Dissertations / Theses on the topic 'Internal loading of nitrogen and phosphorus'

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.

Abstract The eutrophication of the Baltic Sea continues despite decrease of the external phosphorus load by as much as 80% of the target confirmed by HELCOM. The aim of this thesis is to investigate this paradox, critically evaluate previous explanations for the persistent eutrophication, and to introduce a new diagnosis and paradigm for the causes and processes behind eutrophication of the Baltic Sea. According to the current consensus, anthropogenic nutrient loading is nearly the sole cause of eutrophication and regular cyanobacterial blooms. However, this study shows that the areal phosphorus loading rate, when modeled properly, is surprisingly low, and unlikely to be the primary cause of eutrophication. Instead, the frequency of the salt water pulses has decreased dramatically during the past 40 years. This is the root cause of eutrophication, via the hyper-vicious cycle of the hypoxic and finally anoxic conditions of the deeps causing internal phosphorus loading, denitrification, and nitrogen and carbon fixation. Furthermore, this work confirms that nitrogen fixation increases in low nitrogen conditions, further increasing eutrophication and cyanobacterial blooms. Thus, the most effective way to break the cycle of eutrophication is to improve the oxygen conditions of the deeps, which really is impossible to achieve by decreasing external loading alone. A key result of this work is that natural processes, rather than human activity, plays a decisive role in the eutrophication process – a perspective that typically faces substantial resistance. This thesis discusses how sociological and political views have affected the scientific community and its pursuit to model the mechanisms of eutrophication of the Baltic Sea. In conclusion, this study leads to important novel insights by providing new models for calculating the external and internal phosphorus loads of the Baltic Sea, with results highlighting the importance of natural processes of internal loading from the anoxic deeps. Altogether, this thesis introduces a new a paradigm for eutrophication of the Baltic Sea
Tiivistelmä Itämeri rehevöityy edelleen, vaikka fosforikuormitusta on vähennetty 80 % tavoitellusta. On siis syytä tutkia miksi Itämeren tila ei ole parantunut. Syntyneen ristiriidan ratkaisemiseksi tarkoituksena on etsiä aiemmista tulkinnoista ristiriitoja, korjata ne ja uudistaa tietopohja uudeksi ja toimivammaksi paradigmaksi. Virhetulkintojen tunnistamiseen sovelletaan Popperin falsifikaatiomenettelyn periaatteita. Konsensuksen mukaan ihmisperäinen kuormitus on lähes yksinomainen syy (sinilevä)rehevyyteen. Kuitenkin Vollenweiderin mallin mukaan tehty, hydrologialla painotettu fosforin pintakuormitus on ollut 1970 - 1980-luvuilla vain lievää rehevyyttä edellyttävällä tasolla. Ulkoinen kuormitus ei siten voi olla suurin rehevyyden aiheuttaja. Sinilevärehevyyttä ylläpitävänä päätekijänä on syvävesiin happea tuovien suolavesipulssien toistuvuuden romahtaminen. Siitä syntyneet syvävesien ja -pohjien hapettomuudet aiheuttavat Gotlanninmeressä sekä sisäistä kuormitusta että lisärehevöittävää typen- ja hiilensidontaa. Yhdessä näitä prosesseja nimitetään nyt supernoidankehäksi. Johtopäätöksiä: • Ihmistieteelliset ja ympäristöpoliittiset näkemykset vaikuttavat luonnontieteellisiin tulkintoihin luultua enemmän. • Suolavesipulssien harventuminen on hapettomuus- ja rehevyyskierteen perussyy. • Syvänteiden hapettomuus on kaikkialle negatiivisesti säteilevä keskeistekijä. • Itämeren supernoidankehä on purettava saattamalla syvänteet hapellisiksi. • Hapellisuutta ei voida saada aikaan ulkoisen kuormituksen vähentämisellä. • Jäljelle jäävät siten teknologiset keinot, joista toteutuskelvollisimmalta näyttää Itämeren hapellisimman ja kylmimmän veden pumppaus 30 metrin syvyydestä syvänteisiin, mikä tehostaa myös pulssivesien virtausta syvänteiden pohjalle. Tämä väitöskirja sisältää viisi vallitsevasta paradigmasta poikkeavaa tulosta: 1. Itämerelle on kehitetty rehevyyden uusi diagnoosi ja paradigma, 2. Luonto dominoi Itämeren tilaa, ei ihminen, 3. Typensidonnalla on lisärehevöittävä mekanismi, 4. Itämerelle on kehitetty uusi fosforin sisäkuormituksen laskentamenetelmä, 5. Virtuaalisen fosforikuormituksen laskenta. Ilman Gotlanninmeren syvänteiden hapellisuutta Itämeri on tuomittu pysymään rehevyyden supernoidankehässä, ’kaksinkertaisessa takalukossa’

Florida is surrounded by water, and its many internal lakes and rivers have long been recognized for their excellent fishing and boating. This notoriety draws land developers to the lake shores to establish residential and commercial infrastructure. This land development brings with it flood plain alteration, water level stabilization, and increased nutrients which cause adverse impacts to our lakes. In response, the United States Environmental Protection Agency (EPA) passed the Federal Clean Water Act (CWA) in 1972 which set the framework for the water quality standards for the entire United States. As a result of the CWA many point sources were eliminated, but in the process it became apparent that nonpoint source loads represented even more of a threat. To further study the physical and chemical characteristics of urban runoff the Nationwide Urban Runoff Program (NURP) was established in 1978. This research lead to a series of management options, named Best Management Practices (BMPs) which proposed various structural and non-structural methods to reduce nutrient loads. But the research and data collection on the effectiveness of these systems to remove nutrients is in its infancy. The main objective of this study was to generate accurate and effective water quality and water quantity data that future stormwater management decisions can be based upon. More specific, this study established automatic monitoring sites throughout the City of Kissimmee, Florida to determine the pollutant loadings into the tributaries of Lake Tohopekaliga. These monitoring sites are located such that inflows from outside the city limits can be isolated and external pollutant loads quantified. Also, additional internal monitoring sites were established to determine the pollutant loads of internal sections of the city. Data from these internal monitoring sites will also be used to determine the variable pollutant removal efficiencies and hydraulic fluctuations of natural, irregular riverine systems. The secondary objective of this study was to perform a pilot study using the discrete grab samples in tandem with the continuous hydraulic and hydrologic data from the monitoring stations. An existing lake within the project limits was chosen for the pilot study area. Monitoring stations are located at the influent and effluent sections of the lake which provided data on the hydraulic and hydrologic parameters. The pilot study determined the nutrient loads to and from the lake and checked for any seasonal variations in pollutant loading or removal efficiencies. For the purpose of this pilot study, only total nitrogen and total phosphorous were examined for two monitoring sites. The nutrient removal efficiency was performed using both the event mean concentration method and the summation of loads method to check for seasonal variation. There were no storm event concentrations available for used in this analysis, however, there were 25 discrete grab samples collected on a bi-monthly basis over a twelve month period. This data was used with corresponding five-minute rainfall and flow data from both the inflow and outflow points. The results of this study did not reveal any seasonal variation in the nutrient concentrations either flowing into or out from the lake. Although there were some relatively lower values in late spring, the concentration levels of total nitrogen did not seem to vary significantly from its mean value of 0.90 mg/l throughout the year. The concentration levels of total phosphorus did range from 0.02 mg/l to 0.48 mg/l, but not in relation to either season or flow volume fluctuations. The lake showed no net removals of total nitrogen and was actually found to be releasing total phosphorus to the downstream receiving waters. The findings of this study are limited due to the fact that the period of pilot study was only for twelve months and there were no rainfall events used in the analysis. Rainfall events are typically high sources of nutrient loads to a lake. The lower efficiencies were probably due to missing the actual higher nutrient load concentrations during the rainfall event. However, even considering the lack of event data, the nutrient removal efficiency for the pond was still low. This analysis did serve well as a basis for performing future analysis once additional data, including rainfall events, has been collected.
M.S.C.E.
Department of Civil and Environmental Engineering
Engineering and Computer Science
Civil Engineering MSCE

Internal loading of phosphorus (P) from lake sediments can delay the recovery of lakes from eutrophication for years to decades following decreases in external nutrient inputs. While internal P loading is a pervasive problem in freshwater systems, molecular speciation of P in benthic sediments of these systems remains poorly characterized. As different P species will exhibit different responses to changing sediment-water interface (SWI) geochemistry, quantifying P speciation in sediments is a critical step in understanding P dynamics in sediment-water systems. Here, various synchrotron-based techniques were employed to directly probe the bonding environments of P and iron (Fe) in natural and experimentally manipulated lake sediments in order to link chemical speciation to chemical behavior and to identify the geochemical drivers that mediate this linkage. We manipulated SWI redox conditions in mesocosm experiments to investigate the impacts of prolonged anoxia and redox oscillations on P mobility and speciation in sediments. Mesocosm experiments demonstrate that oscillating redox conditions near the SWI may drive accelerated P release from sediments relative to uninterrupted reducing conditions. Sediment P is found to be predominantly associated with Fe oxyhydroxides, calcium carbonate, and apatite minerals in three shallow hyper/eutrophic lakes in northern Vermont. In Missisquoi Bay and Lake Carmi, Fe redox cycling controls P mobility via precipitation and dissolution of Fe oxyhydroxides. In the hypereutrophic Shelburne Pond, the presence of Fe sulfides precludes redox-driven P cycling and P mobility is instead dominated by organic matter mineralization. Our results demonstrate that internal P loading can manifest differently in similar shallow lake systems due to differences in lake configuration, sediment P and Fe speciation, and organic content of sediments. This work demonstrates the potential utility, as well as the limitations, of P K- edge X-ray absorption near edge structure spectroscopy in determining sediment P speciation in freshwater lakes.

Aboveground plant community dynamics in the oligohaline marsh at Big Branch Marsh National Wildlife Refuge, Louisiana, USA, were assessed in response to nutrient loading (3 N x 3 P treatments) and disturbance (both planned lethal disturbance and stochastic tropical storm/hurricane disturbance). Sampling was conducted seasonally from April 2004 to September 2006. Spartina patens and Schoenoplectus americanus are co-dominant plant species in this marsh. Low N-loading additions resulted in increased S. patens cover. However, increased N loading did not result in a shift in plant community composition despite S. americanus consistently having higher leaf tissue N than S. patens. Our results indicate that S. americanus may be more resilient than S. patens to disturbances that do not increase marsh surface elevation. Hurricane Katrina deposited significant amounts of sediment into remaining plots (August 29, 2005). By 2006, this disturbance resulted in a significant increase in both species richness and S. patens cover.

The Earths climate is changing at a higher rate, i.e between 1861 and 1994 the annual mean temperature in Scandinavia increased with 0,68º C and according to recent climate models the annual mean temperature is likely to rise with another 3º C during this century.

A warmer climate in many ways is associated with changing conditions for lake ecosystems. An expected higher water temperature and a stronger summer stratification of the water column increases the risk of anoxic conditions at the lake bottom. Thus anoxic conditions are likely to cause a phosphate leakage from the sediment, i.e. a higher internal loading of phosphate.

In this project, the extremely warm summer of 2002 has been used as an example for a possible scenario for a future climate. By comparing levels of phosphorus in the summer of 2002 with a ten-year median value, a phosphorus related sensitivity to climate change has been analyzed for 55 Swedish lakes. This sensitivity has then been related to several parameters of which in particular the lake morphometry and the land use in the catchment of the lake influenced the climatic sensitivity of the lake to climatic change.

Jordens klimat förändras i en allt snabbare takt. Mellan 1861 och 1994 steg årsmedeltemperaturen i Skandinavien med 0,68º C. Enligt aktuella klimatmodeller förväntas årsmedeltemperaturen i Skandinavien öka med ytterligare 3º C det närmaste seklet.

Ett varmare klimat innebär på flera sätt nya förutsättningar för ekosystemen. Genom höjda vattentemperaturer och en starkare stratifikation sommartid ökar risken för syrefria förhållanden i sjöar. Då sedimentet under syrefria förhållanden kan läcka fosfat innebär detta en ökad internbelastning av fosfor.

I detta projekt har den extremt varma sommaren 2002 använts som ett möjligt framtida klimat. Genom att jämföra fosforhalter sommaren 2002 med ett medianvärde för 10 år har den fosforrelaterade känsligheten för klimatförändringar kunnat analyseras för 55 svenska sjöar. Denna känslighet har sedan relaterats till diverse parametrar så som sjöns morfometri och avrinningsområdets sammansättning.

Nitrate (NO3) loadings from stormwater runoff promote eutrophication in surface waters. Low Impact Development (LID) is a type of best management practice aimed at restoring the hydrologic function of watersheds and removing contaminants before they are discharged into ground and surface waters. Also known as rain gardens, a bioretention system is a LID technology that is capable of increasing infliltration, reducing runoff rates and removing pollutants. They can be planted with visually appealing vegetation, which plays a role in nutrient uptake. A modified bioretention system incorporates a submerged internal water storage zone (IWSZ) that includes an electron donor to support denitrification. Modified (or denitrifying) bioretention systems have been shown to be capable of converting NO3 in stormwater runoff to nitrogen gas through denitrification; however, design guidelines are lacking for these systems, particularly under Florida-specific hydrologic conditions. The experimental portion of this research investigated the performance of denitrifying bioretention systems with varying IWSZ medium types, IWSZ depths, hydraulic loading rates and antecedent dry conditions (ADCs). Microcosm studies were performed to compare denitrification rates using wood chips, gravel, sand, and mixtures of wood chips with sand or gravel media. The microcosm study revealed that carbon-containing media, acclimated media and lower initial dissolved oxygen concentrations will enhance NO3 removal rates. The gravel-wood medium was observed to have high NO3 removal rates and low final dissolved organic carbon concentrations compared to the other media types. The gravel-wood medium was selected for subsequent storm event and tracer studies, which incorporated three completely submerged columns with varying depths. Even though the columns were operated under equivalent detention times, greater NO3 removal efficiencies were observed in the taller compared to the shorter columns. Tracer studies revealed this phenomenon was attributed to the improved hydraulic performance in the taller compared to shorter columns. In addition, greater NO3 removal efficiencies were observed with an increase in ADCs, where ADCs were positively correlated with dissolved organic carbon concentrations. Data from the experimental portion of this study, additional hydraulic modeling development for the unsaturated layer and unsaturated layer data from other studies were combined to create nitrogen loading model for modified bioretention systems. The processes incorporated into the IWSZ model include denitrification, dispersion, organic media hydrolysis, oxygen inhibition, bio-available organic carbon limitation and Total Kjeldahl Nitrogen (TKN) leaching. For the hydraulic component, a unifying equation was developed to approximate unsaturated and saturated flow rates. The hydraulic modeling results indicate that during ADCs, greater storage capacities are available in taller compared to shorter IWSZs Data from another study was used to develop a pseudo-nitrification model for the unsaturated layer. A hypothetical case study was then conducted with SWMM-5 software to evaluate nitrogen loadings from various modified bioretention system designs that have equal IWSZ volumes. The results indicate that bioretention systems with taller IWSZs remove greater NO3 loadings, which was likely due to the greater hydraulic performance in the taller compared to shorter IWSZ designs. However, the systems with the shorter IWSZs removed greater TKN and total nitrogen loadings due to the larger unsaturated layer volumes in the shorter IWSZ designs.

An increasing number of lakes worldwide are impacted by eutrophication and harmful algal blooms due to nutrient inputs. Utah Lake is a unique eutrophic freshwater lake that is naturally shallow, turbid, and alkaline with high dissolved oxygen levels. Recently, the Utah Division of Water Quality has proposed a new limitation of phosphorus (P) loading to Utah Lake from wastewater treatment plants in an effort to mitigate eutrophication. However, reducing external P loads may not lead to immediate improvements in water quality due to the legacy pool of nutrients in lake sediments. The purpose of this study was to characterize the fate and mobility of P in Utah Lake sediments to better understand P cycling in this unique system. We analyzed P speciation, mineralogy, and binding capacity in lake sediment samples collected from 15 locations across Utah Lake. P concentrations in sediment ranged from 306 to 1894 ppm, with highest concentrations in Provo Bay near the major metropolitan area. Sequential leach tests indicate that ~25-50% of P is associated with Ca (CaCO3/ Ca10(PO4)6(OH,F,Cl)2 ≈ P) and 40-60% is associated with Fe (Fe(OOH) ≈ P). Ca-associated P was confirmed by SEM images, which showed the highest P concentrations correlating with Ca (carbonate minerals/apatite). The Ca-associated P fraction is likely immobile, but the Fe-bound P is potentially bioavailable under changing redox conditions. Batch sorption results indicate that lake sediments have a high capacity to absorb and remove P from the water column, with an average uptake of 70-96% removal over the range of 1-10 mg/L P. Mineral precipitation and sorption to bottom sediments is an efficient removal mechanism of P in Utah Lake, but a significant portion of P may be temporarily available for resuspension and cycling in surface waters. Mitigating lake eutrophication is a complex problem that goes beyond decreasing external nutrient loads to the water body and requires a better understanding in-lake P cycling.

In shallow lake systems, phosphorus (P) availability to cyanobacteria populations is often controlled by the release (internal loading) or sequestration of sediment P. This study provides novel insight into the feedbacks between the water column and benthic P pools across multiple time scales and explain how these dynamics influence chemical partitioning of P in lake sediment. Phosphorus partitioning in seasonal sediment core time series collected from a shallow eutrophic bay of Lake Champlain were quantified with SEDEX and enzyme hydrolysis selective extraction schemes. Time series extraction data were interpreted with concurrent water column physical and biogeochemical monitoring data to examine the relationship between water column dynamics and P partitioning of near-surface sediments in this intensively monitored system. Nonmetric multidimensional scaling analysis (NMDS) indicates that both sediment and water column time series cluster seasonally, linking water column variables such as pH, thermal stratification, and dissolved oxygen concentrations to the behavior of sediment P pools over the course of a year. Iron (FeP), exchangeable (Ex-P), calcium carbonate bound P (Ca-P) pools, and enzyme labile P were highly dynamic, especially in spring and summer. The SEDEX concentration data indicated that the sediment was mainly composed of inorganic bound P (De-P), but FeP and Ex-P pools proportionally varied most between sampling dates. Remarkably, while highly dynamic on an intra-annual timescale, the sediment ultimately returned to similar P concentration and chemical partitioning by late fall. The hysteretic nature of this interaction between water column dynamics and sediment P inventory/partitioning was clearly driven by systematic seasonal changes in water column physical, chemical, and ecological conditions governed by northern Vermont’s climate and the physical configuration of the bay and its watershed. This study provides novel insight into the unique challenges associated with improving water quality in lake systems impacted by internal loading of legacy P.

Phosphorus (P) is often the limiting nutrient in lacustrine and brackish eco-systems, and enhanced input of P into an aquatic system might therefore negatively impact the environment. Because modern waste water manage-ment have reduced external P input to surface waters, internal P loading from the sediment has become one of the main P sources to aquatic ecosys-tems, in which relatively unknown organic P compounds seem to be more active in P recycling than previously thought.

This thesis focus is on improving analysis methods for organic P com-pounds in lacustrine and brackish sediments, as well as determining which of these compounds might be degraded, mobilized and subsequently recycled to the water column and on what temporal scale this occur. In both lacustrine and brackish environments, the most labile P compound was pyrophosphate, followed by different phosphate diesters. Phosphate monoesters were the least labile organic P compounds and degraded the slowest with sediment depth. In regulated lakes, it was shown that pyrophosphate and polyphos-phate compound groups were most related to lake trophic status, thus indi-cating their involvement in P cycling. This thesis also indicates faster P turn-over in sediment from the brackish environment compared to sediment from the lacustrine environment.

A comparison of organic P extraction procedures showed that pre-extraction with EDTA, and NaOH as main extractant, was most efficient for total P extraction. Using buffered sodium dithionite (BD) as a pre-extractant and NaOH as main extractant was most efficient for extracting the presuma-bly most labile organic P compound groups, pyrophosphate and polyphos-phate. Furthermore, it was determined that organic P compounds associated with humic substances were more recalcitrant than other P compounds, that the BD step used in traditional P fractionation might extract phosphate monoesters, and that NMR is a statistically valid method for quantification of organic P compounds in sediment extracts.

Eutrophication has proven to be a fundamental ecological problem for lakes and other bodies of water all around the world. The process of eutrophication can be defined as a lake containing increasing concentrations of nutrients from external and/or internal input over time. The increase of nutrients in the lake has several consequences for the lake ecosystem, such as the increase in algal blooms (sometimes containing toxic and harmful cyanobacteria) and the decrease of macrophytes. One nutrient that plays a key role in the eutrophication process is phosphorus. To restore eutrophic waters, the external and internal input of phosphorus needs to be reduced. External input can be decreased by reducing the run-off from industrial areas or agriculture. Internal input can be reduced by disrupting the in-lake phosphorus loading processes, which are connected heavily to the lake sediment. The internal phosphorus loading processes can be caused by several different processes. One is the mineralization of organic biomass on the sediment which releases phosphorus into the water, another is the release of previously iron-bound phosphorus from the sediment. Different treatments can be implemented in a lake system to disrupt these internal processes of phosphorus loading and consequently restore the water quality of the lake. Such treatments also influence the biota of the lake and the ecosystem services, because of their effect on water quality. Biomanipulation treatments and aluminum treatments were implemented in lake Växjösjön in Sweden to restore the lake to a more natural and balanced state. Both treatments were effective in reducing the eutrophic conditions of the lake, improving water quality, biota, and the ecosystem services. Local human populations benefit from these improvements, for example by receiving increased revenue from lake recreation. More research is however needed to discern the long-term effects of the treatments in the Växjö municipality, thereby aiding local government and policy makers in their future decisions regarding restoration.

Nutrients in the sediments of Lake Manitoba’s south basin are resuspended regularly due to its shallow, polymictic nature. In 2009 short sediment core samples were used to determine an internal available nutrient load from sediment of 17,533 tonnes total nitrogen (TN) and 167 tonnes total phosphorous (TP). Water samples were collected at the Whitemud River and Assiniboine River Diversion (ARD) to determine the N and P input to the lake, resulting in an estimate of a total point source input of 3,547 tonnes of TN and 1,130 tonnes of TP. Open water samples were collected to determine a suspended content of 9.2 tonnes of TN /km2 or and 1.7 tonnes of TP/km2. The ARD is the largest contributor of TP to the south basin. The internal sediment pool is a significant source of TN, and when the ARD does not operate, the largest input of TP to the south basin.

Understanding hydrodynamic and biogeochemical processes in lakes is fundamentally important to the management of phytoplankton population and the improvement of water quality. Physical processes such as wind-driven surface mixing, thermal stratification and differential heating and cooling can affect the distribution of water, phytoplankton and sediments and the availability of nutrients and light. These lake processes, which are highly variable in space and time, affect phytoplankton dynamics in the field. This study aims to determine the spatial and temporal variability of phytoplankton and processes that either contribute to or override the variability in the artificially mixed Myponga Reservoir, South Australia. A sediment survey showed that sediments underlying deep water were richer in organic matter, carbon, nitrogen and phosphorus than the sediments underlying shallow water. This may lead to different nutrient release rates between the shallow and deep areas. Both sediment resuspension and anoxic sediment nutrient release were important internal sources of nutrient to support phytoplankton growth in summer when external nutrient supplies were limited by low rainfall in the catchment. An analysis of historical water temperature data revealed the development of micro-stratification at the sediment-water interface in summer, especially during a heatwave (air temperature > 40ºC for several consecutive days). Prolonged micro-stratification could potentially induce anoxic layers at the sediment surface, resulting in the release of nutrients. A risk assessment was conducted to predict the release of phosphorus from anoxic sediments and to evaluate the potential impact of cyanobacterial population (Anabaena circinalis) and the release of secondary metabolites (e.g. saxitoxin and geosmin). Spatial variability of surface mixed layer depths exists between the side-arm and main basin. A simple light model based on the relationships of surface mixed layer depth, daily light dose and phytoplankton growth rate, was developed to estimate the potential variation of phytoplankton population in the two different light habitats (the main basin and side-arm). The model showed that phytoplankton abundance in the main basin was lower than in the side-arm. However, differential heating drove a large basin-scale convection, which circulated the water between the side-arm and main basin within hours. This circulation overrode the time scale of days for the light-dependent growth effect between the two sites and hence there was no observable change in phytoplankton community structure. Although no spatial variability of phytoplankton was observed at community level, significant variations of phytoplankton cellular content and stoichiometry were detected. Higher carbon cellular content in the side-arm than in the main basin was probably due to a greater exposure to light (shallower surface mixed layer in the side-arm) for photosynthesis. In the situation where nutrients were scare, higher phosphorus cellular content was found in the side-arm than in the main basin; this was possibly due to a greater exposure to resuspended nutrients from the lake bottom (shallower water in the side-arm). There was also a strong seasonal pattern in phytoplankton cellular content and stoichiometry between summer and early winter of 2009. The carbon content of phytoplankton increased over time, while the phosphorus content decreased. After the first heavy rain event (70 mm over a four-day period) in early May, carbon cellular content decreased, while phosphorus cellular content increased. These changes in phytoplankton contents were most likely related to the bio-availability of phosphorus in water. This study reviews many complex, interactive processes driving the variability of lake physics and chemistry. The variability can yield rapid biological responses at physiological and cellular levels (e.g. Fv:Fm and cellular content), but does not necessarily appear at community levels (e.g. phytoplankton biomass, diversity). Often, conventional monitoring in lakes and reservoirs overlooks the subtle variability of phytoplankton dynamics. The relative scaling among physical, chemical and biological processes, therefore, is important to adequately describe the spatial and temporal variability in lakes and reservoirs.
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2011

Indiana University-Purdue University Indianapolis (IUPUI)
Phosphorus (P) is essential for aquatic life; cycling between both inorganic and organic forms to maintain an ecological balance. Its addition into P-scarce freshwaters, either through terrestrial (external) or sedimentary (internal) loading, may disrupt this balance causing blooms of phytoplankton to flourish, often resulting in harmful environmental and anthropogenic consequences. Accordingly, reduction of external P loading has been commonly implemented with a recent focus on sediment-bound legacy P that is mobilized into the water column during dynamic redox conditions. Mobile P species have been identified as both inorganic and organic, with the former representing the most bioavailable fraction, and the latter serving as a source for labile P in freshwaters when in high demand, particularly during blooms. Missisquoi Bay in Lake Champlain, VT experiences harmful cyanobacterial blooms driven by internal P loading and has been the target of numerous geochemical and hydrological studies. This thesis describes a high-resolution investigation of both the organic P and organic matter compositions of the bay with respect to mobility, reactivity, and bioavailability using Fourier Transform-Ion Cyclotron Mass Spectrometry (FT-ICR MS). Sediment from Missisquoi Bay was extracted with a diverse set of reagents, resulting in fractionation of both organic matter and organic P, and illustrating the distribution of various labile and recalcitrant compounds. Many of these molecules are associated with porewater or easily extractable mineral surfaces providing a link to the benthic organic matter and phosphorus fractions available to microorganisms. Additionally, the organic chemistry of the bay was investigated seasonally from May 2017 to January 2018 revealing biological processing from the spring runoff season through the post-bloom summer season. The transition from late summer to under ice conditions in winter was less severe with a higher commonality between both organic matter and organic P compounds, suggesting reduced biological and abiotic degradation. Moreover, short-term anoxic incubations of sediment cores from each season revealed the presence of diverse organic signatures from sorption processes, and a significant contribution of benthic microbial activity to the benthic organic geochemistry.

I compared and contrasted nitrogen and phosphorus concentrations and land use differences in two oligotrophic lakes (Sooke and Shawnigan) and two meso-eutrophic lakes (St. Mary and Elk) in order to evaluate nutrient concentrations over time, and evaluate the relationship between in-lake nutrients and land use in the surrounding watershed. I used MapShed© nutrient transport modeling software to estimate the mass load of phosphorus and nitrogen to each lake, and evaluated the feasibility of land use modifications for reducing in-lake nutrients. In comparing nitrogen and phosphorus data in Sooke and Shawnigan Lakes, I determined that natural watershed characteristics (i.e., precipitation, topography, and soils) did not account for the elevated nutrient concentrations in Shawnigan verses Sooke Lake. Natural watershed characteristics indicated that external loads into Shawnigan Lake would be lesser-than or equal to those into Sooke Lake if both watersheds were completely forested. I evaluated trends of in-lake nutrient concentrations for Sooke and Shawnigan Lakes, as well as two eutrophic lakes, St. Mary and Elk. Ten to 30-year trends indicate that nitrogen and phosphorus levels in these lakes have not changed significantly over time. Time-segmented data showed that nutrient trends are mostly in decline or are maintaining a steady-state. Most nutrient concentration data are not precipitation-dependent, and this, coupled with significant correlations to water temperature and dissolved oxygen, indicate that in-lake processes are the primary influence on lake nutrient concentrations -- not external loading. External loading was estimated using, MapShed©, a GIS-based watershed loading software program. Model validation results indicate that MapShed© could be used to determine the effect of external loading on lake water quality if accurate outflow volumes are available. Based on various land-cover scenarios, some reduction in external loading may be achieved through land-based restoration (e.g., reforestation), but the feasibility of restoration activities are limited by private property. Given that most of the causal loads were determined to be due to in-lake processes, land-based restoration may not be the most effective solution for reducing in-lake nitrogen and phosphorus concentrations.
Graduate