Brownification is a collective concept of the processes that leads to brownified water. The driving forces behind it are many, but land use change and climate change are two examples. In this study I have focused on precipitation and how it affects brownification. This has been examined by analyzing the absorbance (420 nm) at eight localities for ten weeks. Besides that, I have examined the link between absorbance and dissolved organic carbon (DOC) at the same localities. The results from this survey show that rainfall and absorbance does not correlate in 75% of the cases. In the remaining 25% of the cases there is a significant correlation, with a varied degree of strength between the localities. When sampling is done the same day as rainfall, MurånDike2 and Murån5 are affected. When sampling is done three days after rainfall, Murån5 and Murån7 are affected. When sampling is done seven days after rainfall, MurånBolmen0 and Murån7 are affected. Absorbance and DOC have a significant correlation (R² =0,938; F =1185; P <0,001) which is in line with prior research.   The lack of significant correlations could be explained by the low amount of precipitation during the period. When there is a significant correlation, the physical design of Murån could be worth looking into. On each occasion when precipitation results in significant correlation with increased absorbance, the localities have similar characteristics. One example is localities Murån5 and Murån7, which are wide (>3 m), have a constant supply of fresh water and are in the main channel. This study contributes with an insight into what effects precipitation has to absorbance and confirms the relationship between absorbance and DOC. / Brunifiering är ett samlingsbegrepp för de processer som tillsammans ledertill att vatten blir allt brunare. Drivkrafterna bakom detta är flera, medändrad markanvändning och klimatförändringar som två exempel. Dennastudie har fokuserat på nederbörd och hur det påverkar brunifieringen avvatten. Detta har undersökts genom att analysera absorbansen (420 nm) pååtta lokaler under tio veckor. Dessutom har sambandet mellan absorbansoch löst organiskt kol (DOC) undersökts vid samma lokaler. Resultatet avundersökningarna visar att nederbörd och absorbans inte korrelerar i 75% avfallen. I de resterande 25% av fallen finns det ett signifikant samband, medvarierande styrka mellan lokalerna. Görs provtagningar samma dag som detregnat påverkas MurånDike2 och Murån5. Görs provtagningar tre dagarefter det regnat påverkas Murån5 och Murån7. Görs provtagningar sju dagarefter det regnat påverkas MurånBolmen0 och Murån7. Absorbans och DOChar ett signifikant samband (R² =0,94; F =1185; P <0,001), vilket följertidigare forskning.Bristen på signifikanta samband skulle kunna förklaras med den låga mängdnederbörd som fallit under perioden. När det finns en signifikant korrelationkan Muråns fysiska utformning vara en faktor, vilket bör undersökas vidare.Vid samtliga tillfällen när nederbörd resulterar i signifikant korrelation medökad absorbans har lokalerna liknande karaktärer. Ett exempel är lokalernaMurån5 och Murån7, vilka är breda (>3 m), ständigt förses med nytt vattenoch ligger i huvudfåran. Den här studien bidrar med en insikt i vilka effekternederbörd har på absorbans och bekräftar förhållandet mellan absorbans ochDOC.

For more than 45 years an extensive liming measure has taken place in the southwest of Sweden in Västra Götaland county. The goal with the liming is to improve the aquatic ecosystems and counteract the acidification that has occurred in a large part of catchments in this area. The acidification problem was discovered in the late seventies and had a global impact but was distinctive in northern Europe. To mitigate the problem, it was necessary to find what was causing the acidification and, in this case, it was the emissions of acidic substances. A legislation about decreasing the emissions of sulfuric acid was established by several countries in Europe and after that a change occurred. Most of the aquatic ecosystems responded well to the decreased amount of sulfuric deposition and the liming. An unexpected response to the improved health of the aquatic ecosystems was the increased water color (mgPt·l-1) in surface waters. The change in water color, often called brownification, can be a natural process and an indication that the surface waters are approaching a more natural state. In this study a total of 120 surface waters in Västra Götaland county were studied under a 40-year period. An increasing browning of surface waters and significant differences in water color between limed lakes and reference lakes was discovered. The area with the highest yearly precipitation had the largest percentual increase in water color particularly for limed lakes. The results showed significant differences in water color in limed lakes between two time periods, 1981-1985 and 2015-2020, that indicate an increasing trend in the study area. The results could not answer why the surface waters had this increase in browning and there was not possible to determine all the driving factors. Other studies have shown that increasing water color relates to a decrease in sulfuric deposition and increasing concentration of dissolved organic carbon (DOC) in the surface waters. Climate change can be a part of the explanation due to warmer temperatures, longer vegetation periods and wetter climate. Further, it is necessary to continue investigating this problem to determine the driving factors and detect possible trends.

During recent decades many lakes have become browner in the northern hemisphere and more specific in Sweden. This process is called brownification. Brownification of lakes makes it more difficult to clean water to drinking water and may have negative ecological effects on biota. Browning of lakes is generally thought to be caused by an increase of humic substances that consist of organic matter which colour the water brown or yellow. However, more recent studies show that dissolved iron can interact with humic substances in browning lakes. Since the concentrations of dissolved iron have increased during recent years and the mechanisms behind brownification and the contribution of iron to this process is not clearly understood it is important to investigate this subject. In this study 17 lakes in south of Sweden were sampled for iron concentration, dissolved organic carbon (DOC), absorbance (420nm) and pH. Further, data was added about atmospheric sulphur deposition and additional data from 17 lakes in the north part of Sweden. Iron had a stronger significant correlation towards absorbance than DOC had in the south of Sweden. A similar amount of dissolved iron seems to colour lakes differently based on their location in Sweden. This indicates that different mechanisms are involved in the interaction between iron and absorbance. However, pH had a stronger relationship with absorbance than either DOC or iron. Overall results suggested that iron do have a strong browning effect on Swedish fresh water lakes in the presence of DOC and that iron-increase driven processes may be due to a change of pH.

Freshwater lakes are among the most important ecosystems for both human and other biological communities. They account for about 87% of surface freshwater in the planet, thus constituting a major source of drinking water. They also provide a wide range of ecosystem services that go from the sustenance of a rich biodiversity to the regulation of hydrological extremes; from the provision of a means for recreation to the support of local economies, e.g., through tourism and fisheries, just to cite a few. Lakes are now also widely recognised as natural early warning systems, their responses potentially being effective indicators of local, regional and global scale phenomena such as acidification and climate change, respectively. This is because of their high sensitivity to environmental factors of the most diverse nature that can rapidly alter the course of their evolution. Examples of this are the observed abrupt shifts between alternative stable states in shallow lakes, which led them to become the archetype, go-to example in alternative stable state theory. Therefore, attaining a good scientific understanding of the many processes that take place within these ecosystems is fundamental for their adequate management. Among the tools that serve this purpose, ecological models are particularly powerful ones. Since their introduction in the 1960s, the development of mechanistic ecological models has been driven by their wide spectrum of potential applications. Nevertheless, these models often fall into one of the two following categories: overly simplistic representations of isolated processes, with limited potential to explain real-world observations as they fail to see the bigger picture; or overly complex and over-parameterised models that can hardly improve scientific understanding, their results being too difficult to analyse in terms of fundamental processes and controls. Moreover, it is now well known that an increased complexity in the mechanistic description of ecological processes, does not necessarily improve model accuracy, predictive capability or overall simulation results. To the contrary, a simpler representation allows for the inclusion of more links between model components, feedbacks which are usually overlooked in highly-complex models that partially couple a hydro-thermodynamic module to a biogeochemical one. However, ecological processes are now known to have the potential to significantly alter the physical response of aquatic ecosystems to environmental forcing. For example, steadily increasing concentrations of coloured dissolved organic carbon, a process known as brownification (also browning), as well as the intense phytoplankton blooms that characterise lakes undergoing severe nutrient enrichment, a process known as eutrophication, have been shown to have the potential to alter the duration of the stratified period, thermal structure and mixing regime of some lakes. In this thesis, with the aim of addressing the limitation of partially-coupled models to account for such feedbacks, we further develop a process-based model previously reported in scientific literature. Subsequent studies have already built upon this model in the last few years. In Chapter 2, we do so too by integrating hydro-thermodynamics and biogeochemistry in a reduced complexity framework, i.e., customising the model so that each version only includes the fundamental processes that, brought together, sufficiently describe the studied phenomena. Two case studies served the purpose of testing the adaptability and applicability of the developed model under different configurations and requirements. Limnological data for these two studies were measured at high spatial and temporal resolutions by means of an automated profiling system and recorded as part of two large-scale mesocosm experiments conducted in 2015 and 2016 at the IGB LakeLab in Lake Stechlin, Brandenburg, Germany. Meteorological datasets were also made available to us for both periods by the German Federal Environment Agency. The scope of the first experiment, which we describe in Chapter 3, was that of detecting any changes attributable to eutrophication and browning, in the competition for nutrients and light between four different groups of lake primary producers. These four groups are phytoplankton, periphyton, epiphyton and macrophytes. The model version for this study, therefore, includes equations for all four groups. By tailoring the model to these very specific needs with relative ease, we demonstrate its versatility and hint at its potential. The second experiment, described in Chapter 4, sought to shed light on the largely unknown effects of an increase in the diffuse luminance of the night sky that is due to artificial light at night (artificial skyglow) on lake metabolic rates, i.e., gross primary productivity, ecosystem respiration and net ecosystem productivity (the difference between the first two). For this purpose, an empirical equation for dissolved oxygen concentration was included, the parameters of which were estimated by means of a Markov Chain Monte Carlo sampling method within a Bayesian statistical framework, showing the compatibility, with these statistical methods, of our otherwise fully deterministic model. In Chapter 5, we present a theoretical study on the ecological controls of light and thermal patterns in lake ecosystems. A series of simulations were performed to determine in which cases ecological processes such as eutrophication and brownification may have an observable effect on the physical response of lakes to environmental forcing, which we assessed along a latitudinal gradient. Results show that, in general, across all examined latitudes, and consistent with previous studies, accounting for phytoplankton biomass results in higher surface temperatures during the warm-up phase, slightly lower water temperatures during the cool-down phase, and a shallower thermocline throughout the entire stratified period. This effect is relatively more important in eutrophic lakes where intense blooms are likely. This importance, however, decreases as lakes get browner. Finally, in line with the overall scope of the SMART EMJD, in Chapter 6 we illustrate the case of Ypacaraí Lake, the most important lake in landlocked Paraguay, hoping to provide an example of how interdisciplinary research and international intersectoral collaboration can help bridge the gap between science and management of freshwater ecosystems. This lake presents very special hydro-ecological conditions, such as very high turbidity that can impair phytoplankton growth despite its nutrient-based trophic state indices having consistently fallen within the hyper-eutrophic range in recent years. A strong interest in its complex functioning, through modelling, was taken early on. This led to a collaborative research line being established among several public and private institutions in Italy, Germany and Paraguay. Results so far include: • three concluded UniTN Master theses in Environmental Engineering, partly developed in Paraguay, the first two in collaboration with the “Nuestra Señora de la Asunción” Catholic University (UCNSA) and the third one with the National University of Asunción (UNA); • a collaborative UCNSA-UniTN research proposal submitted for consideration to receive funding through the PROCIENCIA Programme of the National Council of Science and Technology of Paraguay (CONACYT); and • the first multidisciplinary review that has ever been published about the case of Ypacaraí Lake, which highlights the importance of such a collaborative and integrative approach to further advance scientific knowledge and effectively manage this ecosystem.

Sjön Öjaren i Sandviken är den största dricksvattentäkten i kommunen, färgen på Öjarens vatten har med åren ändrats och blivit allt mörkare. Detta bekräftas vid analys av vattenprover mellan år 1995-2015 som utförts i denna studie. Denna förändring av vattenfärg är inte unik för Öjaren utan har påvisats i flera andra sjöar och vattendrag. Orsakerna till förändrad vattenfärg skiljer sig till viss del beroende på vilken vattenförekomst som undersöks, olika egenskaper som klimat, markbeskaffenhet samt föroreningar bidrar i varierad utsträckning till brunifiering. Gemensamt för Öjaren och andra vattenförekomster är att ökad tillförsel av löst organiskt kol och järn är orsaker till brunifiering. För Öjaren har även ökade manganhalter visat sig påverka brunifieringen. Syftet med denna studie är att undersöka ett eventuellt samband mellan färgen på vattnet i Öjaren och de faktorer som anses påverka färgen. De faktorer som analyseras och enligt tidigare studier anses vara relevanta är järn, pH, COD, mangan, temperatur och nederbörd. Syftet är även att undersöka möjliga åtgärder för att motverka brunifiering. Vattenprover från Öjaren mellan åren 1995-2015 har legat till grund för den statistiska analys som utförts. För att finna eventuella samband har Pearson´s korrelationskoefficient beräknats i programmet Minitab 17. För att ytterligare beskriva det eventuella sambandet mellan faktorerna har en principal component analysis (PCA) utförts. Trender och prognoser har även de tagits fram genom Minitab. Korrelationsanalyserna har visat på mest signifikant samband mellan färgtal och faktorerna järn, COD och mangan. Resultaten av korrelationsanalysen förstärks av PCA:n där det uppvisas att färgtal, järn och COD till störst del samvarierar. Trenden och den framtida prognosen för samtliga undersökta faktorer visar på stigande värden. Detta innebär att dricksvattenberedningen blir mer komplicerad och kostsam samt att dricksvattenkvalitén riskerar att försämras. Denna utveckling är något som förväntas beröra flera ytvattentäkter i bl.a. Europa och Nordamerika. Färgtalet i Öjaren förväntas stiga med 63 % fram till år 2050 vilket innebär att åtgärder för att motverka brunifieringen i Öjaren och andra liknande ytvattenförekomster är nödvändiga för att de ska kunna användas som dricksvattentäkter i framtiden. / The colouring of water in many lakes across the Northern hemisphere is increasing. This is also the case in Lake Öjaren in Sandviken municipality in Sweden. The lake is the main water source in Sandviken and has a high and increasing watercolour. This leads to more complex cleaning methods and higher costs for the society. This study focuses on the changing and increasing watercolour in the lake over a period from 1995-2015. The factors analysed in this study are those who is considered to have an effect on the watercolour in the lake. The reasons for increased brownification are different depending on which type of water body is studied. Climate, soil conditions and pollutants contributes in varying extent to brownification. Increased levels in dissolved organic carbon (DOC) and iron are common reasons for brownification in Lake Öjaren and other water bodies, in Lake Öjaren increased levels of manganese are also shown to affect brownification. The aim of the study is to evaluate the correlation between the watercolour and the factors: iron, pH, COD, manganese, temperature and precipitation. The aim is also to investigate which actions are possible to apply to prevent increased watercolour. Water samples from Lake Öjaren between the years 1995-2015 have been the basis for the executed statistical analysis. Evaluating the correlation between the factors we have used Minitab 17 and calculated Pearson´s correlation coefficient. To further explain the relationship between the factors a principal component analysis (PCA) has been performed. Future scenarios and current trends have also been produced, in order to develop a better foundation for further studies. The correlation analysis reveals that the most significant connection with watercolour are iron, COD and manganese. The result of the PCA also corroborates this connection where three factors are the ones that are most co-variating. The trend and the future scenario for all the factors in Lake Öjaren are that they all are increasing and the watercolour is expected to be increasing by 63 % until year 2050. This scenario means that actions are needed to counteract the increasing colouring of the water.