Lakes Magadi (Kenya) and Manyara (Tanzania) occupy closed basins in the southern Gregory Rift valley. Water in these lakes is presently shallow and saline, testifying to the dominance of evaporation (E) over precipitation (P). Past changes in the P: E ratio, and hence in palaeoclimate, can be reconstructed from evidence of the former extent of these lakes. Lake-level fluctuations engender marked variation in water chemistry, and consequently on the composition of the limnological biota. One approach is to examine the sedimentary record of diatoms (unicellular algae), which are excellent indicators of water chemistry and relative water depth, and whose modem distribution is sufficiently well known to allow the quantitative reconstruction of chemical parameters. Diatom analysis of 116 samples from a series of radiometrically dated (14C and U/Ib) sediment cores has revealed significant changes amongst the diatom assemblages during the Late Quaternary. Conductivity and pH have been estimated from the fossil samples by transfer functions (Gasse unpublished, Gasse 1986b). However, the interpretation of fossil diatom assemblages is often problematical in hypersaline environments. Difficulties arise as a result of the operation of taphonomic and diagenetic processes which can severely alter the composition of the diatom assemblagesfr om the ambient population at the time of deposition. Probably the most important factor responsible for assemblage diagenesis in saline lakes is silica dissolution, and this is explored further by a series of laboratory experiments. Results indicate that silica dissolution acts differentially between species, by removing the smaller, more delicate taxa first, and causing the relative enrichment of large robust forms in the fossil samples. A similar dissolution gradient may be reflected in modem samples studied near hot springs at Magadi. Differential dissolution is potentially an important source of error in palaeoenvironmental reconstructions, but, with the outcome of these experiments, it has been possible to assess the extent to which the dissolution process may have shaped the diatom records from Magadi and Manyara. The bulk of the palaeolimnological evidence is focussed upon two periods, 30,000-20,000 BP and 12,700-9,500 BP. The earlier period is most clearly dated in the core from Manyara, where the diatom record suggests the development of an intermediate level lake between c. 27,500 BP and c. 26,000 BP. This is a more complete representation of the same lake phase found in earlier studies from Manyara by Holdship (1976) based on diatoms, and by Casanova (1986a) on stromatolites 20M above the present lake. This time interval may also be represented by the central portion of the Magadi cores NF1 and NF2 but here dating is more problematical. The period 12,700-9,500 BP was one of major lacustrine transgression across Africa although the fine-structure of this event is less well known. Cores NF1 and NF2 from Magadi provide a detailed register of this phase indicating a major highstand from c. 12,700-11,000 BP when the lake became deep enough to stratify and deposit laminated couplets. At c. 11,000 BP the diatoms show that salinity increased greatly from fresh-oligosaline to meso-hypersaline which was probably a consequence of lake level falling.

The study has attempted to characterise the physicochemical limnology and distribution of algal flora of two salt lake systems in Western Australia, one from the coastal Esperance region and the other from the inland Kambalda region. Climatic conditions, water regimes and physicochemistry were found to differ markedly between the two lake systems and a total of 171 algal taxa, representing five divisions, were recorded. Of these, 82 were members of Bacillariophyta, 48 of Cyanophyta, 33 of Chlorophyta, two of Euglenophyta and six of Dinophyta. The physical limnology of salt lakes in the Esperance region was seasonally variable, defined by climatic conditions. As such, the lakes investigated in the region exhibited a stable cycle of filling during winter and spring, and drying out in summer. Four of the lakes in the region could be classified as near-permanent, and one as seasonal on the basis of predictability and duration of filling. Seasonal fluctuations in water depth resulted in fluctuations in salinity levels. Salinity levels ranged from subsaline to hypersaline, and all the lakes in the region were alkaline. In addition, the lakes were well mixed in terms of oxygen and temperature, and were impacted by eutrophication from their catchments. They were either mesotrophic or eutrophic with respect to both nitrogen and phosphorus. In geological terms, lakes in the Esperance region were separated only recently from the ocean, and two lakes retain a connection with marine waters, one through a creek during years of high rainfall and one through hydrological interactions with groundwater of marine origin. In general, the algal communities of lakes in the Esperance region were similar to those of other Australian coastal salt lakes. / Diatoms and cyanobacteria were dominant in all lakes except the most eutrophic, Lake Warden, in which benthic green algae were most abundant. All algal species recorded were known for their wide geographic distribution and their distribution in Australian coastal waters. Characteristically coastal diatom species included Achnanthes brevipes, Achnanthes coarctata, Achnanthes lanceolata var. dubia, Achnanthidium cruciculum, Campylodiscus clypeus, Cyclotella atomus, Cyclotella meneghiniana, Cyclotella striata, Mastogloia elliptica, Mastoglia pumila, Nitzschia punctata and Thalassiosira weissflogii. The inland salt lakes of the Kambalda region form part of an extensive palaeodrainage system, and were much less predictable in terms water regime than lakes in Esperance. Water depth was determined by seasonal variability in rainfall and evaporation, and by summer cyclonic rainfall events that were unreliable from year to year. In addition, rainfall varied spatially within the region. As such, most lakes were classified as intermittent. Two lakes in the region were not classified on the basis of water regime as they were too highly impacted by mining activities including water diversion and impoundment, water extraction and discharge of groundwater. Salinity varied in accordance with drying and filling cycles in the lakes except the most hypersaline as the volume of water received during rainfall events was insufficient to dilute the extensive surface salt crusts they each supported when dry. Salinities recorded in the region ranged from subsaline to hypersaline, and ionic compositions exhibited the same spectrum as seawater. / Calcium levels were significantly higher than in lakes from the Esperance region due to weathering of calcium rich sediments, and pH ranged from weakly acidic in the most hypersaline lakes to alkaline in the least saline lakes. All were well mixed in terms of oxygen and temperature. Kambalda salt lakes support distinctive algal communities dominated by diatoms and cyanobacteria that are adapted to intermittent water regimes, extended periods of desiccation and variable salinity. Not surprisingly then, none of the algal taxa recorded from the region were regionally restricted, all noted previously in the literature to have wide geographic distributions, and to be tolerant of a range of physicochemical conditions. Canonical correspondence analysis showed that, of the physicochemical parameters that were investigated in this study, both salinity and pH interacted in determining algal community structure. Both of these attributes were correlated with water depth, which varied according to climatic conditions in a seasonal drying and filling cycle. The general relationship between species richness and pH and salinity, and species diversity and pH and salinity was simple and linear; with increasing pH and salinity, species diversity and species richness decreased. What was less simple, and non-linear, was the nature of the relationship between species richness and diversity and salinity within more narrowly defined ranges of salinity. As salinity increased from <1ppt to 30ppt there was a dramatic reduction in species richness and diversity, then, as salinity increased from 30ppt to 100ppt the rate of decrease slowed. Between 100ppt and 250ppt there was almost no relationship between salinity and species richness and species diversity, but after 250ppt both species diversity and species richness declined markedly.

Thesis (Ph. D.)--Miami University, Dept. of Geology, 2007. / Title from second page of PDF document. Includes bibliographical references (p. 192-199).

Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.

Inland salt lakes in Western Australia have been used by the mining industry for the disposal of excess water generated during the mining process. However, the impact of these operations on the salt lakes is poorly understood. This is mainly due to the lack of information on the biota and chemistry for the lakes. The main aim of this project was to develop a classification system for inland salt lakes of Western Australia based on abiotic and biotic factors such as sediment and water quality, invertebrates and algae to determine lakes with unique or significant features. Water and sediments collected from the salt lakes were generally characterised by an alkaline pH, high salinity and the majority of lakes being dominated by sodium and chloride. Concentrations of some metals were also high, particularly in surface water. A high degree of variation in water and sediment quality was demonstrated both within and between the study lakes. In addition, these parameters were shown to be influenced by geography, geology, stage of the hydrocycle within which the lake was sampled and the occurrence of dewatering discharge. Biota in the salt lakes must be able to cope in a harsh environment, adjusting to temporary water regime, high temperature, and high salinity. As such, the species richness of these systems is generally low. Diatoms (a group of algae) and invertebrates were investigated among the biota. A total of 56 diatom species were recorded from 24 lakes. The most common species were Amphora coffeaeformis, Hantzschia aff. baltica and Navicula aff. incertata. These species were shown to have broad tolerances to environmental variations. Sediment chemistry explained variations in diatom community structure, with zinc, moisture content and cobalt having the greatest and negative influence. / In terms of invertebrates, a total of 101 invertebrate taxa were recorded from 13 lakes in this study. Crustacea dominated and the greatest number of taxa was from the genus Parartemia. There were some differences in invertebrate community structure between lakes, most likely reflecting the high degree of speciation, and poor dispersal mechanisms of certain key species. Community structure was influenced by water quality, with phosphorus, bicarbonate and magnesium contributing to the variations in community structure. Among the 43 lakes chosen for this study a total of 17 lakes had received, or are currently receiving dewatering discharge. Sites receiving dewatering discharge generally reported higher concentrations of salts, nutrients and some metals in both water and sediments compared to natural lakes. Species richness of biota such as diatoms and invertebrates was lower at the lakes receiving dewatering discharge. However, the impact was generally localized within the pooled area of dewatering discharge. Also, despite these impacts, there appears to be signs of amelioration by flushing events. Currently there are no guidelines for water and sediment chemistry for inland salt lakes in Western Australia. Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines are the most relevant available. Concentrations of cadmium, cobalt, chromium, copper, lead, nickel and zinc in surface water of the natural inland salt lakes were shown to exceed ANZECC guideline values. / Comparison with the relevant ANZECC sediment guidelines showed that they were applicable to the salt lakes, with the exception of nickel and chromium which were naturally high in the salt lake sediments. Classification of data using multivariate analysis was done for both dry and wet phases of the hydroperiod. Six groups were delineated for the sediment and diatom data, and four groups were defined for the water quality and invertebrate data. It was common for sites from particular lakes to fall in more than one group as a result of the variability in these systems. There are a number of practical applications of this system for the mining industry and it may be used as a predictive tool for determining the impact of dewatering discharge and highlighting unique salt lakes within the Goldfields of Western Australia.

This thesis examines playas as integrated ecosystems, through investigation of long- and short-term processes associated with playa geomorphology, hydrology and diatom ecology, in playas of the Yarra Yarra drainage system of Western Australia. This landscape approach was conducted at a range of scales and consequently revealed a level of heterogeneity not previously described in Australian playas. The key result and common thread linking the three facets of the playas investigated is the importance of hydroperiod in defining the playa environment. The morphology and distribution of playas are described and the results presented of an examination of potential mechanisms determining intra-system variability. Measurements of the physical attributes of the playas, including length, area, shape, density and orientation, were made through Geographic Information System analysis and detailed interpretation of aerial photography. Two main morphological groups are distinguished: small (<10 ha), elliptical to circular playas with a NNW-SSE orientation; and large (>30 ha), elongated playas. Regional patterns in geomorphic attributes are markedly different between the west and east sides of the system. The boundary between the two regions coincides approximately with rainfall distribution. In this respect, climate is an overarching driver of geomorphological variation but sub-catchment characteristics are also critical determinants. Littoral drift and segmentation processes that are common to coastal lagoons but not commonly described in playas were identified from visual interpretation of aerial photographs of spits, bars and cuspate shorelines. Six small playas, representing an hydrological continuum from mostly wet to mostly dry, were selected for investigation of their hydrology and diatom ecology, enabling assessment ii of the variability of hydrology, hydrochemistry and of the effects of different hydrological environments on the distribution of taxa. Hydroperiod was highly variable and central to determining playa geomorphology and biology. In the six playas it ranged from 19 to over 211 days, and filling frequency from 1 to 3 cycles between 2002 and 2004, reflecting rainfall and sub-catchment variability. Monitoring a series of nested piezometers revealed that the playas were net discharge points for ground water over the period of survey. However, small local vertical head variations indicate ground water does not discharge at the same rate across the playa surfaces and that playas may have short-lived phases of ground water recharge. Hydrochemically, the playas are typical of salt lakes in Australia. They displayed a wide range of salinity values, neutral to alkaline pH and ionic composition similar to sea water. It is postulated that the geochemical evolution of waters in the playas follows a pathway where low salinity recharge waters with dilute sea water salts progress to Na-Cl dominated brines through evaporative concentration. Surface waters showed an ionic dominance consistent with sea water with minor variations attributed to transitional phases in the geochemical evolution of the waters. Shallow ground waters showed a common and consistent pattern of ionic dominance: Na+ >Mg2+ >K+ >Ca2+ : Cl- >SO4 2- >HCO3 ->CO3 2-.

published_or_final_version / abstract / Earth Sciences / Master / Master of Philosophy

published_or_final_version / Applied Geosciences / Master / Master of Science

No description available.

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