Browsing by Subject "Lake Malawi"

Now showing 1 - 3 of 3
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    Planktonic archaeal diversity and ammonia-oxidizer abundance change with depth in Lakes Malawi, Kivu and Superior
    (2014-05) Munoz Ucros, Juana
    Planktonic Archaea may play a key role in the nitrogen cycle by oxidizing ammonia, but little is known about these microbes in large lakes of the world. Differences in the abundance of total Archaea, marine group I Archaea (MG-1), and ammonia-oxidizing Archaea (AOA) measured in 2010, 2011, and 2012 were compared during stratified conditions in Lake Malawi, a tropical African great lake, to previous work completed in Lake Superior and Lake Kivu. Total Archaea, MG-1, and AOA abundances increased by more than two orders of magnitude in Lake Malawi from the warm epilimnion to the oxic upper hypolimnion during thermally stratified conditions from November to January, but remained abundant in the deeper anoxic hypolimnion. 16S rRNA clones related to the Thaumarchaeota, possible ammonia oxidizers, and archaeal clones from previous work in Lake Victoria were present in both Lake Malawi and Kivu, and euryarchaeal clones were common in the deeper anoxic waters. The distribution and diversity of planktonic Archaea in this tropical great lake was similar to that in Lake Superior, a temperate great lake of comparable trophic status.While more detailed seasonal work about archaeal abundance and community diversty has been completed in the western basin of Lake Superior, spatial patterns of archaeal distribution have not been evaluated across this lake. Here, I compared the abundance of these 3 archaeal gene markers in the epilimnion and hypolimnion from seven sites across Lake Superior during the stratified period in 2009 and 2010. The abundance of the total Archaea, MG-1, and AOA was consistently lower in the epilimnion, and at least an order of magnitude higher in the hypolimnion at all sites. Although my aim was not to elucidate the causes of such distribution in Lake Superior, this study does provide additional evidence that planktonic Archaea are more abundant in colder waters of the hypolimnion and their abundance is restricted in the surface waters of thermally stratified lakes.
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    Sediment diagenesis in large lakes Superior and Malawi, geochemical cycles and budgets and comparisons to marine sediments
    (2014-09) Li, Jiying
    Large freshwater lakes, despite their socioeconomic importance, are insufficiently characterized in terms of their geochemical cycling. In systems such as Lake Superior, contributions of several important processes, including those affecting biological productivity, remain poorly quantified. To understand the geochemical controls on sediment diagenesis, we investigated sediments in well-oxygenated temperate Lake Superior and tropical meromictic Lake Malawi. We characterized solid-sediment and porewater geochemistry, calculated diagenetic rates and fluxes, and investigated temporal and geographic variability for the cycles of carbon, nitrogen, phosphorus, iron, and sulfur. Revised nutrient budgets (for N and P) were constructed for both sediment and water column, suggesting a significant contribution of sediments to the geochemical cycling in both lakes. Sedimentation rate and the depth of oxygen penetration (OPD) were found to strongly affect the dynamics of carbon and nutrients. In Lake Superior, the deep (>4 cm) oxygenation of sediments in low-sedimentation areas regulates the remineralization rates of carbon and phosphorus, controls denitrification rates, and creates an unusual sulfur cycle driven by the oxidation of organic sulfur to sulfate. It also makes these deeply oxygenated sediments qualitatively distinct from sediments in nearshore high-sedimentation areas, necessitating their separate consideration in geochemical budgets. Comparisons against data from marine environments suggest that sediment processes in large lakes (both temperate and tropical) can be described by the same quantitative relationships as in marine sediments, facilitating the transfer of knowledge.
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    Sources, cycling, and fate of organic matter in large lakes: ingishts from stable isotope and radiocarbon analysis in Lakes Malawi and Superior
    (2014-08) Kruger, Brittany Ruth
    Organic matter (OM) in lake systems is sourced from in situ aquatic primary production (autochthonous), land based plant primary production or detrital material that ultimately originated from photosynthesis (allochthonous), or resuspension of organic rich sedimentary material that was ultimately sourced from a combination of all such sources. Studying the stable and radioisotopic signature of multiple chemical components of lacustrine OM can help elucidate which of the above is the dominant OM source to the lake, as well as how OM is incorporated into and cycles through lake systems. The high organic content and biodiversity in large lakes of the world make them excellent sites to investigate such questions, and this dissertation focuses on such questions in Lake Malawi (SE Africa), and Lake Superior (North America). In Lake Malawi, the organic carbon (OC) recently deposited (within the last 50 years) is largely dominated by aquatic input, and the influence of terrestrial riverine inputs dissipates as distance from shore and water depth increase. This confirms that parameters typically used to investigate historic lake levels (and thereby to infer past climates) can in fact function as robust indicators of distance from shore, and thereby lake level. This is supported by bulk and compound specific stable carbon isotopic and radiocarbon analysis of multiple sediment fractions. Most fractions exhibited isotopic signatures nearshore that were distinct from more offshore, open-lake locations. In Lake Superior, compound specific nitrogen isotope analysis (CSNIA) of specific amino acids from species occupying all levels of the food chain showed that Limnocalanus macrurus, a copepod, occupies a trophic level much higher than expected from known feeding habits, which may indicate the consumption of additional or unique food sources. Bulk radiocarbon analysis of the same suit of species from that lake showed Diporeia, a benthic amphipod, consumes an aged carbon source that does not appear to be significantly incorporated by other (more pelagic) organisms in this study, which rely primarily upon recently synthesized autochthonous organic carbon.