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Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events.

The present thesis focuses on three currently understudied alterations in flow and material dynamics within river networks, namely (i) on the dynamics of floating organic matter (FOM) and its modification in dammed rivers, (ii) on river intermittency and its effects on nutrient and organic matter (OM) dynamics, and (iii) on major future water transfer megaprojects (WTMP). Construction and operation of dams cause modifications in water flow and material fluxes in rivers, such as of FOM. River damming, climate change and water extraction for human needs also lead to a rapid expansion in number and extent of intermittent rivers worldwide, with major biogeochemical consequences on both regional and global scales. Increased intermittency of river networks in turn forces people to implement engineering solutions, such as WTMP, which help to supply water to places of demand. WTMP introduce artificial links among freshwater bodies modifying the hydrological balance. Impacts of abovementioned activities on freshwaters have been assessed in single case studies. Current thesis aimed to integrate the existing knowledge on these impacts and allow a generalization of their globally applicable meaning for ecosystems. The role of FOM in rivers as a geomorphological agent, a resource, a dispersal vector and a biogeochemical component was investigated based on an extensive literature review. Collected information allowed for conceptualizing its natural cycle and dynamics, applicable to a wide range of rivers. Data on FOM accumulations at 31 dams located within catchments of 13 rivers from North America and Europe showed that damming leads to FOM entrapment and modifies its natural cycling. The results of a spatial analysis considering environmental properties revealed that catchment characteristics can explain around 57% in the variation of trapped FOM amounts. Effect of rewetting events on the release of nutrients and OM from bed sediments and course particulate organic materials (CPOM, leaves and biofilms) accumulated in IRES was studied in laboratory experiments using samples from 205 rivers. Results showed that the release of nutrients and OM is substrate and climate specific. On a global scale, dissolved organic carbon, phenolics, and nitrate dominated the flux from riverbeds upon rewetting. The highest amount of nutrients and OM per gram of substrate was released from leaves, but sediments were the main contributors to the total areal flux from rivers due to their higher mass within the riverbeds. Leachates with the highest quality in terms of potential influence on ecosystem processes (low aromaticity, high content of biopolymers) were released from biofilms. A set of nine environmental variables used to explain leachate concentration across climate zones performed best for sediments in the continental and tropical zones. The inventory of the future WTMP compiled in this thesis is based on various peer- and non peer-reviewed literature sources and revealed 60 WTMP planned or under construction. The results showed that North America, Asia and Africa will be the most affected by future WTMP having the highest densities of projects and the largest water transfer distances and volumes. If all projects were completed by 2050, the total water transfer distances would reach 77,063 km transferring more than 1,249 km3 per year. Overall, the results of the current thesis showed the importance of the studied alterations in river flow and organic matter dynamic on the global scale. Studied alterations therefore should be considered in global hydrological and biogeochemical models, as well as in sustainable management of rivers., Die vorliegende Arbeit behandelt drei wissenschaftlich bisher vernachlässigte Veränderungen in Fließ- und Stoffdynamik innerhalb von Flussnetzen, nämlich (i) die Dynamik von schwimmenden organischen Stoffen (FOM) und deren Änderung in gestauten Flüssen, (ii) die Flussperiodizität und deren Effekt auf die Dynamik von Nährstoffen und organischer Materie (OM) und (iii) die großen zukünftigen Wassertransferprojekte (WTMP). Der Bau und der Betrieb von Staudämmen beeinflussen den Wasser- und Stofftransport, zum Beispiel FOM, in Flüssen. Das Stauen von Flüssen, der Klimawandel und die Wasserextraktion zum menschlichen Nutzen führen weltweit zu einer raschen Ausbreitung temporärer Flüsse in Anzahl und Fläche. Das hat beträchtliche biogeochemische Konsequenzen im regionalen und globalen Maßstab. Zunehmende Periodizität von Flussnetzen zwingt Menschen dazu, Baumaßnahmen, wie WTMP, vorzunehmen um die Wasserversorgung zu sichern. WTMP verursachen künstliche Verbindungen zwischen Süßgewässern, die das hydrologische Gleichgewicht verschieben. Die Auswirkungen der oben genannten Faktoren auf Süßgewässer wurden in einzelnen Fallstudien untersucht. Das Ziel dieser Doktorarbeit ist das existierende Wissen über diese Aspekte zu integrieren und die Generalisierung von deren Folgen zu ermöglichen. Die Rolle von FOM in Flüssen als geomorphologisches Element, als Ressource, als Ausbreitungsvektor und als biogeochemischer Bestandteil wurde mit Hilfe einer ausführlichen Literaturrecherche untersucht. Daten über die FOM-Anhäufung an 31 Dämmen innerhalb der Einzugsgebiete von 13 Flüssen in Nord-Amerika und Europa zeigten, dass Stauung von Flüssen zur Akkumulation von FOM führt und deren natürlichen Kreislauf beeinflusst. Das Ergebnis einer räumlichen Analyse, die die Umwelteigenschaften miteinbezieht, zeigte, dass die Eigenschaften des Einzugsgebietes 57% der Mengenunterschiede an akkumuliertem FOM erklären können. Der Einfluss von Wiedervernässungsereignissen auf die Freisetzung von Nährstoffen und OM aus Flussbettsedimenten und grober partikulärer organischer Substanz (CPOM, Blätter und Biofilme), die sich in IRES angehäuft hat, wurde in Laborexperimenten anhand von Proben aus 205 Flüssen untersucht. Es zeigte sich, dass die Eigenschaften von freigesetzten Substanzen von Klimazone und Substrat abhängig sind. Im globalen Maßstab dominieren gelöster organischer Kohlenstoff, Phenole und Nitrat die Stoffflüsse während der Wiedervernässungsereignisse. Die höchste Menge an Nährstoffen und OM pro Gram Substrat wurde von Blättern freigesetzt, wobei Sedimente wegen ihrer höheren Masse den größten Beitrag zur räumlichen Durchflussmenge ausmacht. Die freigesetzten Substanzen mit der höchsten Qualität im Sinne von potenziellem Einfluss auf die Prozesse in Ökosystemen (niedrige Aromatizität, hoher Anteil an Biopolymeren) wurden von Biofilmen freigesetzt. Der Effekt von Umweltvariablen auf Nährstoffkonzentrationen und die Qualität von OM konnte besser für Sedimente als für andere Substrate vorhergesagt werden mit dem größten Anteil der Varianzaufklärung in den kontinentalen und tropischen Zonen. Die globale Zusammenstellung zukünftiger WTMP, die in dieser Doktorarbeit gesammelt wurde, gründet sich auf Daten aus verschiedenen Literaturquellen, die von Experten begutachtet wurden. Insgesamt wurden 60 WTMP, die in Planung oder im Bau sind, identifiziert. Die Ergebnisse zeigten, dass Nordamerika, Asien und Afrika am meisten von den zukünftigen WTMP betroffen sind, weil sie die höchste Dichte an Projekten und die längsten Strecken und größten Volumina im Wassertransfer haben. Wenn alle Projekte bis 2050 fertiggestellt würden, würde die gesamte Wassertransferstrecke 77,063 km betragen und ein Volumen von mehr als 1,249 km3 pro Jahr umgeleitet werden. Insgesamt zeigen die Ergebnisse die Bedeutung der Veränderung von Flussabfluss und Stoffdynamik auf globaler Ebene. Die untersuchten Veränderungen müssen deshalb sowohl in der globalen hydrologischen und biogeochemischen Modellierung als auch im nachhaltigen Management von Flüssen berücksichtigt werden.

Globally, freshwater is unevenly distributed, both in space and time. Climate change, land use alteration, and increasing human exploitation will further increase the pressure on water as a resource for human welfare and on inland water ecosystems. Water transfer megaprojects (WTMP) are defined here as large-scale engineering interventions to divert water within and between river basins that meet one of the following criteria: construction costs >US$ 1 billion, distance of transfer >190 km, or volume of water transferred exceeds 0.23 km3 per year. WTMP represent an engineered solution to cope with water scarcity. These projects are most commonly associated with large-scale agricultural and energy development schemes, and many of them serve multiple purposes. Despite numerous case studies that focus on the social, economic, and environmental impacts of individual water transfer megaprojects, a global inventory of existing, planned and proposed projects is lacking. We carried out the first comprehensive global inventory of WTMP that are planned, proposed or under construction. We collected key information (e.g., location, distance, volume, costs, purpose) on 34 existing and 76 future (planned, proposed or under construction) WTMP. If realized, the total volume of water transferred by future projects will reach 1,910 km3 per year with a total transfer distance of more than twice the length of the Earth's equator. The largest future WTMP are located in North America, Asia, and Africa and the predicted total investment will exceed 2.7 trillion US$. Among future projects, 42 are for agricultural development, 13 for hydropower development and 10 combine both purposes. Future megaprojects are also planned to support mining, ecosystem restoration and navigation. Our results underscore the extent to which humans have and are planning to re-engineer the global hydrological network and flows through WTMP, creating a network of “artificial rivers.” They emphasize the need to ensure the inclusion of these projects in global and basin hydrological models, and to develop internationally agreed criteria to assess the ecological, social and economic impacts of WTMP.

На основании собственных исследований и данных постов мониторинга изучено влияние эвтрофикации на вторичное загрязнение Бугского лимана тяжелыми металлами. Выявлены корреляционные зависимости между проявлениями явления эвтрофикации и колебаниями содержания тяжелых металлов. Исследовательно влияние данных явлений на состояние гидробионтов. На основании полученных результатов предложены механизм и способы преодоления негативного влияния изучаемых процессов загрязнения на состояние поверхностных водных объектов., On the research basis and monitoring data the eutrophication impact on repeated pollution of Bug estuary with heavy metals has been examined. Correlation dependences between eutrophication and variation of heavy metals content have been determined. On the basis of received data the recommendations for preventing negative impact of pollution processes on surface water objects in future have been developed., На підставі власних досліджень та даних постів моніторингу вивчено вплив евтрофікації на вторинне забруднення Бузького лиману важкими металами. Виявлено кореляційні залежності між проявами явища евтрофікації та коливанням вмісту важких металів. Досліджено вплив даних явищ на стан гідробіонтів. На підставі отриманих результатів запропоновано механізм та способи подолання негативного впливу досліджуваних процесів забруднення на стан поверхневих водних об’єктів.

Intermittent rivers and ephemeral streams (IRES) encompass fluvial ecosystems that eventually stop flowing and run dry at some point in space and time. During the dry phase, channels of IRES consist mainly of dry riverbeds (DRBs), prevalent yet widely unexplored ecotones between dry and wet phases that can strongly influence the biogeochemistry of fluvial networks. DRBs are often overlooked because they do not strictly belong to either domain of soil or freshwater science. Due to this dual character of DRBs, we suggest that concepts and knowledge from soil science can be used to expand the understanding of IRES biogeochemistry. Based on this idea, we propose that DRBs can be conceptually understood as early stage soils exhibiting many similarities with soils through two main forces: i) time since last sediment transport event, and ii) the development status of stabilizing structures (e.g. soil crusts and/or vascular plants). Our analysis suggests that while DRBs and soils may differ in master physical attributes (e.g. soil horizons vs fluvial sedimentary facies), they become rapidly comparable in terms of microbial communities and biogeochemical processes. We further propose that drivers of DRBs biogeochemistry are similar to those of soils and, hence, concepts and methods used in soil science are transferable to DRBs research. Finally, our paper presents future research directions to advance the knowledge of DRBs and to understand their role in the biogeochemistry of intermittent fluvial networks., Earth-Science Reviews, 188, ISSN:0012-8252, ISSN:1872-6828

Surface-groundwater interactions in intermittent rivers and ephemeral streams (IRES), waterways which do not flow year-round, are spatially and temporally dynamic because of alternations between flowing, non-flowing and dry hydrological states. Interactions between surface and groundwater often create mixing zones with distinct redox gradients, potentially driving high rates of carbon and nutrient cycling. Yet a complete understanding of how underlying biogeochemical processes across surface-groundwater flowpaths in IRES differ among various hydrological states remains elusive. Here, we present a conceptual framework relating spatial and temporal hydrological variability in surface water-groundwater interactions to biogeochemical processing hotspots in IRES.

Surface-groundwater interactions in intermittent rivers and ephemeral streams (IRES), waterways which do not flow year-round, are spatially and temporally dynamic because of alternations between flowing, non-flowing and dry hydrological states. Interactions between surface and groundwater often create mixing zones with distinct redox gradients, potentially driving high rates of carbon and nutrient cycling. Yet a complete understanding of how underlying biogeochemical processes across surface-groundwater flowpaths in IRES differ among various hydrological states remains elusive. Here, we present a conceptual framework relating spatial and temporal hydrological variability in surface water-groundwater interactions to biogeochemical processing hotspots in IRES.