Living (biotic) and non-living (abiotic) factors drive the function of ecosystems across a variety of scales from the root-soil interface to the watershed. Biotic and abiotic global change pressures such as increasing temperature and invasive species are shifting how ecosystems function. Thus, exploring and understanding how these factors shape function across the landscape is an important research area. For example, climate change both directly and indirectly affects soil microbial functions – such as carbon mineralization and nitrogen transformations – through increasing activity under warming and altering inputs to the soil through species composition changes. Mountains provide a useful tool for studying relationships among biotic and abiotic factors because climate and species diversity shift along gradients. Here, I measured carbon and nitrogen soil processes as well as microbial extracellular enzyme activity along an elevational gradient to explore how changes in climate, edaphic properties, and biotic composition affects ecosystem function. As expected, climate and species composition varied in predictable ways along the gradient – actual evapotranspiration declined, and conifer dominance increased. Soil functions also shifted along the gradient. Potential carbon mineralization increased with elevation and with conifer dominance. Potential nitrogen mineralization rates increased with elevation and with conifer dominance. Surprisingly, there were few predictors for potential soil nitrification, which increased only with soil functional diversity. While temperature and moisture availability drive ecosystem function at broad scales and biotic factors typically drive function at the regional scale, we saw that function of soils at the mountain watershed scale was best explained by a combination of both abiotic and biotic factors.

<p>The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning.</p><p>The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss.</p><p>The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as interspecific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term.</p><p>In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell. </p>

The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning. The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss. The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as interspecific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term. In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell.

Biodiversity rich regions worldwide face threats from various global change agents. This research quantifies environmental influences on vegetation, and the impacts of exotic woody plant invasion and anthropogenic nitrogen (N) deposition in a global biodiversity hotspot. The study was conducted in the montane grasslands of the Nilgiris, Western Ghats, and outlines potential management options for this region. Specifically, I examined (1) the role of environmental factors in influencing native plant distribution and ecosystem properties, (2) the status and impact of exotic shrub (Scotch broom, henceforth broom) invasion, (3) the role of disturbances in the success of broom, (4) the role of fire in restoring invaded grasslands, and (5) the impacts of terrestrial N loading on the grassland ecosystem. I used experiments and surveys to assess these. Distributions of several key species were explained by a few complex environmental gradients. In invaded-grasslands, broom populations consisted mainly of intermediate size and age classes, with no clear indication of population decline. Invasion negatively impacted plant community structure and drastically changed composition, favoring shade-tolerant and weedy species. However, invasion did not greatly alter ecosystem function. Fire successfully eliminated mature broom stands, but resulted in a short-term increase in broom seedling recruitment. At the end of 18 months, the fire effects on uninvaded-grasslands were not apparent, but there was no conclusive evidence of the formerly invaded patches attaining the composition of uninvaded-grasslands following burning. N fertilization strongly influenced soil N dynamics, and shoot N concentrations, but effects on aboveground production were weak. Surprisingly, N enrichment had positive effects on diversity in the short-term. It is clear that these grasslands need immediate management intervention to forestall degradation from invasion. Fire could be used to eliminate mature broom stands and deplete persistent seedbanks, which will facilitate colonization by shade-intolerant grassland plants. Active restoration should be mindful of environmental preferences of framework species. Long-term studies of the impacts of N deposition in the context of disturbances will help determine realistic critical thresholds and utilize disturbances to buffer the potential adverse effects of increasing N loading.

No description available.

<p>Tropical ecosystems play a key role in regulating the global climate and the carbon cycle thanks to the large amounts of water and carbon exchanged with the atmosphere. These biogeochemical fluxes are largely the result of high photosynthetic rates. Photosynthetic activity is highly dependent on climate and vegetation, and therefore can be easily modified along with changes in those two factors. A better understanding of what drives or alters photosynthetic activity in the tropics will lead to more accurate predictions of climate and subsequent effects on ecosystems. The seasonal pattern of photosynthetic activity is one of the main uncertainties that we still have about tropical ecosystems. However, this seasonality of tropical vegetation and its relationship to climate change and land cover is key to understanding how these ecosystems could be affected and have an effect on climate.</p><p>In this dissertation, I present three projects to improve our understanding about tropical ecosystems and how their photosynthetic activity is affected by climate and land cover change. The lack of field-based data has been one of the main limiting factors in our study of tropical ecosystems. Therefore, in these projects I extensively use remote sensing-derived data to analyze large scale and long term patterns. In the first study, I looked at the seasonal relationship between photosynthetic activity and climate, and how model simulations represent it. Vegetation in most of the tropics is either positively correlated with both water and light, or positively correlated with one of them and negatively with the other. Ecosystem models largely underestimate positive correlations with light and overestimate positive correlations with water. In the second study, I focus on the effect of land cover change in photosynthetic activity and transpiration in a highly deforested region in the Amazon. I find that land cover change decreases tropical forests photosynthetic activity and transpiration during the dry season. Also, land cover change increases the range of photosynthetic activity and transpiration in forests and shrublands. These effects are intensified with increasing land cover change. In the last project, I quantify the amount of change in evapotranspiration due to land cover change in the entire Amazon basin. Our remote sensing-derived estimates are well aligned with model predictions published in the past three decades. These results increase our confidence in climate models representation of evapotranspiration in the Amazon.</p><p>Findings from this dissertation highlight (1) the importance of the close relationship between climate and photosynthetic activity and (2) how land cover change is altering that relationship. We hope our results can build on our knowledge about tropical ecosystems and how they could change in the future. We also expect our analysis to be used for model benchmarking and tropical ecosystem monitoring.</p>

Intensive Landwirtschaft ist eine der allgegenwärtigsten anthropogenen Bedrohungen für aquatische Ökosysteme und beeinflusst sowohl die abiotischen als auch die biotischen Eigenschaften von Fließgewässerökosystemen. Diese resultieren aus der Vielzahl von Störungen, die mit Landnutzungsänderungen verbunden sind, wie z.B. Uferkahlschlag und dem damit verbundenen Verlust von allochthonem organischem Material und Beschattung, Nährstoff- und Pestizidkontamination, Feinsedimenteintrag durch Erosion und Oberflächenabfluss, sowie beeinträchtigte Hydromorphologie und damit verbundenem Verlust von vielfältigen Lebensräumen. Umfangreiche Literatur zu den Auswirkungen von Landwirtschaft auf einzelne Ökosystemkomponenten wie Makroinvertebraten, Biofilm oder Fischfauna existiert bereits, wobei der Schwerpunkt hier zumeist auf strukturellen Ansätzen oder einzelnen Ökosystemprozessen liegt. Allerdings haben nur wenige Studien umfassende Auswertungen angestellt, wie intensive Landwirtschaft die Funktion des gesamten aquatischen Ökosystems und die Wechselwirkungen zwischen den Ökosystemkomponenten beeinflusst. Um komplexe Effektmuster landwirtschaftlicher Stressoren auf Fließgewässerstruktur- und funktion sowie mechanistische Wirkzusammenhänge zwischen trophischen Ebenen besser zu verstehen wurden im Rahmen der vorliegenden Arbeit zwei Bäche die innerhalb landwirtschaftlicher Einzugsgebiete liegen und zwei bewaldete Referenzbäche im Harz, Sachsen-Anhalt, hinsichtlich ihrer physiko-chemischen Eigenschaften, der Menge und der Einträge von benthischem organischem Material, der Primärproduktion und organischen Kohlenstoffkreisläufen sowie der Sekundärproduktion von Makroinvertebraten und Fischen über einen Zeitraum von 12-17 Monate miteinander verglichen. Intensive Landwirtschaft hatte starke Auswirkungen auf die physiko-chemischen Eigenschaften der untersuchten Bachoberläufe. Wir ermittelten spezifische Leitfähigkeitswerte in landwirtschaftlich genutzten Bächen, welche die Konzentration gelöster Ionen in bewaldeten Referenzbächen um eine Größenordnung übertrafen. Die Konzentrationen von gelöstem anorganischem Stickstoff und löslichem reaktivem Phosphor waren in den landwirtschaftlichen Bächen im Durchschnitt viermal höher und der pH-Wert war im Durchschnitt eine Einheit niedriger als in den bewaldeten Bächen. Die Wassertemperatur war in landwirtschaftlich genutzten Bächen im Durchschnitt 3°C wärmer und zeigte höhere tageszeitliche Schwankungen. Der Eintrag von partikulärem organischem Material (POM) in die landwirtschaftlichen Bäche war ca. 30-mal niedriger als der Eintrag in die Referenzbäche, und die Menge an benthischem organischem Material (BOM) war in landwirtschaftlichen Bächen signifikant niedriger als in den Referenzbächen. Landwirtschaftliche Fließgewässer wiesen signifikant höhere Mengen an feinem benthischem organischem Material (FBOM) auf, hatten aber geringere Bestände an grobem benthischem organischem Material (CBOM) als die Referenzgewässer. Während in bewaldeten Fließgewässern die zeitliche Dynamik des BOM einem saisonalen Muster folgte, waren die Schwankungen in den landwirtschaftlichen Fließgewässern überwiegend stochastisch und durch anthropogene Aktivitäten, wie das Mähen der Uferrandstreifen, beeinflusst. Auch die Retention von POM war in landwirtschaftlich genutzten Bächen im Vergleich zu bewaldeten Bächen deutlich geringer, was auf eine geringere morphologische Komplexität und die größere hydrologische Variabilität, insbesondere auf die Abflussganglinien, zurückzuführen ist. Innerhalb des Experimentes zur Aufnahme von gelöstem organischem Kohlenstoff (DOC) war die Biofilmqualität in den landwirtschaftlichen Bächen ebenfalls höher. Interessanterweise zeigte jedoch der landwirtschaftliche Bach Hassel einen höheren Grad an Heterotrophie als der bewaldete Bach Wormsgraben. Da die biofilmspezifische DOC-Aufnahme im bewaldeten Bach nur 4 % der Gesamtaufnahme betrug, kann davon ausgegangen werden, dass die mikrobiell vermittelte Verarbeitung von Nährstoffen in der hyporheischen Zone den größten Anteil an der DOC-Aufnahme ausmachte. Vermutlich begünstigte die höhere Verweildauer und der bessere Austausch zwischen Oberflächenwasser und der hyporheischen Zone im bewaldeten Bach Wormsgraben die heterotrophe Nährstoffumsetzung im interstitiellen Porenraum. Die Bewertung der Makroinvertebraten-Gemeinschaft ergab, dass der Artenreichtum in landwirtschaftlichen Bächen im Vergleich zu bewaldeten Bächen um die Hälfte reduziert war. Die Biomasse, Dichte und Sekundärproduktion der Makroinvertebraten (Makroinvertebratensekundärproduktion, MSP) nahm jedoch entlang eines Gradienten der Ressourcenqualität und -quantität zu, mit höchster Produktivität im landwirtschaftlichen Bach Sauerbach, gefolgt vom landwirtschaftlichen Bach Getel und den bewaldeten Bächen Ochsenbach und Wormsgraben. Der Anstieg der MSP war hauptsächlich auf Zunahmen innerhalb der funktionellen Ernährungsgruppen der Sedimentfresser und Weidegänger zurückzuführen. Höhere MSP, Biomasse und Dichte korrelierten positiv mit niedrigeren N:P- und C:P-Verhältnissen des Biofilms und der Ressourcenmenge (höhere Biofilm-Zuwachsrate und Menge an krautiger Vegetation, Chlorophyll a-, TDN- (Summe an gelöstem Stickstoff), SRP- (gelöster reaktiver Phosphor) und POC- (Partikulärer organischer Kohlenstoff) Konzentrationen von Biofilm und Seston). Hohe Interaktionsstärken zwischen Zerkleinerern und grobpartikulärem organischem Material (CPOM) in landwirtschaftlichen Bächen deuteten auf eine Ressourcenlimitierung durch allochthones organisches Material hin. Da die Interaktionsstärken zwischen Weidegängern und Biofilm sowie zwischen feinpartikulärem organischem Material (FPOM) und Sammlern in landwirtschaftlichen Bächen deutlich geringer waren als in bewaldeten Bächen ist anzunehmen, dass die Gruppe der Zerkleinerer diese Limitation durch hohe funktionelle Plastizität und damit alternative Ressourcennutzung ausgleichen konnten. Die Analyse der Fischsekundärproduktion und Interaktionsstärken zwischen Fischen und ihrer Makroinvertebraten-Beute konnte zusätzlich zeigen, dass die landwirtschaftliche Flächennutzung zu einer starken Verschiebung des Fischartenspektrums führen und die Top-down Kontrolle der MSP durch Fische dadurch deutlich zurückgehen kann. Diese Ergebnisse zeigen, dass der Verlust der Ufervegetation und die übermäßige Nährstoffbelastung ökologische Nischen homogenisieren und hochproduktive Generalisten mit hoher Nahrungsplastizität begünstigten, die flexibel zwischen ubiquitären und stochastisch verfügbaren Nahrungsressourcen wechseln können. Insgesamt konnten wir mit dieser Studie darlegen, durch welche Mechanismen aus hoch-diversen, nährstoffbegrenzten, bewaldeten Bachoberläufen, die stark von terrestrischen Laubeinträgen abhängig sind, durch landwirtschaftliche Flächennutzung homogene, nährstoffbelastete und taxonomisch vereinfachte Bäche, mit einem hohen Maß an Autochthonie, werden können. Die gleichzeitige Analyse struktureller und funktioneller Indikatoren zeigte, dass die Landwirtschaft strukturelle Messgrößen wie den Artenreichtum beeinflusst, ohne dass es zu größeren Veränderungen in der Funktion kommen muss (ähnliche MSP im landwirtschaftlich genutzten Bach Getel im Vergleich zu bewaldeten Referenzbächen), und dass die Variabilität in der Funktion (Retention von organischem Material, Laubabbau) nicht automatisch die Informationen widerspiegelt, die die üblicherweise bewerteten chemischen oder biologischen strukturellen Messgrößen liefern. Die Ergebnisse dieser Arbeit zeigen deutlich, dass eine Kombination mehrerer struktureller und funktioneller Indikatoren entlang der Nahrungsnetzhierarchie erforderlich ist, um mechanistisch zu verstehen, wie intensive Landwirtschaft die Ökosysteme von Fließgewässern beeinflusst. So erlaubten z.B. nur die Informationen über die Qualität und Quantität der Primärproduktion, in Kombination mit strukturellen und funktionellen Informationen über die Makroinvertebratengemeinschaft, ein mechanistisches Verständnis darüber, wie landwirtschaftliche Stressoren die Produktivität mehrerer trophischer Ebenen des Nahrungsnetzes von Fließgewässern beeinflussen und damit auch wie diese Auswirkungen gemildert werden könnten. Diese Ergebnisse verdeutlichen die dringende Notwendigkeit, effektive und breite Gewässerrandstreifen zu erhalten oder zu errichten, um Sedimenteinträge durch Erosion- und Oberflächenabfluss zu verringern, sowie Einträge von Nährstoffen und Pestiziden zurückzuhalten, um eine verbesserte Vereinbarkeit von Landwirtschaft und Naturschutz zu erwirken. Diese Maßnahmen hätten eine positive Auswirkung auf Lebensraum- und Ressourcenvielfalt und schützen damit sowohl terrestrische als auch aquatische Ökosysteme und die damit verbundenen essentiellen Ökosystemdienstleistungen dieser Systeme.:Table of contents Table of contents 1 List of tables 5 List of figures 6 List of abbreviations and acronyms 8 Abstract 11 Zusammenfassung 14 1. General introduction 18 1.1 Forested headwater streams - the significance of aquatic-terrestrial coupling 18 1.2 Effects of agricultural land use on forested headwater streams 22 1.3 Integrating measures of ecosystem functioning in stream ecosystem assessment 24 1.4 Aims and objectives 28 1.5 Thesis outline 30 2. Chapter I: Agricultural land use alters temporal dynamics and the composition of organic matter in temperate headwater streams 31 2.1 Introduction 31 2.2 Methods 33 2.2.1 Study sites 33 2.2.2 Sampling of POM inputs and standing crops 37 2.2.3 Organic matter sample processing 37 2.2.4 Physical and chemical stream characteristics 38 2.2.5 C spiraling metrics 39 2.2.6 Litter decomposition 39 2.2.7 Data analyses 41 2.3 Results 43 2.3.1 Organic matter inputs 43 2.3.2 Benthic organic matter 46 2.3.3 Organic matter retention 52 2.3.4 Environmental drivers of BOM dynamics 55 2.3.5 Leaf litter decomposition 55 2.4 Discussion 57 2.4.1 Effects of agricultural land use on POM inputs and BOM dynamics 57 2.4.2 Organic matter retention 60 2.4.3 Organic matter processing 61 2.4.4 Ecosystem-level implications of altered OM dynamics 62 2.5 Conclusions 63 3. Chapter II: Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream 64 3.1 Introduction 64 3.2 Methods 66 3.2.1 Site description 66 3.2.2 Stable isotope tDOC labeling 68 3.2.3 Experimental design 68 3.2.4 Sampling and analysis 69 3.2.5 Statistical analysis 72 3.3 Results 74 3.3.1 Stream characteristics 74 3.3.2 DOC uptake 75 3.3.3 Benthic biofilm attributes 79 3.4 Discussion 84 3.4.1 Mechanisms linking hydromorphology, benthic-biofilm uptake and whole-stream uptake 84 3.4.2 Comparison of whole-stream tDOC uptake with other studies 86 3.4.3 Benthic biofilm attributes 87 3.5 Conclusions 89 4. Chapter III: Resource supply and organismal dominance are associated with high secondary production in temperate agricultural streams 90 4.1 Introduction 90 4.2 Methods 93 4.2.1 Study sites 93 4.2.2 Macroinvertebrate sampling and processing 97 4.2.3 Macroinvertebrate secondary production (MSP) 97 4.2.4 Environmental characteristics of streams 98 4.2.5 Benthic organic matter 99 4.2.6 Biofilm biomass accrual and stoichiometry 99 4.2.7 Fish biomass 100 4.2.8 Ingestion rates and interaction strength 100 4.2.9 Data analyses 101 4.3 Results 104 4.3.1 Environmental characteristics of the study streams 104 4.3.2 Structural descriptors of the macroinvertebrate community 106 4.3.3 Macroinvertebrate secondary production (MSP) 107 4.3.4 Environmental drivers of MSP and biomass 109 4.3.5 Relationships between MSP and species richness and evenness 111 4.3.6 Consumer-resource interactions 111 4.4 Discussion 113 4.4.1 Environmental drivers of MSP and biomass 113 4.4.2 Biological mechanisms associated with MSP 116 4.4.3 Top-down vs. bottom-up control 118 4.5 Conclusion 120 5. General discussion 121 5.1 Environmental conditions 121 5.2 Allochthonous organic matter dynamics 122 5.3 Organic matter processing 124 5.4 Primary production 124 5.5 Drivers of macroinvertebrate secondary production 127 5.6 Fish community 128 6. General conclusion 132 7. Implications for the assessment of running water ecosystems 134 8. Outlook 137 References 139 Appendix 182 Chapter I 182 Chapter II 189 Supplemental methods 189 Chapter III 198 Acknowledgements 212 Curriculum vitae 215 List of publications 217 Publication output during thesis period 217 Further publications 218 Eidesstattliche Erklärung 219 / Intensive crop agriculture is one of the most ubiquitous and pervasive anthropogenic threats to aquatic ecosystems. Important agriculture-related pressures include riparian clearcutting and the associated loss of allochthonous organic matter inputs and shading, nutrient and pesticide contamination, fine sediment inputs due to erosion and run-off, increased surface runoff, flashier hydrographs as well as degraded geomorphology and habitat diversity. The multitude of stressors deriving from agricultural land use often produces concomitant effects on several groups of biological organism and associated functions. While effects on single ecosystem components such as macroinvertebrates, biofilm or fish fauna with primary focus on structural components or single ecosystem processes are well described, only few studies have compiled comprehensive data sets on how agriculture affects the functioning of the entire aquatic ecosystem and interactions among ecosystem components. Thus, this thesis aimed to fill this knowledge gap by comparing agricultural and forested reference streams regarding their physicochemical characteristics, organic matter standing stocks and inputs, primary production, organic carbon spiraling, DOC uptake rates as well as secondary production of macroinvertebrates and fish for a period of 12-17 months. Intensive agricultural land use had strong effects on the physico-chemistry of the studied headwater streams of the main study (two agricultural streams vs. two forested streams in the Harz mountains, Saxony-Anhalt). We ascertained specific conductivity values in agricultural streams that exceeded the ones measured in forested streams by an order of magnitude. Concentrations of dissolved inorganic nitrogen and soluble reactive phosphorus were on average four times higher and pH was on average one unit lower in agricultural streams than in forested streams. Water temperature was on average 3°C higher in agricultural streams and showed higher diurnal variations. Particulate organic matter (POM) inputs to forested streams were approx. 30-times higher than inputs into agricultural streams, and mean standing crops of total benthic organic matter (BOM) were significantly lower in agricultural streams than in forested streams. Agricultural streams had significantly higher standing crops of fine benthic organic matter (FBOM), but less coarse benthic organic matter (CBOM) than forested streams. While in forested streams temporal dynamics of organic matter standing stocks followed a seasonal pattern, variations were predominantly stochastic and influenced by anthropogenic activities such as stream margin mowing in agricultural streams. Also, the retention of POM was distinctly lower in agricultural compared to forested streams due to lower in-stream complexity and flashier hydrographs in agricultural streams indicating that agricultural streams are less efficient to retain organic matter, facilitating the loss of carbon to downstream sections of streams. Within the main study streams, biofilm accrual rates and chlorophyll a content were six and eight times higher, respectively, and biofilm N:P and C:P ratios were three times lower in agricultural than forested streams. In the dissolved organic carbon-uptake experiment (one agricultural stream vs. one forested stream), biofilm quality was similarly higher in the agricultural stream Hassel. Against expectations, the Hassel showed a higher level of heterotrophy than the forested stream Wormsgraben. However, the total and biofilm associated uptake of labeled leaf-leachate dissolved organic carbon (DOC) in the Hassel was six and two times higher than in the forested Wormsgraben, respectively. As the biofilm-specific uptake in the forested stream amounted to only 4% of the whole stream uptake, it seems that microbial-mediated processing of nutrients in the hyporheic zone is responsible for the largest proportion of DOC uptake. Presumably, the higher transient storage and the heterotrophic community resident in the interstitial pore space of the forested stream has a higher demand for labile DOC and hence, retained distinctly more DOC than in the agricultural stream, highlighting the importance of functional hyporheic zones for carbon spiraling. The macroinvertebrate community assessment revealed that species richness was reduced by half in agricultural compared to forested streams. Macroinvertebrate biomass, density, and secondary production (MSP), however, increased along a gradient of resource quality and quantity with highest productivity in the agricultural stream Sauerbach, followed by the agricultural stream Getel, and the forested streams Ochsenbach and Wormsgraben. The increase in MSP was mainly due to higher production within the functional feeding groups (FFGs) of gatherer/collectors and grazers. Higher MSP, biomass, and density correlated positively with lower biofilm N:P and C:P ratios and resource quantity (higher biofilm accrual rate and standing crops of riparian herbaceous vegetation, biofilm and seston chlorophyll a, total dissolved nitrogen, soluble reactive phosphorus, and particulate organic carbon concentrations). High interaction strengths between shredders and coarse particulate organic carbon (CPOM) in agricultural streams indicated a resource limitation of allochthonous organic matter, while this seemed to have no effect on macroinvertebrate productivity as interaction strengths between grazers and biofilm as well as between fine particulate organic carbon (FPOM) and gatherers were distinctly lower in agricultural than in forested streams. These findings show that the loss of riparian canopy and excess nutrient conditions homogenized ecological niches and favored highly productive non-insect generalist species with high feeding plasticity to switch flexibly between ubiquitous and stochastically available food resources. In addition, analyses of fish secondary production and interaction strengths between fish and their macroinvertebrate prey demonstrated that agriculture may cause a major shift in fish species community composition, resulting in a decline in fish top-down control on MSP. Overall, the presented results demonstrate that agriculture induces a shift from nutrient-limited forested headwaters dependent on detrital pathways and closely coupled to riparian subsidies with highly diverse macroinvertebrate communities to homogenous, nutrient contaminated and taxonomically simplified streams with high levels of autochthony. The simultaneous analysis of structural and functional indicators showed that agriculture affects structural measures of community composition such as species richness without major changes in process rates (similar level of macroinvertebrate secondary production in the agricultural stream Getel compared to forested streams) and that variability in function (organic matter retention, decomposition) does not automatically reflect the information provided by commonly assessed structural measures of communities. The results of this thesis clearly showed that a combination of multiple structural and functional indicators along the food web hierarchy is required to mechanistically understand how intensive agricultural land use affects stream ecosystems. For example, only the information on quality and quantity of primary production combined with structural and functional information on the macroinvertebrate community allowed to mechanistically understand how agricultural stressors affect the productivity of multiple trophic levels of the stream food web and thus, how these effects can be mitigated. Given that such mitigation measures would largely address excessive nutrient and sediment inputs, the loss of habitat heterogeneity and natural resource dynamics, the high relevance of riparian buffer zones known for their effective the retention of nutrients, erosional run-off and the provision of resource quantity and quality is evident. Consequently, there is an urgent need to preserve or restore effective riparian zones along stream networks aiding both terrestrial and aquatic ecosystems while integrating protection measures for other anthropogenic pressures such as habitat degradation and climate change.:Table of contents Table of contents 1 List of tables 5 List of figures 6 List of abbreviations and acronyms 8 Abstract 11 Zusammenfassung 14 1. General introduction 18 1.1 Forested headwater streams - the significance of aquatic-terrestrial coupling 18 1.2 Effects of agricultural land use on forested headwater streams 22 1.3 Integrating measures of ecosystem functioning in stream ecosystem assessment 24 1.4 Aims and objectives 28 1.5 Thesis outline 30 2. Chapter I: Agricultural land use alters temporal dynamics and the composition of organic matter in temperate headwater streams 31 2.1 Introduction 31 2.2 Methods 33 2.2.1 Study sites 33 2.2.2 Sampling of POM inputs and standing crops 37 2.2.3 Organic matter sample processing 37 2.2.4 Physical and chemical stream characteristics 38 2.2.5 C spiraling metrics 39 2.2.6 Litter decomposition 39 2.2.7 Data analyses 41 2.3 Results 43 2.3.1 Organic matter inputs 43 2.3.2 Benthic organic matter 46 2.3.3 Organic matter retention 52 2.3.4 Environmental drivers of BOM dynamics 55 2.3.5 Leaf litter decomposition 55 2.4 Discussion 57 2.4.1 Effects of agricultural land use on POM inputs and BOM dynamics 57 2.4.2 Organic matter retention 60 2.4.3 Organic matter processing 61 2.4.4 Ecosystem-level implications of altered OM dynamics 62 2.5 Conclusions 63 3. Chapter II: Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream 64 3.1 Introduction 64 3.2 Methods 66 3.2.1 Site description 66 3.2.2 Stable isotope tDOC labeling 68 3.2.3 Experimental design 68 3.2.4 Sampling and analysis 69 3.2.5 Statistical analysis 72 3.3 Results 74 3.3.1 Stream characteristics 74 3.3.2 DOC uptake 75 3.3.3 Benthic biofilm attributes 79 3.4 Discussion 84 3.4.1 Mechanisms linking hydromorphology, benthic-biofilm uptake and whole-stream uptake 84 3.4.2 Comparison of whole-stream tDOC uptake with other studies 86 3.4.3 Benthic biofilm attributes 87 3.5 Conclusions 89 4. Chapter III: Resource supply and organismal dominance are associated with high secondary production in temperate agricultural streams 90 4.1 Introduction 90 4.2 Methods 93 4.2.1 Study sites 93 4.2.2 Macroinvertebrate sampling and processing 97 4.2.3 Macroinvertebrate secondary production (MSP) 97 4.2.4 Environmental characteristics of streams 98 4.2.5 Benthic organic matter 99 4.2.6 Biofilm biomass accrual and stoichiometry 99 4.2.7 Fish biomass 100 4.2.8 Ingestion rates and interaction strength 100 4.2.9 Data analyses 101 4.3 Results 104 4.3.1 Environmental characteristics of the study streams 104 4.3.2 Structural descriptors of the macroinvertebrate community 106 4.3.3 Macroinvertebrate secondary production (MSP) 107 4.3.4 Environmental drivers of MSP and biomass 109 4.3.5 Relationships between MSP and species richness and evenness 111 4.3.6 Consumer-resource interactions 111 4.4 Discussion 113 4.4.1 Environmental drivers of MSP and biomass 113 4.4.2 Biological mechanisms associated with MSP 116 4.4.3 Top-down vs. bottom-up control 118 4.5 Conclusion 120 5. General discussion 121 5.1 Environmental conditions 121 5.2 Allochthonous organic matter dynamics 122 5.3 Organic matter processing 124 5.4 Primary production 124 5.5 Drivers of macroinvertebrate secondary production 127 5.6 Fish community 128 6. General conclusion 132 7. Implications for the assessment of running water ecosystems 134 8. Outlook 137 References 139 Appendix 182 Chapter I 182 Chapter II 189 Supplemental methods 189 Chapter III 198 Acknowledgements 212 Curriculum vitae 215 List of publications 217 Publication output during thesis period 217 Further publications 218 Eidesstattliche Erklärung 219

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Biological invasion has become a major threat to economy, ecology, global biodiversity and ecosystem function of aquatic ecosystems. The main aim of the thesis was to study the effects of the zebra mussel (Dreissena polymorpha), a versatile invasive species, on phytoplankton dynamics and ecosystem function of lakes. In a first attempt, I compared the density of Dreissena and the physicochemical data of ecosystems that it invaded among North American and European lakes to identify important factors in its invasion success. Secondly, I investigated the impact of zebra mussels on phytoplankton community composition in a natural lake. Thirdly, I evaluated whether zebra mussel feeding behavior were affected by the presence of predatory waterborne cues. Finally, I examined the effect of Dreissena on seston stoichiometry. A Generalized Additive Model revealed that a joint effect of surface area, mean depth, total phosphorus and calcium concentrations can explain the variability in Dreissena density. Selective grazing by zebra mussels varied in relation to seasonal phytoplankton dynamics. Risk cues released by predators affected both feeding rate and prey selection of the mussels and had cascading indirect effects on phytoplankton biomass and community structure. I found that the flux in nutrients caused by differences in zebra mussel consumption lead to a variation in phytoplankton nutrient limitation. The flexibility of zebra mussel feeding behavior and variation in susceptibility among phytoplankton groups to mussel ingestion indicate that invading zebra mussels could alter phytoplankton community composition of lakes and have important ecosystem consequences. The results of this thesis contribute to the growing evidence that predators indirectly affect resource dynamics and food web structure through their non-lethal effects on consumers. The results suggested that zebra mussel can indirectly both reduce and increase the energy transfer efficiency from primary producers to upper trophic levels in the pelagic and benthic food webs, respectively.