll a and physico-chemical parameters were also measured in situ during collection. In the lab,
samples were settled using the Utermöhl method and species were identified to at least genus level,
counted and cell measurements taken under an inverted microscope for biovolume calculations and
biomass thereof. Abundance in cells per liter and biomass (carbon) in pg/L was then analyzed from the
counts.
The nano-and microplankton groups recorded in the system were cyanobacteria, chlorophytes,
cryptophytes, dinoflagellates, ciliates and diatoms. Seventy eight phytoplankton taxa were identified
composed of 56 diatoms, eight green algae, one cryptophyte, seven cyanobacteria and six dinoflagellate
taxa. Nineteen ciliate taxa were also found. Only 12 of the diatom taxa identified in this study were listed
by Johnson (1977), none of the taxa in the other phytoplankton groups was listed by Johnson (1977). The
Johnson (1977) study conducted in the system from 1975-1977 listed the phytoplankton taxa occurring at
that time. There was no significant difference in the community composition, biovolume and biomass
between seasons hence no seasonal trend however, there were significant differences in the nano- and
microplankton community composition, biovolume and biomass at the three different sites of the system.
Cyanobacteria were the main taxa in the northern embayments dominating in abundance, biovolume and
biomass (biological variables), green algae and cryptophytes dominated in abundance, biovolume and
biomass in the Channels while in South Lake, green algae dominated in abundance but diatoms
dominated in biovolume and biomass. Ciliate biological variables were higher in the northern regions
than in the other parts of the estuary. The absence or limited grazing pressure of ciliate predators in the
northern region due to their inability to cope with the extreme salinities compared to the other parts of the
system explains why the northern embayments had the highest abundance, biovolume and biomass of
ciliates. Ciliates and heterotrophic dinoflagellates were the heterotrophs in this study.
Autotrophic:heterotrophic biomass ratio was lowest in the northern regions as heterotrophs had a higher
biomass there. This ratio was higher in the South Lake and the Channel. The lack of stratification and
generally high turbidity in the system made the system unfavourable for dinoflagellate growth. The higher
presence of ciliate predators in the South Lake and Channel probably accounts for the low heterotrophic
biomass hence high autotrophic:heterotrophic biomass ratio in South Lake and the Channel.
Nutrients were not limiting during this study and salinity was the main physico-chemical characteristic
accounting for the differences in nano- and microplankton biological variables. The Cyanothece bloom in
the northern region was primarily due to high salinities (>150) which also indicated unfavourable
conditions for other plankton types. The high diatom biomass in the southern lake was due to low
salinities (<28) which favoured their growth, whereas chlorophytes and cryptophytes dominated in the
Channel mainly due to low turbidity (median of 11.4NTU) and fresh water input from the Mfolozi
lowering salinities (<5).
The South Lake and Channel thus had the highest available energy for higher trophic level organism since
1) diatoms and green algae are the most favoured food source for phytoplankton grazers while the
cyanobacteria though most abundant are the least favoured food source leaving the northern lake with
smaller energy source for higher trophic level organisms and 2) The low autotrophic:heterotrophic
biomass ratio in the northern region leaves the region with a lower net carbon biomass than the other parts
of the system with a higher autotrophic: heterotrophic biomass. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.
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Dietary dynamics of two key fish species in the St Lucia estuarine system, South Africa.Dyer, David Clifford. January 2014 (has links)
Among the 155 species of fish recorded so far in the St Lucia estuarine lake, Oreochromis
mossambicus and Ambassis ambassis are the two most prominent. Although originally
endemic to southern Africa, O. mossambicus is now one of the most widely distributed exotic
fish species worldwide. Together with A. ambassis, they have become the dominant fish
species in the St Lucia estuarine lake since the closure of the mouth in 2002 and are,
therefore, a crucial component of the food webs throughout the system. After a decade
dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically
in terms of prevailing environmental conditions, as a result of higher than average rainfall at
the end of 2011 and the onset of a new wet phase at the start of 2012. In response, A.
ambassis, which prefers lower salinity regimes, has expanded its distribution range
throughout the estuarine lake. Stable δ¹⁵N and δ¹³C isotope analysis was used in conjunction
with gut content analysis to elucidate the diet of these species at sampling localities spanning
the geographical range of the system and determine whether these species shift their diet in
response to environmental or climatic shifts. From both studies it is evident that from a
temporal and spatial scale these two species adopt similar, yet very different, dietary tactics.
Oreochromis mossambicus was shown to adopt a generalist feeding strategy,
opportunistically feeding on dietary items that are available thus allowing this species to alter
its diet according to the environment that it inhabits. Trophic positioning of this species was
found to be controlled by salinity in St Lucia as dietary composition differed greatly between
sites. In contrast, Ambassis ambassis displayed a more specialist dietary composition, feeding
predominantly on zooplankton. However, this species also opportunistically supplements its
diet with additional sources when available. Trophic position of A. ambassis was higher in
the dry season owing to the increased productivity of the system during the wet season. The
success and dominance of both species in the St Lucia system can therefore be attributed to
their dietary strategies. Under extreme environmental conditions, O. mossambicus has the
added advantage of its wide tolerance of different environmental conditions, particularly
salinity, thus allowing it to proliferate. / M.Sc. University of KwaZulu-Natal, Durban 2014.
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Diversity of bivalve molluscs within the St Lucia estuarine system, with emphasis on the ecophysiology of Solen cylindraceus and Brachidontes virgiliae.Nel, Holly Astrid. 17 June 2014 (has links)
The St Lucia estuarine system, Africa’s largest estuarine lake, is characterised by cyclic changes from
hypersaline to oligo/mesohaline conditions in response to alternations between drought and wetter
than average years. In addition, St Lucia also experiences stochastic disturbances, such as flooding
events that rapidly decrease salinity levels. Due to their sessile and slow moving nature, bivalves are
particularly vulnerable to rapid or prolonged changes in the physico-chemical environment. The
recent freshwater deprivation crisis that prevailed for the last decade resulted in a significant loss in
bivalve species richness within the system. An annotated and illustrated bivalve census revealed the
occurrence of twenty-four species within St Lucia between the years 1925 and 2011. However, only
six species were recorded during the most recent survey in March 2011. The infaunal razor clam,
Solen cylindraceus, and the epifaunal brackwater mussel, Brachidontes virgiliae, are currently the
dominant bivalve species within St Lucia. This study, therefore, aimed to record the species richness
of bivalves found in Lake St Lucia and to investigate key biological aspects of the two dominant
bivalve taxa within the system, under different salinity regimes. Experiments revealed that S.
cylindraceus can tolerate salinities between 15 and 65, while B. virgiliae prefers salinity levels
ranging from freshwater to 20. The varying tolerance limits, therefore, dictate the distribution of these
species during different climatic conditions within the estuarine lake. During wet periods, S.
cylindraceus is restricted to the northern reaches, unable to tolerate the oligohaline conditions present
in the rest of the system. Conversely, B. virgiliae, often restricted to the Narrows, becomes ubiquitous
throughout the system under such conditions. Solen cylindraceus can reach a maximum length of 95
mm. However, in the St Lucia estuarine system, specimens seldom exceed a length of 55 mm,
probably because prevailing/re-occurring harsh conditions prevent them from reaching maximum
size. In situ measurements of this species also revealed less growth during the first year of life than for
the same species in different systems. While B. virgiliae is substantially smaller than S. cylindraceus,
the high densities that this species is able to attain makes it an important grazer with the potential to
have significant feeding impacts on the local phytoplankton biomass. Results showed that in localised
areas, B. virgiliae populations may consume up to eight times the available phytoplankton biomass.
These key bivalve species are strongly influenced by the fluctuation in climatic conditions from wet to
dry phases. Thus, understanding the effects that climatic shifts have on key estuarine species is
essential, as flood and drought events are predicted to increase in frequency, intensity and duration as
a result of global climate change. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2014.
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