Academic literature on the topic 'Silicon cycle (Biogeochemistry)'

Abstract. The availability of dissolved silica (Si) in the ocean provides a major control on the growth of siliceous phytoplankton. Diatoms in particular account for a large proportion of oceanic primary production. The original source of the silica is rock weathering, followed by transport of dissolved and biogenic silica to the coastal zone. This model study aims at assessing the sensitivity of the global marine silicon cycle to variations in the river input of silica on timescales ranging from several centuries to millennia. We compare the performance of a box model for the marine silicon cycle to that of a global biogeochemical ocean general circulation model (HAMOCC2 and 5). Results indicate that the average global ocean response to changes in river input of silica is comparable in the models on time scales up to 150 kyrs. While the trends in export production and opal burial are the same, the box model shows a delayed response to the imposed perturbations compared to the general circulation model. Results of both models confirm the important role of the continental margins as a sink for silica at the global scale. Our work also demonstrates that the effects of changes in riverine dissolved silica on ocean biogeochemistry depend on the availability of the other nutrients such as nitrogen, phosphorus and iron. The model results suggest that the effects of reduced silica inputs due to river damming are particularly pronounced in the Gulf of Bengal, Gulf of Mexico and the Amazon plume where they negatively affect opal production. While general circulation models are indispensable when assessing the spatial variation in opal export production and biogenic Si burial in the ocean, this study demonstrates that box models provide a good alternative when studying the average global ocean response to perturbations of the oceanic silica cycle (especially on longer time scales).

Abstract. The silicon biogeochemical cycle has been studied in the Mediterranean Sea during fall 1999 and summer 2008. The distribution of nutrients, particulate carbon and silicon, fucoxanthin (Fuco) and total chlorophyll-a (Tchl-a) were investigated along an eastward gradient of oligotrophy during two cruises (PROSOPE and BOUM) encompassing the entire Mediterranean Sea during the stratified period. At both seasons, surface waters were depleted in nutrients and the nutriclines gradually deepened towards the East, the phosphacline being the deepest in the easternmost Levantine basin. Following the nutriclines, correlated deep maxima of biogenic silica (DSM), fucoxanthin (DFM) and Tchl-a (DCM) were evidenced during both seasons with maximal concentrations of 0.45 μmol L−1 for BSi, 0.26 μg L−1 for Fuco, and 1.70 μg L−1 for Tchl-a, all measured during summer. Contrary to the DCM which was a persistent feature in the Mediterranean Sea, the DSM and DFMs were observed in discrete areas of the Alboran Sea, the Algero-Provencal basin, the Ionian sea and the Levantine basin, indicating that diatoms were able to grow at depth and dominate the DCM under specific conditions. Diatom assemblages were dominated by Chaetoceros spp., Leptocylindrus spp., Pseudonitzschia spp. and the association between large centric diatoms (Hemiaulus hauckii and Rhizosolenia styliformis) and the cyanobacterium Richelia intracellularis was observed at nearly all sites. The diatom's ability to grow at depth is commonly observed in other oligotrophic regions and could play a major role in ecosystem productivity and carbon export to depth. Contrary to the common view that Si and siliceous phytoplankton are not major components of the Mediterranean biogeochemistry, we suggest here that diatoms, by persisting at depth during the stratified period, could contribute to a large part to the marine productivity and biological pump, as observed in other oligotrophic areas.

Abstract. The silicon biogeochemical cycle has been studied in the Mediterranean Sea during late summer/early autumn 1999 and summer 2008. The distribution of nutrients, particulate carbon and silicon, fucoxanthin (Fuco), and total chlorophyll-a (TChl-a) were investigated along an eastward gradient of oligotrophy during two cruises (PROSOPE and BOUM) encompassing the entire Mediterranean Sea during the stratified period. At both seasons, surface waters were depleted in nutrients and the nutriclines gradually deepened towards the East, the phosphacline being the deepest in the easternmost Levantine basin. Following the nutriclines, parallel deep maxima of biogenic silica (DSM), fucoxanthin (DFM) and TChl-a (DCM) were evidenced during both seasons with maximal concentrations of 0.45 μmol L−1 for BSi, 0.26 μg L−1 for Fuco, and 1.70 μg L−1 for TChl-a, all measured during summer. Contrary to the DCM which was a persistent feature in the Mediterranean Sea, the DSM and DFMs were observed in discrete areas of the Alboran Sea, the Algero-Provencal basin, the Ionian sea and the Levantine basin, indicating that diatoms were able to grow at depth and dominate the DCM under specific conditions. Diatom assemblages were dominated by Chaetoceros spp., Leptocylindrus spp., Pseudonitzschia spp. and the association between large centric diatoms (Hemiaulus hauckii and Rhizosolenia styliformis) and the cyanobacterium Richelia intracellularis was observed at nearly all sites. The diatom's ability to grow at depth is commonly observed in other oligotrophic regions and could play a major role in ecosystem productivity and carbon export to depth. Contrary to the common view that Si and siliceous phytoplankton are not major components of the Mediterranean biogeochemistry, we suggest here that diatoms, by persisting at depth during the stratified period, could contribute to a large part of the marine primary production as observed in other oligotrophic areas.

Marine protists are a polyphyletic group of organisms playing major roles in the ecology and biogeochemistry of the oceans, including performing much of Earth’s photosynthesis and driving the carbon, nitrogen, and silicon cycles. In addition, marine protists occupy key positions in the tree of life, including as the closest relatives of metazoans. Despite all the reasons to better understand them, knowledge of the cell biology of most marine protist lineages is sparse. This is beginning to change thanks to vibrant growth in the development of new model organisms. Here, we survey some recent advances in studying the cell biology of marine protists toward understanding the functional basis of their unique features, gaining new perspectives on universal eukaryotic biology, and for understanding homologous biology within metazoans and the evolution of metazoan traits.

Abstract. Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated, with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.

Abstract. Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water, and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.

This review examines the links between pelagic ecology and ocean biogeochemistry with an emphasis on the role of the Southern Ocean in global cycling of carbon and silica. The structure and functioning of pelagic ecosystems is determined by the relationship between growth and mortality of its species populations. Whereas the key role of iron supply in conditioning the growth environment of land-remote oceans is now emerging, the factors shaping the mortality environment are still poorly understood. This paper addresses the role of grazing as a selective force operating on the structure and functioning of pelagic ecosystems within the larger conceptual framework of evolutionary ecology. That mortality due to grazing decreases with increasing cell size is widely taken for granted. We examine the impact of this principle across the range of size classes occupied by Southern Ocean plankton and show that relatively few species play crucial roles in the trophic structure and biogeochemical cycles of the Southern Ocean. Under iron-sufficient conditions, high growth rates of weakly silicified diatoms and Phaeocystis result in build-up of blooms that fuel “the food chain of the giants” (diatoms-krill-whales) and drive the carbon pump. In contrast, high grazing pressure of small copepods and salps on the regenerating microbial communities characteristic of the iron-limited Southern Ocean results in accumulation of large, heavily silicified diatoms that drive the silicon pump. The hypotheses we derive from field observations can be tested with in situ iron fertilization experiments.

We investigate the silicon (Si) cycle in the Southern Ocean through two isotopic approaches: (1) 30Si-incubation experiments and (2) natural silicon isotopic composition (ä30Si). 30Si-spiked incubation allows to discriminate the short-term (~ 1 day) net Si-uptake flux in bSiO2 production and dissolution. ä30Si of both biogenic silica and dissolved silicon integrates at seasonal/annual scale bSiO2 production or dissolution and mixing.

(1) A new mass spectrometer method (HR-SF-ICPMS) has been developed for 30Si-isotopic abundance measurements. This methodology is faster and easier than the previous available methodologies and has the same precision. A complete set of incubation was coupled with parallel 32Si-incubations and the two methodologies give not significantly different bSiO2 production rates. In the Southern Ocean, especially in the southern Antarctic Circumpolar Current, the large silicic acid concentration degrades the sensitivity of the method with Si dissolution fluxes staying generally below the detection limit. In contrast, the 28Si-isotopic dilution was sensitive enough to assess low biogenic silica dissolution rates in silicic acid poor waters of the northern ACC. We show that large accumulation of detrital dissolving biogenic silica after productive period implies really efficient silicon loop with integrated (euphotic layer) dissolution:production ratio equal or larger than 1.

(2) We largely expand the silicic acid isotopic data in the open ocean. Relatively simple mass and isotopic balances have been performed in the Antarctic Zone and have allowed to apply for the first time ä30Si in a quantitative way to estimate regional net silica production and quantify source waters fueling bSiO2 productivity. We observe that at the end of the productive period as suggested with 30Si-incubation, large accumulation of detrital biogenic silica in the surface waters increase the D:P ratio and subsequently dampens the bSiO2 production mediated isotopic fractionation with residual biogenic silica carrying heavier ä30Si than expected. Seasonal isotopic evolution is simulated and seems in agreement with our observations. These simulations strongly suggest working with non-zero order equations to fully assess the seasonal expression of the different processes involved: mixing, uptake, dissolution. Si-isotopes are also tracking the origin and fates of the different ACC pools across the Southern Ocean meridional circulation. Moreover during the circumpolar eastward pathway, the bSiO2 dissolution in deep water decreases the corresponding ä30Si values and this imprint is further transmitted via the upper limb of the meridional circulation in the intermediate water masses.

Doctorat en Sciences
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Silicon (Si) is one of the most abundant elements in the dissolved phase in rivers and is a key nu-trient in riverine and marine ecosystems. The continental cycle of Si is complex and involves interactions with many secondary reservoirs such as clay minerals and biogenic silica (BSi), making the Si fluxes hard to constrain. Stable isotopes provide a way to trace and describe element cycling. The natural isotopic fractionations that accompany the transfer of the element from one reservoir to another lead to specific isotopic signatures that can be used to reconstruct its source and the pathway during its biogeochemical cycle. The aim of this thesis is therefore to explore the potential of Si isotopes as a tracer of the factors controlling the dissolved Si (DSi) concentration in rivers and more specifically in tropical rivers.

Key issues treated in this thesis are the improvement of our understanding of 1° the spatial and seasonal variability of Si isotopic signatures in rivers, 2° the biological influence on the riverine isotopic signatures and on DSi and BSi fluxes, and 3° the impact of the type of weathering on the riverine isotope signatures.

The isotopic composition of different tropical basins such as the Congo River (Central Africa), the Tana River (Kenya), the Amazon (South America) and its tributaries, were determined along with other physico-chemical parameters. In order to achieve this, the water sample purification processing, necessary before isotope analyses, required specific improvements that are also pre-sented here. The average of all the riverine δ30Si signatures available so far is +1.11 ‰ (n = 253). The impact of diatom growth on the isotopic signatures of the rivers can be clearly shown in the different systems studied, and especially in the Congo River where the isotopic signature could be used in order to estimate the diatom production. The impact of anthropic perturbations through dam construction is also clearly shown in the Tana River. On a global scale the biological influ-ence on the riverine isotopic signatures is estimated to induce an increase of 0.18 ‰ of the δ30Si signature in rivers. This study also confirms the preponderant influence of weathering and secondary clay formation on dissolved Si isotope signatures in the studied rivers. Finally, isotopic signatures from these rivers are compared to data available for other rivers around the world in order to draw large trends on a global scale.

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Le silicium (Si) est l’un des éléments les plus abondants sous forme dissoute dans les rivières et est un nutriment fondamental tant dans les rivières que dans les écosystèmes marins. Le cycle continental du Si est complexe et inclut des interactions avec de nombreux réservoirs secondaires, comme les argiles et la silice biogénique (BSi), rendant les flux de Si difficiles à quantifier. Les isotopes stables fournissent un moyen de tracer et de décrire le cycle d’un élément. Le fractionnement isotopique qui accompagne le transfert de l’élément d’un réservoir à un autre induit des signatures isotopiques spécifiques qui peuvent être utilisées pour retracer la source et la trajectoire suivie par cet élément au cours de son cycle biogéochimique. Le but de cette thèse est d’explorer le potentiel des isotopes du Si en tant qu’indicateur des facteurs contrôlant la concentration en Si dissous (DSi) dans les rivières et plus spécifiquement dans les rivières tropicales.

Les questions principales traitées dans cette thèse sont l’amélioration des connaissances de :1° la variabilité spatiale et saisonnière des signatures isotopiques du Si dans les rivières, 2° l’influence biologique sur les signatures isotopiques des rivières et sur les flux de DSi et BSi et 3° l’impacte du type d’altération sur les signatures isotopiques des rivières.

Les compositions isotopiques de différents bassins tropicaux tels que le Fleuve Congo (Afrique Centrale), le Fleuve Tana (Kenya), l’Amazone (Amérique du Sud) et ses principaux affluents ont été déterminées en même temps que d’autres paramètres physicochimiques. Pour ce faire, le pro-cédé de purification des échantillons d’eau, préalable aux analyses isotopiques, a nécessité des améliorations spécifiques qui sont également présentées ici. La moyenne de toutes les signatures δ30Si accessibles à l’heure actuelle est de +1.11 ‰ (n = 253). L’impact de la croissance des diatomées sur les signatures isotopiques des rivières est démontré dans les différents systèmes étudiés, spécialement pour le Fleuve Congo où la signature isotopique a pu être utilisée afin de déterminer la production de diatomées. L’influence de perturbations anthropiques telles que la construction de barrages a pu être démontrée pour le Fleuve Tana. À l’échelle globale, on estime que l’influence biologique sur la signature isotopique des rivières mène à une augmentation de 0.18 ‰ de la signature δ30Si moyenne des rivières. Cette étude confirme également l’influence prépondérante de l’altération et de la formation d’argiles secondaires sur les signatures isotopiques du DSi dans les rivières étudiées. Enfin, les signatures isotopiques de ces rivières sont comparées aux données accessibles pour d’autres rivières à travers le monde afin d’en déduire les grandes tendance à l’échelle mondiale.

Doctorat en Sciences
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The external silicon cycle during the Precambrian (4.5-0.5 Ga) is not well understood despite its key significance to apprehend ancient dynamics at the surface of the Earth. In the absence of silicifying organisms, external silicon cycle dramatically differs from nowadays. Our current understanding of Precambrian oceans is limited to the assumption that silicon concentrations were close to saturation of amorphous silica. This thesis aims to bring new insights to different processes that controlled the geochemical silicon cycle during the Archaean (3.8-2.5 Ga). Bulk rock Ge/Si ratio and Si isotopes (δ30Si) offer ideal tracers to unravel different processes that control the Si cycle given their sensitivity to fractionation under near-surface conditions.

First, this study focuses on Si inputs and outputs to ocean over a limited time period (~2.95 Ga Pongola Supergroup, South Africa) through the study of a palaeosol sequence and a contemporaneous banded iron formation. The palaeosol study offers precious clues in the comprehension of Archaean weathering processes and Si transfer from continent to ocean. Desilication and iron leaching were shown to be the major Archaean weathering processes. The occurrence of weathering residues issued of these processes as major component in fine-grained detrital sedimentary mass (shales) attests that identified weathering processes are widely developed and suggest an important dissolved Si flux from continent to the ocean. In parallel, banded iron formations (BIFs), typically characterised by alternation of iron-rich and silica-rich layers, represent an extraordinary record of the ocean-derived silica precipitation throughout the Precambrian. A detailed study of a 2.95 Ga BIF with excellent stratigraphic constraints identifies a seawater reservoir mixed with significant freshwater and very limited amount of high temperature hydrothermal fluids as the parental water mass from which BIFs precipitated. In addition, the export of silicon promoted by the silicon adsorption onto Fe-oxyhydroxides is evidenced. Then, both Si- and Fe-rich layers of BIFs have a common source water mass and a common siliceous ferric oxyhydroxides precursor. Thus, both palaeosols and BIFs highlight the significance of continental inputs to ocean, generally under- estimated or neglected, as well as the close link between Fe and Si cycles.

In a second time, this study explores secular changes in the Si cycle along the Precambrian. During this timespan, the world ocean underwent a progressive decrease in hydrothermal inputs and a long-term cooling. Effects of declining temperature over the oceanic Si cycle are highlighted by increasing δ30Si signatures of both chemically precipitated chert and BIF through time within the 3.8-2.5 Ga time interval. Interestingly, Si isotope compositions of BIF are shown to be kept systematically lighter of about 1.5‰ than contemporaneous cherts suggesting that both depositions occurred through different mechanisms. Along with the progressive increase of δ30Si signature, a decrease in Ge/Si ratios is attributed to a decrease in hydrothermal inputs along with the development of large and widespread desilication during continental weathering.

Le cycle externe du silicium au précambrien (4.5-0.5 Ga) reste mal compris malgré sa position clé dans la compréhension des processus opérant à la surface de la Terre primitive. En l’absence d’organismes sécrétant un squelette externe en silice, le cycle précambrien du silicium était vraisemblablement très différent de celui que nous connaissons à l’heure actuelle. Notre conception de l’océan archéen est limitée à l’hypothèse d’une concentration en silicium proche de la saturation en silice amorphe. Cette thèse vise à une meilleure compréhension des processus qui contrôlaient le cycle géochimique externe du silicium à l’archéen (3.8-2.5 Ga). Dans cette optique, le rapport germanium/silicium (Ge/Si) et les isotopes stables du silicium (δ30Si) représentent des traceurs idéaux pour démêler les différents processus contrôlant le cycle du Si.

Dans un premier temps, cette étude se focalise sur les apports et les exports de silicium à l’océan sur une période de temps restreinte (~2.95 Ga Pongola Supergroup, Afrique du Sud) via l’étude d’un paléosol et d’un dépôt sédimentaire de précipitation chimique quasi-contemporain. L’étude du paléosol apporte de précieux indices quant aux processus d’altération archéens et aux transferts de silicium des continents vers l’océan. Ainsi, la désilicification et le lessivage du fer apparaissent comme des processus majeurs de l’altération archéenne. La présence de résidus issus de ces processus d’altération en tant que composants majeurs de dépôts détritiques (shales) atteste de la globalité de ces processus et suggère des flux significatifs en silicium dissout des continents vers l’océan. En parallèle, les « banded iron formations » (BIFs), caractérisés par une alternance de niveaux riches en fer et en silice, représentent un enregistrement extraordinaire et caractéristique du précambrien de précipitation de silice à partir de l’océan. Une étude détaillée d’un dépôt de BIFs permet d’identifier une contribution importante des eaux douces dans la masse d’eau à partir de laquelle ces roches sont précipitées. Par ailleurs, un mécanisme d’export de silicium via absorption sur des oxyhydroxydes de fer est mis en évidence. Ainsi, les niveaux riches en fer et riche en silice constituant les BIFs auraient une même origine, un réservoir d’eau de mer mélangée avec des eaux douces et une contribution minime de fluides hydrothermaux de haute température, et un même précurseur commun. Dès lors, tant les paléosols que les BIFs mettent en évidence l’importance des apports continentaux à l’océan, souvent négligés ou sous estimés, ainsi que le lien étroit entre les cycles du fer et du silicium.

Dans un second temps, cette étude explore l’évolution du cycle du silicium au cours du précambrien. Durant cette période, l’océan voit les apports hydrothermaux ainsi que sa température diminuer. Dans l’intervalle de temps 3.8-2.5 Ga, les effets de tels changements sur le cycle du silicium sont marqués par un alourdissement progressif des signatures isotopiques des cherts et des BIFs. Le fort parallélisme entre l’évolution temporelle des compositions isotopiques des deux précipités met en évidence leur origine commune, l’océan. Cependant, les compositions isotopiques des BIFs sont systématiquement plus légères d’environ 1.5‰ que les signatures enregistrées pas les cherts. Cette différence est interprétée comme le reflet de mécanismes de dépôts différents. L’alourdissement progressif des compositions isotopiques concomitant à une diminution des rapports Ge/Si reflètent une diminution des apports hydrothermaux ainsi que la mise en place d’une désilicification de plus en plus importante et/ou généralisée lors de l’altération des continents.

Doctorat en Sciences
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Microbes, living on the boundary between the sediment and the water-column in lakes, can play a pivotal role in governing the magnitude and frequency of nutrient cycling. The purpose of this research was to focus on the role of benthic microalgae in regulating such processes and to identify spatial and temporal characteristics in their function. Approaches included the quantification of sediment nutrient concentrations (particularly P fractionation), estimates of equilibrium phosphate concentrations (EPC0) (resuspended and undisturbed sediment estimates), and assessment of the benthic microalgal community composition, biostabilisation capacity, and its ability to regulate diffusive-nutrient flux. This thesis highlighted the importance of biological regulation of benthic/pelagic nutrient cycling, especially the role of benthic microautotrophs. Release sensitive sediment-P fractions were observed to be highly variable (both with depth and season) and correlated well with indicators of benthic photosynthesis (e.g. DO, chlorophyll, pH). Understanding the seasonality of whole-system P partitioning can enhance future lake management programmes. EPC0 estimates were significantly higher during undisturbed as opposed to disturbed sediment conditions. Epipelon constituted < 17 % of the total sediment chlorophyll signal and was highest in the clearer winter months and at intermediate depths at which a trade off between wind-induced habitat disturbance and light limitation existed. In intact core experiments, the benthic microalgal community significantly reduced the diffusive nutrient (especially PO₄-P and SiO₂) flux. NH₄ -N release was highest under light conditions at high temperatures. The mechanisms for regulation included direct uptake, photosynthetic oxygenation of the sediment surface, and regulation of nitrification/denitrification processes. Sediment stability increased with colloidal carbohydrate concentration (extruded by benthic microbes) at 4.1 m water-depth but not at 2.1 m overlying water depth, probably indicating the role of habitat disturbance in shallow areas acting to reduce epipelic production. Additionally, in an ecosystem comparison, the nature and extent of the biotic mediation of sediment stability varied between freshwater and estuarine ecosystems.

Globally, human activities are altering nutrient biogeochemical cycles. The impact of humans on silicon (Si) cycles remains largely unexplored. Understanding the cycle of Si is important because weathering of siliceous rocks is a substantial sink of atmospheric carbon. Additionally, Si is required by diatoms. Diatoms form the base of important socioeconomic food webs, responsible for ~50% of oceanic net primary production, and deliver atmospheric carbon to ocean sediments as part of the ocean’s biological pump. My dissertation aims to assess the role of anthropogenic activities in altering Si cycling across the land-ocean continuum. Chapter 2 focuses on how assimilation of biogenic silica (BSi) by trees may be impacted by projected changes in climate. Using samples collected during a multi-year, snow removal experiment, I show that increased frequency and duration of soil freezing in winter significantly decreased (-28%) BSi in sugar maple (Acer saccharum) fine roots compared to control plots. Importantly, I observed that fine roots are a previously undescribed pool of BSi within sugar maples, accounting for 29% of total sugar maple BSi while only 4% of sugar maple biomass. Chapter 3 examines the origin and fate of Si within wastewater for the City of Boston. I determined the total dissolved silica (DSi) load in wastewater influent (69,500 kmol DSi year-1), then parsed the total DSi flux between Si contributions of sewage (49%), groundwater infiltration (39%), and surface runoff inflow (12%). In Chapter 4, I study the DSi load carried by treated effluent. I determined that effluent load (67,800 kmol DSi year-1) is not statistically different from influent load, indicating that wastewater treatment does not remove DSi. In Chapter 5 I demonstrate how humans impact concentrations of DSi in urban groundwater. Groundwater DSi increases with human presence and urban areas have significantly higher concentrations of DSi compared to groundwater conditions along the Massachusetts coast. I demonstrate that historic variables defining fill techniques, fill material, and pre-fill land-use out preform geologic variables in predicting urban groundwater DSi concentrations. This dissertation highlights human alterations to biological assimilation, fate, and effects of Si in sewage, and centuries-long subsurface Si impacts that perturb the distribution and availability of a nutrient intimately tied to water quality and climate.

Phosphorus (P) is one of the most well-studied nutrients in aquatic ecosystems because of its role in limiting primary production on ecological and geological timescales (van Capellen and Berner, 1989; Holland, 1994; Tyrell, 1999; van Cappellen and Ingall, 1996). Other key linkages to biological systems include the role of P as an essential constituent of genetic material (RNA and DNA) and cellular membranes (phospholipids), as well as in energy-transforming molecules (e.g., ATP, etc.). Consequently, marine P has received considerable attention in recent decades, with particular emphasis on source and sink terms in budgets (Froelich et al., 1982; Meybeck, 1982; Ruttenberg, 1993; Sutula et al., 2004). Excessive loading of N to estuarine waters can result in P limitation in systems that are generally considered to be N limited. In such cases where primary production is limited by P, N:P ratios are expected to exceed the Redfield value of 16:1 but can be replenished by sediment efflux of P due to redox changes. For example, after the initial N loading of a system there will be an increase in primary production, which can cause the system to become P limited. Then, the phytodetritus from these early stages of N loading can be remineralized in sediments resulting in anoxic conditions in surface sediments, which can then enhance P release from sediments to the overlying waters where primary production is once again enhanced. Evidence for the role of sediment-derived P on primary production in estuaries with high N loading has been shown to occur particularly in shallow water systems (Timmons and Price, 1996; Cerco and Seitzinger, 1997). On the other hand, many coastal areas have also been subjected to high P loading from anthropogenic sources, where in some cases inputs of P are 10 to 100 times greater than in preindustrial times (Caraco et al., 1993). In many cases, P and N loading to estuarine systems will occur simultaneously and decoupling or isolating their individual effects can be difficult (e.g., HELCOM, 2001). The cycling and availability of P in estuaries is largely dependent upon P speciation.