The global silicon (Si) cycle is of fundamental importance for the global carbon cycle. Diatom growth in the oceans is a major sequestration pathway for carbon on a global scale (often referred to as the biological pump). Patterns of diatoms preserved in marine sediment records can reveal both natural and anthropogenic driven environmental change, which can be used to understand silicon dynamics and climate change. Si isotopes have been shown to have great potential in order to understand the Si cycle by revealing both past and present patterns of dissolved Si (DSi) utilization, primarily when diatoms form their siliceous frustules (noted as biogenic silica, BSi). However, studies using Si isotopes are still scarce and only a few studies exist where stable Si isotopes are used to investigate the biogeochemical Si cycle in aquatic systems. Therefore, this thesis focuses on developing analytical methods for studying BSi and DSi and also provides tools to understand the observed Si isotope distribution, which may help to understand impacts of climate change and human perturbations on marine ecosystems. The Baltic Sea, one of the biggest estuarine systems in the world, was chosen as the study site. BSi samples from a sediment core in Bothnian Bay, the most northern tip of the Baltic Sea, and diatom samples from the Oder River, draining into the southern Baltic Sea were measured and reported in Paper II and III, after establishing a method for Si isotope measurements (Paper I). Si isotope fractionation during diatom production and dissolution was also investigated in a laboratory-controlled experiment (Paper IV) to validate the observations from the field. The major result is that Si isotope signatures in BSi can be used as an historical archive for diatom growth and also related to changes in climate variables. There is isotopic evidence that the Si cycle has been significantly altered in the Baltic Sea catchment by human activities. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

Water chemistry and bioindicators (diatoms and invertebrates) were examined for freshwater lakes, ponds and streams in two regions within Sirmilik National Park, northern Baffin Island, Nunavut. Significant differences were recorded between the water chemistry and diatom and invertebrate assemblages of the two regions. Modern diatom assemblages were explained mainly by specific conductivity, ORP, pH, temperature, elevation and distance from the coast. Paleolimnological techniques were applied to a sediment core from Lake BY14 on Bylot Island. Fossil diatom assemblages indicate increases in nutrients and temperature since 1935 AD. The diatom biostratigraphy does not show as large an increase in diversity and production since the middle 20th century as has been noted elsewhere, and this may be a reflection of the more nutrient-rich status of the lake relative to other Arctic lakes.

Water chemistry and bioindicators (diatoms and invertebrates) were examined for freshwater lakes, ponds and streams in two regions within Sirmilik National Park, northern Baffin Island, Nunavut. Significant differences were recorded between the water chemistry and diatom and invertebrate assemblages of the two regions. Modern diatom assemblages were explained mainly by specific conductivity, ORP, pH, temperature, elevation and distance from the coast. Paleolimnological techniques were applied to a sediment core from Lake BY14 on Bylot Island. Fossil diatom assemblages indicate increases in nutrients and temperature since 1935 AD. The diatom biostratigraphy does not show as large an increase in diversity and production since the middle 20th century as has been noted elsewhere, and this may be a reflection of the more nutrient-rich status of the lake relative to other Arctic lakes.

Cattle farming is vital to the economy of the United States. Frequently, cattle are given access to ponds and streams for water. The relative impacts of cattle access in natural water sources on the periphyton community have been rarely investigated. Periphyton is the basis of the aquatic food web, and community composition can serve as a bioindicator of pollution. Thus, my objectives were to quantify the effects of cattle access in aquatic lentic systems on periphyton community structure and biovolume, identify taxa that were associated with cattle access, and identify abiotic mechanisms that might be driving assemblage changes. I conducted my research in 4 cattle-access (CA) and 4 no-access (NA) farm ponds on the University of Tennessee Plateau Research and Education Center from May 2005 – April 2006. Periphyton community composition and water quality were measured every 2 weeks using standard environmental monitoring procedures. I documented 181 new periphyton taxonomic records (7 phyla, 52 families, and 132 genera) in Cumberland County, Tennessee. Periphyton species richness was greatest in NA ponds. Mean biovolume of pollution-sensitive diatoms (e.g., Achnanthidium minutissimum, Cymbella sp., Eunotia sp., Fragilaria crotonensis and Tabellaria fenestrata) was greater in NA ponds. In contrast, pollution-tolerant diatoms (e.g., Gomphonema sp. and Navicula sp.) and non-diatoms (e.g., Oscillatoria sp. and Scenedesmus sp.) were more abundant in CA ponds. Turbidity, pH, conductivity, and concentrations of the total Kjeldahl nitrogen, total phosphorus, potassium, periphyton phosphorus, silicon, iron, magnesium and aluminum were greater in CA ponds. Thus, changes in water quality associated with cattle-access likely mediated changes in the periphyton community. Considering that changes in periphyton community composition can destabilize higher trophic levels, I recommend that cattle farmers take advantage of USDA conservation programs that provide funds for fencing cattle from watersheds and developing alternate water sources. My results also provide evidence that monitoring the periphyton community is a reliable technique to detect water pollution from cattle.

Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2005. / Committee Chair: Kenneth Sandhage; Committee Members: Joe Cochran, Robert Snyder and Tom Sanders. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The main goal of this study was to assess the impact that nitrate and light have on the relationship between the gene expression of the enzyme nitrate reductase and the incorporation of nitrate in the cosmopolitan diatom Thalassiosira pseudonana, both in laboratory experiments and in natural environments. Continuous cultures were grown at different nitrate (NO3-) concentrations (i.e., 60, 120, and 400 µM) to evaluate their effects on the expression levels of different genes of the nitrogen metabolic pathway (i.e., nitrate and ammonium transporters, nitrate and nitrite reductases, glutamine synthetases II and III). Semi-continuous cultures were grown under different irradiances (i.e., 50, 110, 200, and 320 µmol photon cm-2 s-1) to assess the influence of light intensity (irradiance) on the relationship between the expression of those genes, uptake, and assimilation of nitrate. The expression of all of the genes that were tested decreased significantly (p < 0.05) at the highest concentration of NO3- (i.e., 400 µM), with nitrate transporters showing the most pronounced change from 27.97 to 0.59 fold change cell-1 x 10-6, at 60 and 400 µM NO3- concentrations respectively. Ammonium transporters were detected at all concentrations of NO3-, suggesting that cells are always ready to metabolize ammonium. Growth was limited (µ = 0.99 d-1) by the low irradiance treatment, was maximum (µ = 2.04 d-1) at 200 µmol photon cm-2 s-1 and was inhibited (µ = 1.54 d-1) at the highest irradiance. These trends were reflected in gene expression and uptake rates, with minimum values at the lowest and highest irradiance levels. However, results from the enzymatic assay did not show any significant differences between treatments (p > 0.05). The trends observed in the enzymatic rates could be explained by the gene expression of NO3- reductase and the uptake and growth rates in a multiple regression analysis (R2 = 0.66, p < 0.05). The results of this study show that uptake is independent of gene expression, probably because of a decoupling between transcription and protein synthesis. Not all of the newly synthesized transcripts will inevitably be translated into proteins. And even if they were, there could be post-translational mechanisms preventing the enzymes to become active. This indicates that uptake can be independent of the expression. It was attempted to measure the expression of T. pseudonana genes involved in the metabolism of NO3- in natural diatom assemblages. The use of gene expression as a proxy for metabolic processes carried out by a phytoplankton assemblage in the field is limited and depends on environmental factors, since the current methods of assessing expression rely on genomic sequences that are particularly variable in phytoplankton. The assessment of gene expression provides a useful insight into physiological studies of phytoplankton, and it should be complemented with other measurements, such as the biomass and taxonomic composition of the assemblage for a more complete picture of marine ecosystem nutrient dynamics. / Graduate

Phytoplankton dynamics and chemical characteristics of the euphotic zone were measured from 1998-2000 (an El Niño/La Niña cycle) at the 5 major stations along Line P. Near-shelf and offshore stations exhibited low seasonality in chlorophyll and moderate seasonality in particulate organic carbon (POC) production. During the 1998 El Niño, June was characterized by low chlorophyll and POC productivity due to nitrate depletion. In contrast, during the 1999 La Niña, and in 2000, higher POC productivity and nitrate occurred in June. During 1999, chlorophyll and POC productivity were similar to 1998 in late summer. Near-shelf biomass was highest in June and lowest in Feb. for the near-shelf stations. High nitrate, low chlorophyll (HNLC) stations had the highest chlorophyll in Feb. followed by June. The coccolithophore assemblage was usually numerically dominated by Emiliania huxleyi, particularly in June. Along the transect, coccolithophore abundance was much higher in June during the 1998 El Niño than in the 1999 La Niña, with Aug./Sept. abundance of both years being very low. Higher abundances were measured along the transect in June and the late summer of 2000 with sporadic ‘blooms’ of >1000 cells ml⁻¹ at some stations. Particulate inorganic carbon (PIC) production was high along the transect during June 1998, and low during both winters, June 1999 and during late summers of 1998 and 1999. There was an increase in diatom biomass and >20 µm POC production during the 1998 El Niño, specifically in the farthest offshore HNLC stations, yet diatoms were rarely found to dominate total phytoplankton biomass or production. However, there were some sporadic examples of anomalously high diatom biomass (carbon and abundance) as well as >20 µm POC production, specifically at P12 in Aug./Sept 2000. The same major diatom species were found throughout Line P (near-shelf, P16, and HNLC). Integrated silica production measured by ³²Si ranged from 0.2 to 4.7 mmol Si m⁻² d⁻¹ between 1999-2000. Silicic acid and nitrate were never limiting at all stations in Feb. and generally increased in concentration along Line P during all seasons.

The current study developed a paleolimnological approach to assess changes in diatom assemblages (class Bacillariophyceae) from present-day lake sediments in comparison to those deposited before significant human impact (ca. pre-1850) from 51 Halifax (Nova Scotia, Canada) region lakes in conjunction with a regional diatom-based transfer functions for pH and total phosphorus. All 51 lakes showed changes in diatom assemblages between the present-day and pre-industrial assemblages that was greater than would be expected (i.e. similarity between ‘top’ and ‘bottom’ samples was much less than the similarities within triplicate ‘top’ and ‘bottom’ samples). To help identify the most important environmental stressors impacting diatom assemblages in these lakes, diatom-based reconstructions of inferred changes in pH (DI-pH) and total phosphorus (DI-TP) that were greater than the Root Mean Squared Error (RMSE) of the respective inference models were reconstructed for each of the 51 lakes. For example, a decrease in DI-pH greater than the RMSE of the pH model would be a strong indication of recent acidification, whereas an increase in DI-TP greater than the RMSE of the model indicates nutrient enrichment. Based on this approach, 4% of the lakes are showing acidification-related trends occurring in lakes with low pre-industrial pH values and relatively undisturbed watersheds. Almost 14% of the study lakes have been impacted by nutrients and characterized by watershed development and high concentrations of TP. Approximately 4% of lakes showed oligotrophication and acidification. Diatom assemblages from almost 20% of the study lakes that were relatively unimpacted by the afore-listed environmental stressors show trends consistent with climate warming. These lakes show an increase in DI-pH greater than the RMSE of the inference model, and floristic changes typically showed a decrease in the relative abundance of Aulacoseira distans paralleled with increase in Cyclotella stelligera and other planktonic diatoms in the modern sediments. In addition, Diatoma tenue and Diploneis parma, diatoms tolerant of high conductivity, increased in 45% of the study lakes suggesting road salt as an additional stressor. This thesis provides a rapid paleolimnological-based technique to assess regional water-quality changes, and further demonstrates the complexity of ecological changes within freshwater resources. / Thesis (Master, Biology) -- Queen's University, 2009-09-10 15:18:07.01

Despite the documented sensitivity of polar environments, long-term monitoring data are especially sparse in these regions. Diatom-based paleolimnology has contributed significantly to understanding the response of Arctic lakes to climate change, but most studies have been conducted in regions with relatively high lakewater buffering capacity. As such, Pim Island (Nunavut, Canada) is a region of limnological interest because, due to the local geology, its surface waters are of relatively lower pH and previous research suggests that such softwater lakes may be especially responsive to climate fluctuations and therefore provide the best paleoclimate records. This thesis has two separate but related chapters, as well as an exploratory study included in appendix. First, a novel approach using visible-near-infrared spectroscopy (VNIRS) was used to infer lakewater dissolved organic carbon (DOC), from a 160-lake calibration set from the Canadian Arctic. Historically sound and similar trends were reconstructed when compared against a Canadian diatom-based DOC and Swedish VNIRS-based total organic carbon (TOC) model on Arctic Holocene sediment records. Second, a diatom and spectroscopically-based multi-proxy approach was utilized on Holocene sedimentary records from two lakes on Pim Island to assess long-term environmental change from this region. Benthic and epiphytic diatom taxa dominated the pre-19th century assemblages, although marked shifts in dominant species were recorded during the mid-Holocene. The mid-Holocene diatom assemblages underwent an abrupt ecological shift from alkaliphilous Fragilaria sensu lato to slightly acidophilous Achnanthes and Navicula. The post-19th century was characterized by an increase in the planktonic species (Cyclotella radiosa), indicating marked lakeice reductions. Third, the limnological properties and modern diatom assemblages of ponds and lakes surveyed from 1979 to 2009 on Pim Island were examined as part of an exploratory study. The ponds and lakes displayed typical characteristics observed in high Arctic lacustrine environments (i.e. oligotrophic, very dilute) but with overall relatively low alkalinity. Poorly-buffered sites had diatom assemblages that were distinct from well-buffered lakes elsewhere in the High Arctic. Our findings contribute to an improved understanding of the interactions between local environments and limnological changes, and also provide insight on the biological responses of lakes to Holocene environmental change and allow us to compare responses to those in more alkaline sites. / Thesis (Master, Biology) -- Queen's University, 2010-12-22 19:53:45.03

Recent paleolimnological research in the eastern Canadian high Arctic on the ecological impact of the Thule c.1000-1500 AD has documented the influence of prehistoric anthropogenic activities. Six lake and pond sites (three pairs) on the south-western coast of Baffin Island, Nunavut, were used to compare impacted and non-impacted sites in the southern-most region of Thule occupation, as well as to compare Thule occupation sites with sites occupied by another early Arctic population group, the Dorset. Tanfield 1 and Tanfield 2 (impacted and control, respectively) are adjacent to several multiple-occupation Dorset sites on Cape Tanfield; Juet 1 and 2 (impacted and control, respectively) are adjacent to a short-term occupation Dorset site on Juet Island; McKellar 1 is adjacent to a multiple-occupation Thule site near McKellar Bay. A nearby site (McKellar 2) was also studied, but it was clearly an anomalous, eutrophic site, rather than a control for McKellar 1. Diatom assemblages and sedimentary 15N profiles were analyzed in sediment cores from all study sites. Selected paired sediment intervals were AMS radiocarbon dated using both humic acids and terrestrial macrofossils in an attempt to establish basal dates for each core. Significant differences between several of the paired AMS radiocarbon dates serve as a cautionary note for dating Arctic sediments using either humic acids or terrestrial macrofossils. Paleolimnological analyses revealed that at both multiple-occupation sites (Tanfield 1 and McKellar 1), the activities of the Dorset and the Thule influenced lake ecology, while at the short-term occupation site (Juet 1), the Dorset occupation was not sufficiently large to have a discernible impact. McKellar 1 showed a greater impact compared to Tanfield 1, consistent with the intense marine mammal hunting by Thule at the former, compared to the moderate marine mammal hunting by Dorset at Tanfield 1. The origin of marine-derived nutrients at McKellar 2 could not be ascertained with certainty. The influence of early Arctic population groups remains obvious in present-day nutrient- and production-related water chemistry variables. This research points to the value of collaborations between paleolimnologists and archaeologists and may provide insight into the future implications of current anthropogenic activities in the Arctic. / Thesis (Master, Biology) -- Queen's University, 2011-10-04 15:52:18.29