In river ecology the description and understanding of near-natural ecosystem functionality is a difficult task to achieve as the majority of river floodplains have been intensively impacted by human activities. This work addresses ecological functionality of a relatively unimpacted large river system, focussing on the lateral dynamic and connectivity mechanisms driving aquatic vegetation processes. Macrophytes were found to be very patchily distributed at the riverscape scale, being mainly confined to low energy lateral habitats in the floodplain, such as backwaters. Backwaters provided favourable conditions for plants to colonise and recruit and contributed highly to species diversity and productivity at the floodplain scale. Differences between backwaters were attributed to the frequency of connectivity with the main channel during flood events. Nevertheless, the ecological mechanism driving diversity through flooding appears not to be related to flow disturbance. Biomass produced in backwaters was found to remain stable after potentially scouring floods. Therefore the hypothesis that flood disturbances promote species diversity through the removal and destruction of biomass and rejuvenate communities such that species coexistence is increased was rejected. Rather, it appears that diversity in backwaters increases along a temporal gradient as a response to the input of colonists and their accumulation overtime through successive flood inputs. Despite the apparently non-destructive effect of floods on macrophyte biomass, backwaters appear to have a significant role in exporting large amounts of plant propagules from the site of production. Backwaters represented a net source of propagules which highly enriched the main channel pool of potential colonists. However, whereas propagules could be dispersed for long distances in flood flows the probability for them to reach a suitable downstream habitat was extremely low. This work showed that dispersal at baseflow and entry to backwaters through the downstream end after short dispersal drift provided a greater chance of successful colonisation despite the individually much shorter distance moved. Backwaters were demonstrated to be rather isolated aquatic habitats, even though they experience hydrological connectivity, suggesting that primary colonisation of these sites is a limiting step. Instead, colonisation was shown to rely primarily on propagules generated internally by established plants. Whereas colonisation could occur via internal re-organisation of existing plant propagules, the backwater seed bank could also contribute to the macrophytes species established in backwaters. Such contribution was consistently low to medium along a gradient of disturbances and connectivity and showed independence from such river flow processes. Species richness was found to be higher in the established species than in the seed bank, suggesting that asexual reproduction is prioritised by aquatic vegetation in riverine backwaters. The occurrence or persistence of macrophyte species in backwaters depends upon rhizome and plant shoot regeneration. The lack of influence of connectivity revealed that plants may originate from both in situ and externally waterborne vegetative propagules derived from other upstream backwaters. This research demonstrated that the lateral dynamic and associated connectivity are major components of river floodplain ecology which generate a wide spectrum of habitats and have a controlling effect on vegetation processes. Therefore a naturally dynamic ecological state is required to support ecosystem functionality in large river floodplains and especially to maintain a high level of species diversity, productivity and colonisation of backwaters by macrophytes.

Understanding energy fluxes within and between floodplain water bodies, and between rivers and their floodplains is essential for comprehending the dynamics of modern, altered river systems. Floodplain aquatic invertebrates may move between habitats deliberately (through emergence and dispersal) or through passive transport during flooding. This movement may represent a significant flux of energy and an essential food web subsidy. I assessed the usefulness of the stable isotopes of hydrogen and oxygen (D and 18O respectively) for identifying the origins and movements of macroinvertebrates in Mississippi River floodplain water bodies. I sampled water and invertebrates from the Mississippi River, intermittent and permanent floodplain wetlands, and tributaries during 2007 and 2008. Results showed consistent relationships between δD and δ18O signatures in invertebrate tissues and their home water bodies. I also investigated whether δD and δ18O could be used as a multivariate "fingerprint" to trace a captured invertebrate back to its environment of origin. Results showed that δD can be a useful tracer of the movement of floodplain invertebrates in some cases, although δ18O is likely not suitable for that purpose.

Shore zones of large rivers are hot spots of biodiversity and contribute significantly to riverine ecosystem functioning. Today, shore degradation and other structural impair-ments like river straightening and channelization are strong impact factors on river ecosystem health. However, we still lack a thorough understanding of how structural shore zone degradation affects benthic community composition and their inherent ecosystem functions. In this thesis I tested the influence of training structure induced environmental factors on benthic macroinvertebrate community composition and the share of non-native species. Moreover, I assessed the community-associated ecosystem functions in terms of secondary production and resource utilization. In the main channel, communities were composed of only a few specialized taxa with low abundances, which contributed little to riverine secondary production. This is probably due to the harsh conditions produced by constantly high flow velocities and relocation of the fine sandy sediment. Main channel habitats were hardly affected by the adjacent training structure. Hence, species compositions and productivities were similar at all investigated main channel sites. By contrast, each of the shore communities was diverse, highly abundant and productive in comparison to the main channel. However, variations between shore structure communities and their ecosystem functions were prominent. One particular training structure, i.e. the off-bankline revetment, bore the most diverse and by far most productive benthic community, which utilized vast total amounts of basal resources (1,323 g DM m-² y-1). Varying sediment compositions, availability of macrophytes and diverse flow velocities, including lentic conditions, were revealed as key factors for increasing biodiversity, secondary production and resource utilization. Allochthonous boulder habitats were generally highly prone to non-native species invasion. Neozoa proved less productive than many native community members and consumed minor relative and total amounts of the prevailing resource pelagic algae. The present quantitative comparison of shore type specific effects on biodiversity, biomass and productivity provides managers with a tool to improve the ecological attributes of large river ecosystems with an unchangeable, impaired macrostructure. In its entirety, this thesis constitutes a sound basis to increase the mechanistic understanding of the way in which shore zone manipulation can affect riverine benthic communities and their associated ecosystem functions. / Die Uferbereiche großer Flüsse sind für die Biodiversität und Funktion dieser Ökosys-teme von enormer Bedeutung. Uferdegradierung und Kanalisierung gehören zu wichti-gen Faktoren, die Flussökosysteme nachhaltig beeinträchtigen. Wie sich anthropogene Umstrukturierungen der Uferbereiche auf die Zusammensetzung benthischer Ge-meinschaften und deren Ökosystemfunktion auswirken ist jedoch weitestgehend unbe-kannt. In dieser Arbeit werden die Effekte von uferspezifischen Habitatfaktoren auf die Komposition des Makrozoobenthos und den Anteil an Neozoen getestet. Zudem wurden die gemeinschaftsassoziierten Ökosystemfunktionen in Bezug auf Sekundärproduktion und Ressourcennutzung erfasst und bewertet. Die benthische Gemeinschaft des Hauptstroms bestand lediglich aus wenigen spezialisierten Arten, welche nur geringfügig zur Gesamtsekundärproduktion des Flusses beitrugen. Dies war vermutlich auf die rauen Bedingungen im Hauptstrom (insbesondere hohe Strömungsgeschwindigkeiten und damit verbundener Sedimenttransport) zurückzuführen. Die Makrozoobenthosgemeinschaften des Hauptroms wurden von den jeweils angrenzenden Ufertypen nicht beeinflusst. Im Gegensatz zum Hauptstrom zeigte jeder der untersuchten Ufertypen hohe Dichten an benthischen Invertebraten wobei sich Zusammensetzung und Funktion der Artengemeinschaften zwischen den Ufertypen stark unterschieden. Dabei brachte das Parallelwerk die diverseste und produktivste Makroinvertebratenfauna hervor, welche enorme Mengen basaler Ressourcen ingestierte (1,323 g DM m-2y-1). Hauptfaktoren für eine Erhöhung von Biodiversität, Sekundärproduktion und Ressourcennutzung waren neben variablen Sedimentzusammensetzungen und Makrophytenbeständen sich oft ändernde Fließgeschwindigkeiten mit lentischen Phasen. Allochthone Steinhabitate waren generell anfällig für die Invasion nicht heimischer Arten. Diese Neozoen waren weniger produktiv als viele heimische Taxa und nutzten nur geringe Mengen pelagischer Algen, welche die meist verfügbare Ressource darstellten. Aus dem ökologischen Vergleich der verschiedenen Ufertypen lassen sich Managementempfehlungen zur Verbesserung des ökologischen Zustandes in Bezug auf Biodiversität und Ökosystemfunktionen ableiten, welche für große Flüsse mit degradierter und unveränderbarer Makrostruktur in Betracht gezogen werden können. In ihrer Gesamtheit liefert diese Arbeit ein fundiertes mechanistisches Verständnis über die Effekte von Ufermanipulationen auf benthische Gemeinschaften und deren assoziierte Ökosystemfunktionen in großen Flüssen.

Shore zones of large rivers are hot spots of biodiversity and contribute significantly to riverine ecosystem functioning. Today, shore degradation and other structural impair-ments like river straightening and channelization are strong impact factors on river ecosystem health. However, we still lack a thorough understanding of how structural shore zone degradation affects benthic community composition and their inherent ecosystem functions. In this thesis I tested the influence of training structure induced environmental factors on benthic macroinvertebrate community composition and the share of non-native species. Moreover, I assessed the community-associated ecosystem functions in terms of secondary production and resource utilization. In the main channel, communities were composed of only a few specialized taxa with low abundances, which contributed little to riverine secondary production. This is probably due to the harsh conditions produced by constantly high flow velocities and relocation of the fine sandy sediment. Main channel habitats were hardly affected by the adjacent training structure. Hence, species compositions and productivities were similar at all investigated main channel sites. By contrast, each of the shore communities was diverse, highly abundant and productive in comparison to the main channel. However, variations between shore structure communities and their ecosystem functions were prominent. One particular training structure, i.e. the off-bankline revetment, bore the most diverse and by far most productive benthic community, which utilized vast total amounts of basal resources (1,323 g DM m-² y-1). Varying sediment compositions, availability of macrophytes and diverse flow velocities, including lentic conditions, were revealed as key factors for increasing biodiversity, secondary production and resource utilization. Allochthonous boulder habitats were generally highly prone to non-native species invasion. Neozoa proved less productive than many native community members and consumed minor relative and total amounts of the prevailing resource pelagic algae. The present quantitative comparison of shore type specific effects on biodiversity, biomass and productivity provides managers with a tool to improve the ecological attributes of large river ecosystems with an unchangeable, impaired macrostructure. In its entirety, this thesis constitutes a sound basis to increase the mechanistic understanding of the way in which shore zone manipulation can affect riverine benthic communities and their associated ecosystem functions. / Die Uferbereiche großer Flüsse sind für die Biodiversität und Funktion dieser Ökosys-teme von enormer Bedeutung. Uferdegradierung und Kanalisierung gehören zu wichti-gen Faktoren, die Flussökosysteme nachhaltig beeinträchtigen. Wie sich anthropogene Umstrukturierungen der Uferbereiche auf die Zusammensetzung benthischer Ge-meinschaften und deren Ökosystemfunktion auswirken ist jedoch weitestgehend unbe-kannt. In dieser Arbeit werden die Effekte von uferspezifischen Habitatfaktoren auf die Komposition des Makrozoobenthos und den Anteil an Neozoen getestet. Zudem wurden die gemeinschaftsassoziierten Ökosystemfunktionen in Bezug auf Sekundärproduktion und Ressourcennutzung erfasst und bewertet. Die benthische Gemeinschaft des Hauptstroms bestand lediglich aus wenigen spezialisierten Arten, welche nur geringfügig zur Gesamtsekundärproduktion des Flusses beitrugen. Dies war vermutlich auf die rauen Bedingungen im Hauptstrom (insbesondere hohe Strömungsgeschwindigkeiten und damit verbundener Sedimenttransport) zurückzuführen. Die Makrozoobenthosgemeinschaften des Hauptroms wurden von den jeweils angrenzenden Ufertypen nicht beeinflusst. Im Gegensatz zum Hauptstrom zeigte jeder der untersuchten Ufertypen hohe Dichten an benthischen Invertebraten wobei sich Zusammensetzung und Funktion der Artengemeinschaften zwischen den Ufertypen stark unterschieden. Dabei brachte das Parallelwerk die diverseste und produktivste Makroinvertebratenfauna hervor, welche enorme Mengen basaler Ressourcen ingestierte (1,323 g DM m-2y-1). Hauptfaktoren für eine Erhöhung von Biodiversität, Sekundärproduktion und Ressourcennutzung waren neben variablen Sedimentzusammensetzungen und Makrophytenbeständen sich oft ändernde Fließgeschwindigkeiten mit lentischen Phasen. Allochthone Steinhabitate waren generell anfällig für die Invasion nicht heimischer Arten. Diese Neozoen waren weniger produktiv als viele heimische Taxa und nutzten nur geringe Mengen pelagischer Algen, welche die meist verfügbare Ressource darstellten. Aus dem ökologischen Vergleich der verschiedenen Ufertypen lassen sich Managementempfehlungen zur Verbesserung des ökologischen Zustandes in Bezug auf Biodiversität und Ökosystemfunktionen ableiten, welche für große Flüsse mit degradierter und unveränderbarer Makrostruktur in Betracht gezogen werden können. In ihrer Gesamtheit liefert diese Arbeit ein fundiertes mechanistisches Verständnis über die Effekte von Ufermanipulationen auf benthische Gemeinschaften und deren assoziierte Ökosystemfunktionen in großen Flüssen.