Submitted by Biblioteca de Pós-Graduação em Geoquímica BGQ (bgq@ndc.uff.br) on 2017-09-04T17:31:07Z No. of bitstreams: 1 TESE_Viviane Figueiredo Souza versão final.pdf: 1973892 bytes, checksum: 8b1ac31fbe25c8a6584e68f8e3b8ff71 (MD5) / Made available in DSpace on 2017-09-04T17:31:07Z (GMT). No. of bitstreams: 1 TESE_Viviane Figueiredo Souza versão final.pdf: 1973892 bytes, checksum: 8b1ac31fbe25c8a6584e68f8e3b8ff71 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Universidade Federal Fluminense. Instituto de Química. Programa de Pós-Graduação em Geoquímica, Niterói, RJ / O nitrogênio (N) é um elemento imprescindível para todos os organismos do nosso planeta, entretanto o composto nitrogenado mais abundante, o gás dinitrogênio (N2), é assimilável apenas por poucos micro-organismos. Isso torna o N limitante, refletindo na sua disponibilização via mineralização de matéria orgânica (MO) e a nitrificação, que produzem amônio (NH4+) e nitrato (NO3-), respectivamente. Em solos, esses processos são regulados por fatores como conteúdo de MO e água no solo, pH e temperatura. Em ecossistemas florestais, tropicais e boreais, esses processos são muito relevantes e ainda pouco estudados, principalmente em áreas de floresta secundária sob influência de manejo. A mudança de uso do solo causa alterações na ciclagem e disponibilidade do N, nos fatores reguladores, e na emissão de óxido nitroso (N2O), um gás de efeito estufa. Padrões de recuperação florestal são bastante distintos entre diferentes florestas tropicais, como visto para floresta de Mata Atlântica e Amazônica, com taxas de mineralização em florestas jovens (10 anos) muito elevadas na Amazônia (20,9 μg N g-1 SWD d-1) e mais baixas na Mata Atlântica (3,2 μg N g-1 SWD d-1). Já a nitrificação teve um padrão semelhante, com baixas taxas em ambas as florestas jovens (0,6 6,8 μg N g-1 SWD d-1 na Mata Atlântica e Amazônia, respectivamente), o que indica uma ciclagem de N conservativa, evitando perdas via emissão de gás e lixiviação de NO3-. Entretanto, a emissão de N2O em área de restauração na Mata Atlântica foi maior do que na pristina (22 e 2,5 μg N2O-N g-1 SWD h-1, respectivamente) devido à maior temperatura do solo que estimula os processos microbianos produtores de N2O. Em termos de floresta pristina, a taxa de nitrificação foi bastante distinta entre a floresta de Mata Atlântica e Amazônica (0,08 e 15,9 μg N2O-N g-1 SWD h-1, respectivamente). Isso evidencia os diferentes fatores reguladores de cada região tropical, como regime de chuvas, composição vegetal, tipos de solo, etc. Em florestas boreais, foi visto que o conteúdo de MO e água do solo e o pH do solo são os principais reguladores, limitando a disponibilidade de N em florestas bem drenadas dominadas por coníferas, em comparação com florestas pouco drenadas dominadas por turfa. Dessa forma, verificamos que a mineralização e, principalmente, a nitrificação, são processos muito relevantes no controle de N, e mesmo em ecossistemas diferentes, os fatores reguladores muitas vezes são os mesmos. Isso evidencia a necessidade de mais estudos acerca da dinâmica do N nesses ambientes, principalmente em florestas em restauração / Nitrogen (N) is an essential element for all organisms; however, the most abundant nitrogen compound, dinitrogen (N2), is assimilable only by a few microorganisms. This makes N limiting, which reflects in its availability via organic matter (OM) mineralization and nitrification, which produce ammonium (NH4+) and nitrate (NO3-), respectively. In soils, these processes are regulated by factors such as OM and soil water content, pH and temperature. In forest ecosystems, tropical and boreal, these processes are very relevant and still poorly understood, especially in secondary forest previously deforested. The land use change causes alterations in cycling and availability of N, regulating factors, and emission of nitrous oxide (N2O), a greenhouse gas. Forest recovery patterns are quite distinct among different rainforests, as seen for Atlantic and Amazon rainforest, with very high levels of mineralization in young forests (10 years) in Amazonia (20.9 μg N2O-N g-1 SWD h-1) and low in the Atlantic Forest (3.2 μg N2O-N g-1 SWD h-1). Nitrification had a similar pattern between them, with low rates in both young forests (0.6 and 6.8 μg N2O-N g-1 SWD h-1 in the Atlantic and Amazon forest, respectively), indicating a conservative N cycling, avoiding losses through gas emission and NO3- leaching. However, the N2O emission in restoration area in the Atlantic Forest was higher than in pristine (22 and 2.5 μg N2O-N g-1 SWD h-1, respectively) due to the higher soil temperature, which stimulates microbial production of N2O. In terms of pristine forest, the nitrification rate was very different between the Atlantic forest and Amazonian forest (0.08 and 15.9 μg N2O-N g-1 SWD h-1, respectively), showing the different regulatory factors of each tropical region, such as rainfall regime, vegetal composition, soil types, etc. In boreal forests, OM, soil water content and soil pH were the main regulators, limiting the availability of N in conifer-dominated well-drained forests compared to peat-dominated poorly drained forests. In this way, we verified that mineralization and, mainly, nitrification are very relevant processes in the control of N, and even in extremely different ecosystems, the regulating factors are often the same. This evidences the need for more studies about N dynamics in these environments, especially in restoration forests

Trembling aspen (Populus tremuloides Michx.) is one of the most common tree species in the boreal and temperate forests of North America. It grows on many different sites and associates with a variety of tree species. In BC, aspen is frequent throughout all submontane and montane continental forested zones. Relationships between environmental factors and forest productivity have been the subjects of many studies. Most of these studies, using various topographic, soil, physical and chemical properties as independent variables, had limited success in accounting for the variation in SI over a large geographic area. The objectives of this study were (1) to quantify relationships between aspen SI and environmental factors at two spatial scales, and (2) to develop predictive SI models from easily measurable environmental factors.

Communicating science in three-dimensional (3D) multimedia is an immersive and interactive way to explore scientific processes (Signals and Communication Technology, 2019). Geovisualization is an emerging 3D multimedia method for visual analysis, synthesis, and presentation of geospatial, geologic, and geophysical data (MacEachren & Kraak, 2001). There is an identified need to develop scientific communication tools to further understand boreal peatland evolution, hydrogeology, ecology, and geochemistry (Bubier et al., 2003) since the International Union of Conservation of Nature asserts that peatlands are among the most valuable ecosystems on Earth, critical for preserving global biodiversity, providing drinking water, minimising flood risk, preventing wildfire, and mitigating climate change (Hama et al., 2000). The intention of this thesis is to communicate a novel approach to geovisualize boreal peatland architecture using Light Detection and Ranging (LiDAR) and Ground Penetrating Radar (GPR). GPR and LiDAR have been used to create 3D subsurface geovisualizations for archaeology (Kenady et al., 2018; Schultz & Martin, 2011) and resource geology (Corradini et al., 2020; Koyan & Tronicke, 2020) although application to peatland hydrogeology is uncommon. Point-source hydrogeological and geochemical data were integrated with 3D geological models to estimate carbon and nitrogen storage in an archetypal boreal peatland near Mariana Lakes, Alberta. Peatland geometry resembled a shallow lake basin with depths greatest in fens (>10 m) and thinnest in bogs (<2 m). Hydraulic conductivity was only a few meters per year and vertical groundwater movement was limited. Sequestered carbon and nutrients increased with depth. The average concentration of dissolved ammonium was 3 grams per cubic metre of peat (g/m3), 5g/m3of Total Kjeldahl Nitrogen, 60g/m3 of dissolved organic carbon and 200g/m3 of dissolved inorganic carbon. Tritium detection from atmospheric atomic weapons radionuclide fallout revealed that in deeper anaerobic peat (catotelm), tritium was absent, signifying groundwater was older than 50 years and not mixed with meteoric waters. Fen catotelm channels are likely acting as gravity-driven hydraulic traps (Tóth, 1999). / Graduate

Den biologiska mångfalden i Sverige såväl som globalt minskar kraftigt och har gjort under en längre tid. Utdöendetakten idag är upp till 1000 gånger snabbare än den naturliga och många arter har redan dött ut delvis på grund av klimatförändringar. Syftet med översikten är att sammanställa vilka metoder som i Sverige används för att bevara den biologiska mångfalden i skogen och se hur effektiva dessa är. Även möjliga vägar framåt kommer att diskuteras. Bevarandemetoderna och -verktygen som används i Sverige är bland annat spridningskorridorer, naturvårdsbränning, naturreservat, det svenska artprojektet, åtgärdsprogram, frivilliga avsättningar och naturvårdshänsyn vid avverkningar. Samtliga metoder och verktyganvänds för att bevara den biologiska mångfalden i svenska boreala skogar. I forskningsöversikten beskrivs varje metod eller verktyg och vilka effekter som dessa ger. Resultatet fokuserar på metoderna naturvärdsbränning och spridningskorridorer, även om formellt avsatta områden så som naturreservat är den mest använda metoden i Sverige i nuläget. Alternativa metoder som borde implementeras i svenska skogar presenteras också. Dessa är mikroklimatisk buffring, identifikation av nyckelbiotoper samt utökad planering och samarbete. Slutligen presenteras ett didaktiskt perspektiv om elevers förståelse av biologisk mångfald, vilka undervisningssätt som vanligen används för att lära ut om biologisk mångfaldsamt vilka undervisningssätt som framöver bör användas för att öka elevernas kunskaper. Litteraturöversikten visar att dagens bevarandemetoder i Sverige ger betydligt större positiva effekter än negativa och att dessa bör fortsätta användas och i vissa fall utökas. Angående alternativa metoder finns det stöd för att dessa skulle fungera väl även inom Sverige. Den didaktiska undersökningen visar att elever och lärarstudenter visar en nedåtgående kunskapskurva om biologisk mångfald och att detta har pågått under en längre tid. Undervisningsmetoder som oftast används är grupparbeten och -diskussioner, lärarledda presentation och fältstudier. Dessa metoder bör fortsätta användas, gärna tillsammans med lektioner eller projekt utomhus och besök till verksamma laboratorier, för att öka elevernas förståelse kring biologisk mångfald. / The biodiversity in Sweden as well as globally is decreasing at an alarming rate and has been for a rather long time. The extinction rate is up to 1000 times higher than the normal rate and many species has already gone extinct partly due to climate change. The purpose of this summery is to compose commonly used conservation methods and tools in Sweden and see how efficient these are. Possible ways forward will also be discussed. Conservation methods and tools commonly used in Sweden are corridors, conservation burning, nature reserves, the Swedish Species Project and actions programs. All the mentioned methods and tools are used for biodiversity conservation in Swedish boreal forests. In this literature review each method or tool is described as well as its effects on biodiversity. Corridors and conservation burning are the focus for the report even though nature reserves is the most used method in Sweden today. Alternative methods for biodiversity conservation that we believe should be implemented in Swedish forests will also be presented. These methods are microclimatic buffering, identification of key biotopes and extended planning and communication between the owners and the state or municipality and other involved organizations. Lastly, a didactive perspective is presented in which students’ knowledge of biodiversity, common teaching methods regarding biodiversity and optimal teaching methods for an increased understanding will be discussed. The review show that the commonly used methods in Sweden are much more positive than negative and should therefore continue, and in some cases be extended. Regarding alternative methods, there is significant support for positive results in Sweden if used. The didactic analysis show that students and teacher students show a decreasing knowledge with respect to biodiversity and that this has gone on for quite some time now. Teaching methods often used in biodiversity education is groupwork and -discussions, presentations and field studies. These methods should continue being used as well as lessons or projects outdoor and field trips to laboratories to increase the students’ knowledge about biodiversity.

Biologisk mångfald minskar kraftigt i hela världen som ett resultat av mänskliga handlingar. Den biologiska mångfalden hos svenska boreala skogar hotas av det moderna skogsbruket där trakthyggesbruk som är en vanlig skogsbruksmetod skapar en miljö som är svår att överleva i för naturvårdsarter. I denna studie är syftet att undersöka om observationer av sex stycken naturvårdsarter är högre i skyddade värdekärnor bestående av gammal barrskog än i produktionsbarrskog i Dalarnas län, Sverige. Detta genom att analysera andelen av de totala observationerna i proportion till andelen av den totala arealen för respektive skogstyp samt andelen observationer av varje naturvårdsart för sig i proportion till andelen av den totala arealen för respektive skogstyp. Då tidigare studier visat att artantalet och den biologiska mångfalden är högre i skyddade områden, inkluderat gammal skog, förväntades det att resultatet för denna studie skulle följa samma trend. Rumsliga data för skyddade värdekärnor och produktionsskog erhålls från Naturvårdsverket, Skogskarta från Sveriges lantbruksuniversitet (SLU) och rumsliga data för artobservationer från SLU Artdatabanken. Den rumsliga analysen utfördes i QGIS och en statistisk analys genom ett Pearson Chi square-test i SPSS. Resultatet i studien visade att det inte finns en signifikant skillnad i proportionen mellan andelen observationer och andelen areal för respektive skogstyp vilket är skyddade värdekärnor och produktionsskog. För varje enskild naturvårdsart var det endast för rynkskinn (Phlebia centrifuga) som det fanns en signifikant skillnad i proportionen mellan andelen observationer och andelen areal för respektive skogstyp vilket indikerar på att den arten föredrar skyddade värdekärnor över produktionsskogar. Studiens resultat överensstämmer inte med tidigare studier men trots detta är bevarandet och främjandet av gamla skogar som är skyddade en viktig del i arbetet mot att stoppa minskningen av den biologiska mångfalden. / Biodiversity is declining rapidly worldwide as a result of human actions. The biodiversity of Swedish boreal forests is threatened by modern forestry, where clearfelling, which is a common forestry practice, creates an environment that is difficult for conservation species to survive in. In this study, the aim is to investigate whether observations of six conservation species are higher in protected high conservation value forests consisting of old coniferous forest than in production coniferous forest in Dalarna County, Sweden. This by analyzing the share of total observations in proportion to the share of the total area for each forest type, and also the share of observations of each conservation species separately in proportion to the share of the total area for each forest type. As previous studies have shown that species numbers and biodiversity are higher in protected areas including old-growth forests, it was expected that the results of this study would follow the same trend. Spatial data for protected high conservation value forests and production forests were obtained from the Swedish Environmental Protection Agency, Forest Map from the Swedish University of Agricultural Sciences (SLU) and spatial data for species observations from the SLU Swedish Species Information Centre. The spatial analysis was performed in QGIS and a statistical analysis through a Pearson Chi square test in SPSS. The results of the study showed that there is no significant difference in the proportion of observations to area for each forest type that is protected high conservation value forests and production forests. For each individual conservation species, only for Phlebia centrifuga fungus was there a significant difference in the proportion between the share of observations and the share of area for each forest type which indicates that this species prefers high conservation value forests over production forests. The results of the study are not consistent with previous studies, but despite this the conservation and promotion of protected old-growth forests is an important part of efforts to halt biodiversity loss.

Boreal peatlands store carbon sequestered from the atmosphere over millennia and the importance of this and the other ecosystem services these areas provide is now widely recognised. However, a changing climate will affect these environments and, consequently, the services they provide to the global population. The rate and direction of environmental change to peatlands is currently unclear and they have not yet been included in many climate models. This may in part be due to the ecological heterogeneity and spatial extent of these areas and the sparse sampling survey methods currently adopted. Hyperspectral remote sensing from satellite platforms may in future offer an approach to surveying and do so at the high spectral and spatial resolutions necessary to infer ecological change in these peatlands. However, work is required to develop methods of analysis to determine if hyperspectral data can be used to measure the overstorey vegetation of these areas. This will require an understanding of how annual and inter-annual cyclical changes affect the peatland plant canopy reflectances that would be recorded by hyperspectral sensors and how these reflectances can be related to state variable of interest to climate scientists, ecologists and peatland managers. There are significant areas of peatland within Scotland and, as it is towards the southern extreme of the boreal peatlands, these may be an early indicator of environment change to the wider boreal region. Calluna vulgaris, a hardy dwarf shrub, is the dominant overstorey species over much of these peatlands and could serve as a proxy for ecological, and consequently, environmental change. However, little has been done to understand how variations in leaf pigments or canopy structural parameters influence the spectral reflectance of Calluna through annual and inter-annual growth and senescence cycles. Nor has much work been done to develop methods of analysis to enable images acquired by hyperspectral remote sensing to be utilised to monitor change to these Calluna dominated peatlands over time. To advance understanding of the optical properties of Calluna leaves and canopies and develop methods to analyse hyperspectral images laboratory, field and modelling studies have been carried out in time series over a number of years. The leaf and canopy parameters significantly affecting reflectance have been identified and quantified. Differences between published Chlorophyll(a+b) in vivo absorption spectra and those determined were found. Carotenoids and Anthocyanins were also identified and quantified. The absorption spectra of these pigments were incorporated into a canopy reflectance model and this was coupled to a Calluna growth model. This combined model enabled the reflectance of Calluna canopies to be modelled in daily increments through annual and inter-annual growth and senescence cycles. Reasonable results were achieved in spectral regions where reflectance changed systematically but only for homogeneous Calluna stands. However, it was noted during this research that the area of support for the spectral measurements appeared to differ from that assumed from the specification provided by the spectroradiometer manufacturers. The directional response functions (DRFs) of two spectroradiometers were investigated and wavelength, or wavelength region, specific spatial dependences were noted. The effect that the DRFs of the spectroradiometers would have on reflectances recorded from Calluna canopies was investigated through a modelling study. Errors and inaccuracies in the spectra that would be recorded from these canopies, and commonly used biochemical indices derived from them, have been quantified.

Methylmercury (MeHg) is a neurotoxic compound that threatens the well-being of humans and wildlife. It is formed through the methylation of inorganic mercury (HgII) under suboxic/anoxic conditions in soils, sediment and waters. The chemical speciation of HgII, including specific HgII species in aqueous and solid/adsorbed phases, plays a key role in MeHg formation. Chemical forms of HgII which have been reported to be available for uptake in methylating bacteria include neutral HgII–sulfide complexes, HgII complexes with specific low molecular mass (LMM) thiols, and nanoparticulate HgS(s). Accurate determination of the chemical speciation of HgII is thus crucial when elucidating the mechanism of MeHg formation. The concentration of HgII–LMM thiols complexes is predicted to be extremely low (sub fM range). Current analytical methods do not allow direct quantification of HgII complexes due to the very low concentration of these complexes, and therefore determination rely on thermodynamic modeling. Accurate stability constants for HgII–LMM thiols complexes and quantification of LMM thiol ligands in environments are thus required to precisely determine the concentration of such complexes. In this thesis, a novel analytical method was developed based on online pre-concentration coupled with liquid chromatography tandem mass spectrometry to determine the concentration of 16 LMM thiols (Paper I). This method was successful in detecting 8 LMM thiols in boreal wetland porewaters, with mercaptoacetic acid and cysteine being the most abundant. The total concentration of individual detected LMM thiols ranged from sub nM (LOD=0.1 nM) to 77 nM. Moreover, the stability constant (β2) for HgII complexes with 15 LMM thiols were directly determined for the first time by competing ligand exchange experiments combined with liquid chromatography ICPMS analysis (Paper II). Values of log β2 for the reaction Hg2+ + 2LMM-RS- = Hg(LMM-RS)2 ranged from 34.6 for. Based on the determined constants of Hg(LMM-RS)2 complexes and state-of-the-art constants from literature for other HgII complexes, we established comprehensive thermodynamic speciation models for MeHg and HgII in boreal wetlands (Paper III). The speciation of HgII was coupled with the HgII methylation rate constant (km) determined with different enriched Hg isotope tracers (Paper IV). There was a good correlation (R2=0.88) between the km determined by a HgII(aq) tracer added as Hg(NO3)2 with high bioavailability and a tracer where HgII was bond to thiol groups in natural organic matter (HgII-NOM(ads)) and has a lower bioavailability. The HgII(aq) tracer was consistently methylated at 5 times higher rate than the HgII-NOM(ads) tracer. A good correlation was observed between the concentration of biologically produced LMM thiols and km in the boreal wetlands. In a mesocosm study of estuarine sediment-brackish water systems, increased concentration of phytoplankton chlorophyll α due to macro nutrient additions led to an increase in HgII methylation rate of the HgII(aq) but not of the HgII-NOM(ads) tracer or ambient HgII species (Paper V). Furthermore, simulated newly deposited HgII species from atmospheric and terrestrial sources were exhibited significantly higher HgII methylation rates when compared with simulated aged sediment HgII pools. Through the development and adoption of novel analytical methods, this thesis reveals the significance of LMM thiols in Hg biogeochemistry by precise determination of HgII–LMM thiol complexes in natural environmental systems.

Anthropogenic activities perturb the global carbon and nitrogen cycle with large implications for the earth’s climate. Land use activities deliver excess carbon and nitrogen to aquatic ecosystems. In the boreal biome, this is mainly due to forestry and atmospheric deposition. Yet, impacts of these anthropogenically mediated inputs of carbon and nitrogen on the processing and emissions of greenhouse gases from recipient streams and lakes are largely unknown. Understanding the ecosystem-scale response of aquatic greenhouse gas cycling to land use activities is critical to better predict anthropogenic effects on the global climate system and design more efficient climate change mitigation measures. This thesis assesses the effects of forest clearcutting and nitrate enrichment on greenhouse gas emissions from boreal inland waters. It also advances methods to quantify sources and sinks of these emissions. Short-term clearcut and nitrate enrichment effects were assessed using two whole-ecosystem experiments, carried out over four years in nine headwater catchments in boreal Sweden. In these experiments, I measured or modeled air-water fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), combining concentration, ebullition and gas-transfer velocity measurements in groundwater, streams and lakes. By using Swedish national monitoring data, I also assessed broad-scale effects of forest clearcutting by relating CO2 concentrations in 439 forest lakes to the areal proportion of catchment forest clearcuts. To improve quantifications of CO2 sources and sinks in lakes, I analyzed time series of oxygen concentrations and water temperature in five lakes on conditions under which whole-lake metabolism estimates can be inferred from oxygen dynamics given the perturbing influence of atmospheric exchange, mixing and internal waves. The experiments revealed that aquatic greenhouse gas emissions did not respond to nitrate addition or forest clearcutting. Importantly, riparian zones likely buffered clearcut-induced increases in groundwater CO2 and CH4 concentrations. Experimental results were confirmed by monitoring data showing no relationship between CO2 patterns across Swedish lakes and clearcut gradients. Yet, conclusions on internal vs. external CO2 controls largely depended on whether spatially or temporally resolved data was used. Partitioning CO2 sources and sinks in lakes using time series of oxygen was greatly challenged by physical transport and mixing processes. Conclusively, ongoing land use activities in the boreal zone are unlikely to have major effect on headwater greenhouse gas emissions. Yet, system- and scale specific effects cannot be excluded. To reveal these effects, there is a large need of improved methods and design of monitoring programs that account for the large spatial and temporal variability in greenhouse gas dynamics and its controls by abiotic and biotic factors.

Boreal old-growth forests are key determinants in the global carbon cycle. It is unknown how the role of persistent old-growth forests will be in the carbon cycle in the face of predicted climatic changes. Eddy-covariance measurements are commonly used to quantify carbon exchange between ecosystems, such as forests, and the atmosphere. Error due to gap-fill method is of particular interest in these datasets. Here we filled a 15-year eddy covariance dataset from the Northern Old-Growth Boreal Black Spruce (Picea mariana) site located near Thompson, in central Manitoba, Canada using four different gap-fill methods. Our objectives were to determine if choice of gap-fill method affected annual NEP and if these errors compounded to even greater differences over the 15-year study period. Most significant differences in NEP among methods occurred from September to December, but variations during the growing season were responsible for most of the annual differences. / October 2016

Terrestrial inputs of organic matter contribute greatly to the functioning of aquatic ecosystems, subsidizing between 30-70% of secondary production. This contribution of terrestrial resources is especially important in boreal lakes that are largely nutrient-poor and thus more responsive to these additions. Yet the mechanisms underlying initial processing of terrestrial resources by microbial communities at the base of lake food webs remain poorly understood. With this in mind, this thesis aims to advance our understanding of lake sediment microbial community assembly and functioning along abiotic gradients, primarily reflecting variation in terrestrial organic matter inputs that are predicted to increase with future environmental change. Chapter 1 reviews current knowledge on the terrestrial support of lake food webs and highlights gaps in understanding the factors influencing the microbial processing of terrestrial resources. It also provides an overview of metagenomics methods for microbial community analysis and their development over the course of the thesis. Chapter 2 tests how much of ecosystem functioning is explained by microbial community structure relative to other ecosystem properties such as the present-day and past environment. Theory predicts that ecosystem functioning, here measured as CO2 production, should increase with diversity, but the individual and interactive effects of other ecosystem properties on ecosystem functioning remain unresolved. Chapter 3 further questions the importance of microbial diversity for ecosystem functioning by asking whether more diverse microbial communities stabilize ubiquitous functions like CO2 production and microbial abundances through time. It also aims to identify the biotic and abiotic mechanisms underlying positive diversity-stability relationships. Chapter 4 then explores how microbial communities assemble and colonize sediments with varying types and amounts of terrestrial organic matter in three different lakes over a two-month period. Understanding how microbial communities change in relation to sediment and lake conditions can help predict downstream ecosystem functions. Finally, Chapter 5 discusses the main findings of the thesis and ends with proposed avenues for future research.