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Lake Ellesmere/Te Waihora is one of New Zealand’s largest coastal, brackish water lakes. It has nationally significant wetland bird populations and is regionally important for iwi. The lake regularly experiences fluctuations in water level, resulting in a continually expanding and contracting littoral zone. This study investigated the impacts of these water level changes on the ecology of the lake. Water chemistry results collected over 12 months, confirm the lake is hypertrophic, due to high nutrient (nitrogen and phosphorus) concentrations resulting in high chlorophyll a levels and low water clarity. Water chemistry conditions were collected at five locations around the lake and showed marked spatial variation, with the eastern most end (Kaituna Lagoon) having generally the best water quality and lowest salinity (mean 4.9 ppt). Mean concentrations of total nitrogen ranged from 1.63 to 2.4 mg/L, chlorophyll a from 50 to 148 ug/L and total suspended solids from 151 – 248 mg/L. Seasonally, highest nutrient concentrations (mean, total nitrogen = 2.625 mg/L, dissolved reactive phosphorus = 0.059 mg/L and total phosphorus = 0.365 mg/L) occurred in late summer months (February – March), slightly decreasing but remaining high throughout winter. The benthic invertebrate community was surprisingly diverse, Crustacea (Paracorophium excavatum), Oligochaeta, Mollusca (Potamopyrgus antipodarum) and Chironomidae (Chironomus zealandicus) were dominant community members in the littoral zone, although 24 other taxa were collected. At high water levels, taxonomic richness increased in the eulittoral zone, while decreasing in the mid-littoral and lower littoral zones. In contrast, density decreased with higher water level in the eulittoral and mid-littoral zones, while increasing in the lower littoral zone. Benthic invertebrate communities appeared to be adapted to periods of intermittent dewatering, and even sustained dewatering under cooler temperatures. Despite the relatively high diversity of benthic invertebrates, invertebrate predators are generally absent from the lake. My results suggest multiple factors and interactions from predation pressure, salinity and lack of macrophytes are likely responsible for the absence of predatory invertebrates such as damselfly (Xanthocnemis zealandica) and dragonfly (Procordulia grayi) larvae. The lack of significant relationships between water quality variables and water level, and the positive relationship between chlorophyll a and salinity, suggests that current lake opening events do not have a positive effective on either water quality or phytoplankton biomass in Lake Ellesmere/Te Waihora. However, the current lake opening regime seems to be favourable to benthic invertebrate survival in the littoral zone, as the lake is predominantly open over winter when temperatures are lower, reducing the risk of desiccation. Anthropogenic activities which modify hydrodynamic and water quality conditions can potentially have a large negative impact on the structure and diversity of the littoral invertebrate community as well as flow on effects through the lake food web. Based on results from this study, I suggest a minimum lake level at Taumutu of 0.6 m during the months from November – April in order to protect benthic invertebrate communities in the eulittoral zone from extensive loss of habitat, extreme temperature and reduced risk of desiccation. Having a minimum set at ~0.6 m would provide sufficient littoral zone habitat for the lakes extensive bird life and fish populations. In addition, immediate efforts are needed into reducing nutrient loads into the lake, through improved farm management (nutrient and stocking budgets) and riparian fencing. Furthermore, physical and chemical water quality properties would benefit from an increased water level over summer months, by reducing water temperatures, diluting readily available nutrient concentrations and potentially reducing phytoplankton (and potentially toxic cyanobacterial) blooms.

Lake Malawi is the second largest lake in Africa, supporting diverse populations of endemic cichlids and supplying essential water resources to Malawi, Mozambique and Tanzania. However, population growth, rapid deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion during the last half century. These anthropogenically-disturbed catchments have caused rivers to transport greater sediment loads into Lake Malawi than rivers within forested catchments. Lake Malawi?s immense size and oligotrophic nature may retard detection of inputs of external contaminants. Reversing the effects of increased nutrient loading to Lake Malawi once observed would likely take generations, as the residence time of water is over 140 years. Therefore, sensitive metrics are required to assess the effects of land use change and climate variability in Lake Malawi in advance of deleterious effects. In this study, paleolimnological analyses of four sediment cores collected in 1997 and 1998 along a longitudinal transect of Lake Malawi, dated with ²¹⁰Pb analyses and analyzed for biogenic silica and sedimentary diatom assemblages, were used to create a long-term water quality dataset. These four sites span gradients of land use and latitude in order to reconstruct limnological conditions over the whole lake during the last 300 years. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake wide. Southern cores contain evidence of nutrient enrichment starting as early as ca. 1940, indicated by increased silica, carbon and nitrogen burial. By ca. 1970, increased rates of sedimentation, diatom influx and changes in diatom community composition, characterized by increased percent abundance of eutrophic diatom taxa, are attributable to accelerated enrichment by terrestrial soil erosion. The succession of diatoms in southern Lake Malawi begins with high percent abundance of <i>Aulacoseira nyassensis</i> and <i>Fragilaria africana</i>, which thrive in nutrient-rich waters, followed by a shift towards diatom taxa with reduced silica requirements by ca. 1980 (e. g. <i>Stephanodiscus nyassae, S. minutulus, S. muelleri, Cyclostephanos</i> and small <i>Nitzschia</i> species. ), a pattern comparable to the eutrophication-induced decline in silica to phosphorus ratios in Lake Victoria. In Lake Malawi, evidence of eutrophication extends to the mid lake as indicated by similar diatom assemblage changes in the sediment core from the central region. Diatom stratigraphies from the north end of the lake indicate no observable impacts of land use change on the northern basin of Lake Malawi during the past 350 years. However, a nine-meter rise in water level ca. 1860 AD appears to have resulted in elevated diatom influxes at that time comparable to the recent eutrophication-induced diatom influxes of the southern cores. The effects of this rise in water level was recorded in all three measured sites, southern, central and northern Lake Malawi, indicating lake-wide increased productivity, yet changes to the diatom community composition were imperceptible. This study shows evidence of recent cultural eutrophication altering limnological conditions with impacts to the biogeochemical cycling of silica, the available silica to phosphorus ratios and the biotic communities of a large portion of Lake Malawi. Thus, providing an early warning that proper stewardship of Lake Malawi requires effective management of land-use practices within the catchment to reduce soil erosion and avoid widespread water quality deterioration of this great lake.

Lake Malawi is the second largest lake in Africa, supporting diverse populations of endemic cichlids and supplying essential water resources to Malawi, Mozambique and Tanzania. However, population growth, rapid deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion during the last half century. These anthropogenically-disturbed catchments have caused rivers to transport greater sediment loads into Lake Malawi than rivers within forested catchments. Lake Malawi?s immense size and oligotrophic nature may retard detection of inputs of external contaminants. Reversing the effects of increased nutrient loading to Lake Malawi once observed would likely take generations, as the residence time of water is over 140 years. Therefore, sensitive metrics are required to assess the effects of land use change and climate variability in Lake Malawi in advance of deleterious effects. In this study, paleolimnological analyses of four sediment cores collected in 1997 and 1998 along a longitudinal transect of Lake Malawi, dated with ²¹⁰Pb analyses and analyzed for biogenic silica and sedimentary diatom assemblages, were used to create a long-term water quality dataset. These four sites span gradients of land use and latitude in order to reconstruct limnological conditions over the whole lake during the last 300 years. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake wide. Southern cores contain evidence of nutrient enrichment starting as early as ca. 1940, indicated by increased silica, carbon and nitrogen burial. By ca. 1970, increased rates of sedimentation, diatom influx and changes in diatom community composition, characterized by increased percent abundance of eutrophic diatom taxa, are attributable to accelerated enrichment by terrestrial soil erosion. The succession of diatoms in southern Lake Malawi begins with high percent abundance of <i>Aulacoseira nyassensis</i> and <i>Fragilaria africana</i>, which thrive in nutrient-rich waters, followed by a shift towards diatom taxa with reduced silica requirements by ca. 1980 (e. g. <i>Stephanodiscus nyassae, S. minutulus, S. muelleri, Cyclostephanos</i> and small <i>Nitzschia</i> species. ), a pattern comparable to the eutrophication-induced decline in silica to phosphorus ratios in Lake Victoria. In Lake Malawi, evidence of eutrophication extends to the mid lake as indicated by similar diatom assemblage changes in the sediment core from the central region. Diatom stratigraphies from the north end of the lake indicate no observable impacts of land use change on the northern basin of Lake Malawi during the past 350 years. However, a nine-meter rise in water level ca. 1860 AD appears to have resulted in elevated diatom influxes at that time comparable to the recent eutrophication-induced diatom influxes of the southern cores. The effects of this rise in water level was recorded in all three measured sites, southern, central and northern Lake Malawi, indicating lake-wide increased productivity, yet changes to the diatom community composition were imperceptible. This study shows evidence of recent cultural eutrophication altering limnological conditions with impacts to the biogeochemical cycling of silica, the available silica to phosphorus ratios and the biotic communities of a large portion of Lake Malawi. Thus, providing an early warning that proper stewardship of Lake Malawi requires effective management of land-use practices within the catchment to reduce soil erosion and avoid widespread water quality deterioration of this great lake.

Recent drought over West Africa (1970s-present) has been a global concern, and the ability to predict the frequency and severity of future droughts is important to mitigate the devastating socio-economic effects of drought. The Sahel region, situated at 10-20°N just south of the arid Sahara Desert and north of the forested Guinea Coast, is particularly vulnerable to drought periodicity because rainfall is already low at 400 mm yr-1. The ability to predict future climate variability depends on adequate knowledge of fluctuations in the past. In West Africa, meteorological records are too sparse and too short in duration to characterize the drought frequency. Consequently, climate reconstructions from lacustrine sediment records are increasingly recognized as an important source of information on past climate variability. Lake Bosomtwe, Ghana (6o30N and 1o25W) was formed over one million years ago by a meteorite impact crater in the Guinea Coast region, just south of the Sahel region. Lake Bosomtwe has a closed-drainage hydrology and lake levels are known to fluctuate with the net flux in rainfall inputs relative to evaporative outputs. In 2004, the International Continental Scientific Drilling Program recovered the complete sediment record for paleoclimatic reconstructions. However, very little has been studied of the limnological conditions that lead to the formation of laminated sediments in Lake Bosomtwe. This thesis has set out to understand the influence climate has on the physical, chemical and biological in-lake processes that generate sedimenting materials, which are preserved as laminated sediment layers. Two years of water column sampling of temperature, oxygen and nutrients at a central deep-water site (78 m water depth maximum) found that this quiescent crater lake is thermally stratified during much of the year, with anoxia persisting below 35 m water depth. During the short dry season of July and August, the monsoon rains that are associated with the intertropical convergence zone (ITCZ) are displaced northwards over the Sahel region (and away from lake Bosomtwe), and cool air temperatures and clear night skies lead to the disruption of the thermocline and circulation of dissolved nutrients nitrogen (N) and phosphorus (P) in Lake Bosomtwe. Phytoplankton primary productivity, as measured by particulate carbon and chlorophyll a concentrations, was found to increase markedly following the nutrient upwelling event in August. Sediment trap samplers deployed at 20 and 30 m water depth captured the pattern of organic matter deposition and a high flux of organic sediment was deposited shortly after the nutrient upwelling episode in August. The composition of these organic-rich sediments was distinguished by a marked depletion in δ13C and enrichment of δ15N, as compared to sediments deposited before and after this event. Spatial assessment of sediment cores identified that presently, visible laminations were preserved at and below 35 m water depth, but, not at shallower depths. Water depth was also positively correlated with the organic matter content in sediment records and could be used to reconstruct pre-historic lake levels down core. The relationship between lake level and organic content in sediments predicted that water levels were likely 22 m lower than present levels during the period ~1425-1610 CE, which corresponds with a climatic periods known as the Little Ice Age (LIA). The spatial sediment trends also revealed that inorganic sedimentation rates had increased since the onset of recent land clearance and road construction in the catchment, particularly to the north, near the town of Abono. For this reason, two cores from the central deep-water region of Lake Bosomtwe were analysed for organic and carbonate content, δ13C and δ15N, nutrients (C, N, P), magnetic susceptibility, greyscale imagery of the x-radiograph and micro-X-ray analysis of elemental constituents. Paleoenvironmental reconstructions during the past 550 years found that climate-driven lake level change was a prominent factor contributing to the organic content of sediments. High inorganic content, iron concentrations and depleted δ13C distinguished a low stand during the LIA (~1425-1610) when pelagic sediments were likely exposed to periodic oxygenation. High concentrations of organic matter, calcium (Ca) and strontium (Sr), enrichment of δ13C and low C:N ratios were indicative of wet years that likely increased lake levels and the depth of water column mixing. However, sediments with high organic content, depleted δ13C signatures and reduced Ca and Sr concentrations were suggestive of drought years that restricted the depth of seasonal water column mixing and nutrient circulation and did not necessarily result in pronounced lake-level change. During the past century, δ13C of bulk matter was positively correlated with the rainfall anomalies (r2 = 0.45, P < 0.002), indicating that droughts can result in reduced primary productivity, which may ultimately lower fishery yields. The communities living within the crater are dependent on subsistence fishing and farming, and predicting the drought frequency and magnitude in this region is essential to protecting both the ecosystem and the human population. Long-range climate forecasts for West Africa predict greater drought and increasing air temperature. However, with a detailed long-term paleoclimatic reconstruction from Lake Bosomtwe sediment records, potentially the accuracy of these predictions can be improved and better equip policy makers to enact a viable action plan in the best interests of the people.

Recent drought over West Africa (1970s-present) has been a global concern, and the ability to predict the frequency and severity of future droughts is important to mitigate the devastating socio-economic effects of drought. The Sahel region, situated at 10-20°N just south of the arid Sahara Desert and north of the forested Guinea Coast, is particularly vulnerable to drought periodicity because rainfall is already low at 400 mm yr-1. The ability to predict future climate variability depends on adequate knowledge of fluctuations in the past. In West Africa, meteorological records are too sparse and too short in duration to characterize the drought frequency. Consequently, climate reconstructions from lacustrine sediment records are increasingly recognized as an important source of information on past climate variability. Lake Bosomtwe, Ghana (6o30N and 1o25W) was formed over one million years ago by a meteorite impact crater in the Guinea Coast region, just south of the Sahel region. Lake Bosomtwe has a closed-drainage hydrology and lake levels are known to fluctuate with the net flux in rainfall inputs relative to evaporative outputs. In 2004, the International Continental Scientific Drilling Program recovered the complete sediment record for paleoclimatic reconstructions. However, very little has been studied of the limnological conditions that lead to the formation of laminated sediments in Lake Bosomtwe. This thesis has set out to understand the influence climate has on the physical, chemical and biological in-lake processes that generate sedimenting materials, which are preserved as laminated sediment layers. Two years of water column sampling of temperature, oxygen and nutrients at a central deep-water site (78 m water depth maximum) found that this quiescent crater lake is thermally stratified during much of the year, with anoxia persisting below 35 m water depth. During the short dry season of July and August, the monsoon rains that are associated with the intertropical convergence zone (ITCZ) are displaced northwards over the Sahel region (and away from lake Bosomtwe), and cool air temperatures and clear night skies lead to the disruption of the thermocline and circulation of dissolved nutrients nitrogen (N) and phosphorus (P) in Lake Bosomtwe. Phytoplankton primary productivity, as measured by particulate carbon and chlorophyll a concentrations, was found to increase markedly following the nutrient upwelling event in August. Sediment trap samplers deployed at 20 and 30 m water depth captured the pattern of organic matter deposition and a high flux of organic sediment was deposited shortly after the nutrient upwelling episode in August. The composition of these organic-rich sediments was distinguished by a marked depletion in δ13C and enrichment of δ15N, as compared to sediments deposited before and after this event. Spatial assessment of sediment cores identified that presently, visible laminations were preserved at and below 35 m water depth, but, not at shallower depths. Water depth was also positively correlated with the organic matter content in sediment records and could be used to reconstruct pre-historic lake levels down core. The relationship between lake level and organic content in sediments predicted that water levels were likely 22 m lower than present levels during the period ~1425-1610 CE, which corresponds with a climatic periods known as the Little Ice Age (LIA). The spatial sediment trends also revealed that inorganic sedimentation rates had increased since the onset of recent land clearance and road construction in the catchment, particularly to the north, near the town of Abono. For this reason, two cores from the central deep-water region of Lake Bosomtwe were analysed for organic and carbonate content, δ13C and δ15N, nutrients (C, N, P), magnetic susceptibility, greyscale imagery of the x-radiograph and micro-X-ray analysis of elemental constituents. Paleoenvironmental reconstructions during the past 550 years found that climate-driven lake level change was a prominent factor contributing to the organic content of sediments. High inorganic content, iron concentrations and depleted δ13C distinguished a low stand during the LIA (~1425-1610) when pelagic sediments were likely exposed to periodic oxygenation. High concentrations of organic matter, calcium (Ca) and strontium (Sr), enrichment of δ13C and low C:N ratios were indicative of wet years that likely increased lake levels and the depth of water column mixing. However, sediments with high organic content, depleted δ13C signatures and reduced Ca and Sr concentrations were suggestive of drought years that restricted the depth of seasonal water column mixing and nutrient circulation and did not necessarily result in pronounced lake-level change. During the past century, δ13C of bulk matter was positively correlated with the rainfall anomalies (r2 = 0.45, P < 0.002), indicating that droughts can result in reduced primary productivity, which may ultimately lower fishery yields. The communities living within the crater are dependent on subsistence fishing and farming, and predicting the drought frequency and magnitude in this region is essential to protecting both the ecosystem and the human population. Long-range climate forecasts for West Africa predict greater drought and increasing air temperature. However, with a detailed long-term paleoclimatic reconstruction from Lake Bosomtwe sediment records, potentially the accuracy of these predictions can be improved and better equip policy makers to enact a viable action plan in the best interests of the people.

<p> Isostatic rebound and climatic changes throughout the Holocene have resulted in several periods of lowered water-levels in the Great Lakes watershed. The Early Holocene lowstand phase corresponds with the Paleoindian period in the Great Lakes Region (11,000-9000 YBP), and subsequent water-level rise has inundated Paleoindian archaeological sites. This research sought to reconstruct the water-level history of Rice Lake (located north of Lake Ontario) in order to identify areas of high potential for submerged prehistoric sites. </p> <p> Over 750 line km of detailed geophysical data (single-beam bathymetry) and 16 sediment cores were collected over a 30 km2 area of Rice Lake. Sediment cores were visually logged and analyzed for particle size, microfossils and microdebitage. Water-level reconstructions accounting for sediment infill and isostatic rebound of the lake record a post Lake Iroquois (after 12 ka BP) Early Holocene lowstand (~10-12 m below present level (bpl)) (EH-1). At 10 ka BP, gradually rising water-levels and establishment of wetlands as indicated by thecamoebian assemblages coincide with a newly discovered Paleoindian occupation of the Mcintyre basin, where quartz microdebitage was found. Waterlevel continued to rise to almost 2 m bpi until 6.5 ka BP, when warmer and drier temperatures caused a sudden drop in water-levels as recorded by a pollen hiatus. After 4 ka BP, water-levels quickly recovered and stabilized as shown by rapid recovery of pre-hiatus thecamoebian biofacies and the establishment of wild rice stands. </p> <p> The small number of known, well-preserved Great Lakes Paleoindian sites has limited analysis of Early Holocene population densities, migration patterns, cultural diffusion, or the chronology of settlement. The method of modelling water-level fluctuations and associated archaeological potential developed in this thesis represents a substantial advance in our understanding of Early Holocene archaeology in the Great Lakes. These methods will have broader application to exploration of submerged terrestrial landscapes elsewhere in the Great Lakes and will allow for future regional synthesis of archaeological site distribution and characteristics. </p> / Thesis / Doctor of Philosophy (PhD)

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Multiple linear regression models were developed to calculate lake fluctuation that occurs between 10 percent, 50 percent, and 90 percent of the time lake surface elevation is exceeded. A total of 48 lakes were selected from Hillsborough, Pasco, Highlands and Polk counties, which were identified as natural lakes through the study the Southwest Florida Water Management District (SWFWMD) conducted in 1999 and 2002 to develop the models. "Natural lake" refers to lakes that were not impacted by ground water pumping. Among these 48 lakes, 22 lakes from Hillsborough and Pasco counties sit in the coastal lowlands area. 26 lakes from Highlands and Polk counties are located in the Upland and Highlands Ridge area. In developing multiple regression models, the 48 lakes were divided into two groups, the same group of lakes that SWFWMD used to develop the Reference Lake Water Regime, the method that is used to set the minimum lake levels in the region. Further, these two groups of data were subdivided into four categories based on their physical characteristics. 22 lakes were divided into surface water flow through lakes (SWF) and surface water drainage lakes (SWD). 26 lakes used their county line as the divider to separate them into Highlands County lakes and Polk County lakes. A total of six sets of multiple regression models were developed to predict the lake stage fluctuation for lakes that have no or limited lake stage data. The Polk County date set provides the best model with R2 at 0.9. However, due to the lack of available information on lake basin characteristics, the models that were developed for Hillsborough and Pasco counties do not provide a good prediction.