Riverine Floodplain Heterogeneity As a Controller of Organic Matter Dynamics and Terrestrial Invertebrate Distribution
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Eawag 05141 Doctoral Thesis ETH No. 16997 RIVERINE FLOODPLAIN HETEROGENEITY AS A CONTROLLER OF ORGANIC MATTER DYNAMICS AND TERRESTRIAL INVERTEBRATE DISTRIBUTION A dissertation submitted to SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Sciences presented by SIMONE DANIELA LANGHANS Dipl. Natw. ETH born 18.09.1976 citizen of Nussbaumen (Thurgau) and Jona (St. Gallen) accepted on the recommendation of Prof. Dr. Bernhard Wehrli, examiner Prof. Dr. Stuart Findlay, co-examiner Prof. Dr. Klement Tockner, co-examiner Dr. Urs Uehlinger, co-examiner 2006 TABLE OF CONTENTS Summary 1 Zusammenfassung 5 Introduction 9 Chapter 1: Environmental heterogeneity controls organic-matter dynamics in river-floodplain ecosystems 23 Chapter 2: The role of timing, duration, and frequency of inundation in controlling leaf litter decomposition in a river-floodplain ecosystem (Tagliamento, northeastern Italy) 33 Chapter 3: Leaf decomposition across aquatic and terrestrial habitat types in a riverine floodplain mosaic 53 Chapter 4: Cotton strips as a leaf surrogate to measure decomposition in river floodplain habitats 77 Chapter 5: Aerial input, lateral transport, and surface storage of coarse particulate organic matter in a riverine floodplain mosaic (Tagliamento, NE-Italy) 93 Chapter 6: Seasonal variation of riparian arthropods along lateral and vertical gradients in a braided gravel-bed river 119 Synthesis and Outlook 151 Curriculum Vitae 157 Acknowledgements 161 SUMMARY Rivers and their fringing floodplains are among the most complex, dynamic, and therefore, diverse ecosystems worldwide. Flow and flood pulses are responsible for the formation and structure of different habitats, and these pulses define the degree of connectivity among habitats. Floods and droughts represent two extremes of the flow regime. Between these extreme states, riverine floodplains undergo distinct cycles of expansion and contraction along longitudinal, lateral, and vertical dimensions, and therefore, shift between terrestrial and aquatic phases. In Europe, most rivers and floodplains have been severely altered by human activities, with dramatic consequences for their biodiversity and functioning. To successfully manage riverine floodplains, we need to understand how temporal and spatial heterogeneity, including expansion and contraction dynamics, and the composition and configuration of habitats determine ecosystem processes. Research for this thesis was conducted along the Tagliamento River in northeastern Italy. This system represents a unique opportunity to study ecosystem processes under near-natural conditions, as it has retained an essentially pristine morphological and hydrological character. The present thesis investigates the importance of environmental heterogeneity in determining organic matter dynamics and the distribution of terrestrial invertebrates in a river-floodplain ecosystem (Tagliamento River). The thesis begins with a general introduction that sets the context of the work, followed by a conceptual model that unifies input and storage of organic matter, with leaf quality and decomposition in a complex and dynamic system (Chapter 1). Currently, individual components of organic matter dynamics are mostly treated separately in the literature. My conceptual model takes an advanced and holistic perspective that links natural heterogeneity with ecosystem processes, and provides a framework for the scientific information necessary for the sustainable management of floodplains. The role of the flow regime, defined by its components duration, frequency, and timing of inundation, in controlling leaf decomposition was tested by field manipulation experiments in the Tagliamento River (Chapter 2). Leaf bags (Populus nigra) were exposed to 10 different treatments imitating a dynamic flow regime. Experiments were conducted in two seasons to account for timing of inundation. After 30 days, 12 - 49% of leaf litter was decomposed, with significantly faster rates in winter than in summer. Duration of inundation was -1- Summary the main hydrological component that determined leaf decomposition, whereas frequency of inundation induced leaf decomposition heterogeneity. Shredding macroinvertebrates played a significant role in leaf decomposition only under permanent aquatic and terrestrial conditions. Fungi were responsible for the faster leaf decay in winter. My results indicate that modifications of the inundation regime will directly modify decomposition heterogeneity. The role of habitat heterogeneity in leaf decomposition was quantified within the floodplain mosaic of the Tagliamento River (Chapter 3). Leaf bags (P. nigra) were placed in seven contrasting habitat types, including the river channel, different pond types, exposed gravel surface, large wood accumulations, vegetated islands, and riparian forest. Based on decomposition rates, habitats separated into three groups: river channel with fast rates, ponds with medium rates, and terrestrial sites with slow rates. Microbes and detritivores drove leaf decomposition in aquatic sites, whereas microbial activity was the main driver in terrestrial sites. Habitat heterogeneity seems to be strongly linked with heterogeneity of leaf decomposition rates. Hence, morphological simplification of the ecosystem, in concert with alterations to the flow regime is expected to homogenize decomposition rates, with subsequent consequences for overall ecosystem functioning. In Chapter 4, I tested the applicability of cotton strips as a surrogate to measure leaf decomposition in contrasting aquatic and terrestrial habitats. Cotton-strip decomposition, measured as loss of tensile strength and mass, was correlated with leaf mass loss. Across river channels, ponds, and terrestrial sites, I found comparable patterns between loss in cotton strip tensile strength and leaf mass. The results suggest that cotton strips have the potential to mimic leaf decomposition in fluvial settings, and therefore, to serve as a functional indicator for stream assessment. In correspondance with leaf decomposition experiments along the Tagliamento River, litterfall and lateral transport of organic matter was quantified with spatially distributed litter traps over a 10 month period. Organic matter standing stock was determined in four habitats and three seasons (Chapter 5). Direct litterfall dominated litter input close to vegetation, whereas lateral transport was the major pathway in the floodplain 10 to 20 m away from vegetation. Islands contributed more than 95% to the annual direct input, and 65% to lateral transport. However, riparian forests were identified as hot spots of litter standing stock. -2- Summary In Chapter 6, I explored the connection between environmental heterogeneity and terrestrial invertebrate distribution using a four-dimensional field experiment, including the air space, the sediment surface, the unsaturated zone, and the temporal dimension, along a large gravel bank in the Tagliamento River. Invertebrate abundance, at two taxonomic levels (family and species) responded to all four dimensions. Beetles assemblages were correlated with various environmental parameters indicating that spatial and temporal heterogeneity plays a major role in their behaviour. From my research, I propose that both, spatial and temporal heterogeneity shape and determine organic-matter dynamics and terrestrial invertebrate distribution in riverine floodplains. Therefore, morphological and/or hydrological alterations have consequences for the functioning of organic matter dynamics and the abundance and diversity of terrestrial invertebrates. This can result in changes in nutrient cycling, sediment respiration, primary and secondary production, or decreasing terrestrial and aquatic biodiversity. -3- -4- ZUSAMMENFASSUNG Flüsse gehören zusammen mit ihren Auenflächen zu den komplexesten, dynamischsten und daher vielseitigsten Ökosystemen der Erde. Schwankungen im Abflussregime mit Hochwasserereignissen bestimmen in erster Linie das Vorhandensein, die Struktur und den Verbindungsgrad der verschiedenen Habitate. Hochwässer die den gesamten Auenbereich überfluten bzw. Phasen in denen komplette Flussabschnitte trocken fallen, stellen Extremereignisse des Abflussregimes dar. Zwischen diesen beiden Extremen durchlaufen flussnahe Auenbereiche einen steten Kreislauf der Expansion und Kontraktion. Da sie diesen Prozessen sowohl in longitudinaler, lateraler als auch vertikaler Dimension ausgesetzt sind, befinden sich Auenbereiche in einem ständigen Wechsel zwischen terrestrischem und aquatischem Zustand. In Europa wurden die meisten Fliessgewässer und Flussauen aufgrund der intensiven Nutzung durch den Menschen stark verändert. Dies hatte dramatische Konsequenzen für deren Biodiversität und Funktionsfähigkeit zur Folge. Um gezielte Massnahmen zur Erhaltung der flusseigenen Dynamik ergreifen zu können, benötigen wir ein präzises Verständnis des Zusammenhangs zwischen zeitlicher und räumlicher Heterogenität und dem Funktionieren von Ökosystemprozessen. Das heisst wir benötigen sowohl ein Verständnis über die Ausdehnung und Kontraktion von Flüssen als auch über die Zusammensetzung und -stellung von Habitaten in mosaikartigen Landschaften, die die ökosystemaren Prozesse bestimmen. Die Untersuchungen für meine Dissertation habe ich entlang des Tagliamento (Italien) durchgeführt. Aufgrund seines ursprünglich gebliebenen morphologischen und hydrologischen Charakters, ermöglicht der Tagliamento das Studium von ökosystemaren Prozessen unter natürlichen Bedingungen.