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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Ecohydrological Interfaces Comprehensive overview of the process dynamics and interactions governing ecohydrological interfaces Summarizing the interdisciplinary investigation of ecohydrological interface functioning, Ecohydrological Interfaces advances the understanding of their dynamics across traditional subject boundaries. It offers a detailed explanation of the underlying mechanisms and process interactions governing ecohydrological interface functioning from the micro scale to the ecosystem and regional scale. The multidisciplinary team of authors integrates and synthesises the current understanding of process dynamics at different ecohydrological interfaces to develop a unifying concept of their ecosystem functions. The work introduces novel experimental and model-based methods for characterizing and quantifying ecohydrological interface processes, taking account of innovative sensing and tracing technologies as well as microbial and molecular biology approaches. Key questions addressed in the book include: * Which conditions stimulate the transformative nature of ecohydrological interfaces? * How are ecohydrological interfaces organized in space and time? * How does interface activity propagate from small to large scales? * How do ecohydrological interfaces react to environmental change and what is their role in processes of significant societal value? As a research level text on the functionality and performance of ecohydrological interfaces, Ecohydrological Interfaces is primarily aimed at academics and postgraduate researchers. It is also appropriate for university libraries as further reading on a range of geographical, environmental, biological, and engineering topics.
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Ecohydrological Interfaces
Edited by
Stefan Krause University of Birmingham, Birmingham, UK
David M. Hannah University of Birmingham, Birmingham, UK
Nancy B. Grimm Arizona State University, Tempe, AZ, US
This edition first published 2024
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Cover Images: Courtesy of Stefan Krause; Courtesy of David M. Hannah; Caio Pederneiras/Shutterstock; Nattapon Ponbumrungwong/Shutterstock
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Contents
Cover
Title Page
Copyright Page
Preface
List of Contributors
Section 1
1 Ecohydrological Interfaces as Hotspots of Ecosystem Processes
2 Biological Activity as a Trigger of Enhanced Ecohydrological Interface Activity
Section 2
3 The Four Interfaces’ Components of Riparian Zones
4 Organizational Principles of Hyporheic Exchange Flow and Biogeochemical Cycling in River Networks across Scales
5 Groundwater–Lake Interfaces
6 Coastal–Groundwater Interfaces (Submarine Groundwater Discharge)
Section 3
7 Identifying and Quantifying Water Fluxes at Ecohydrological Interfaces
8 Heat as a Hydrological Tracer
9 Sampling at Groundwater–Surface Water Interfaces
10Automated Sensing Methods for Dissolved Organic Matter and Inorganic Nutrient Monitoring in Freshwater Systems
11 Tracing Hydrological Connectivity with Aerial Diatoms
12 Measurement of Metabolic Rates at the Sediment–Water Interface Using Experimental Ecosystems
13 Using Diel Solute Signals to Assess Ecohydrological Processing in Lotic Systems
14 Evolving Molecular Methodologies for Monitoring Pathogenic Viruses in Ecohydrological Interfaces
Section 4
15 Global Environmental Pressures
16 Restoring the Liver of the River: Actionable Research Insights to Guide the Restoration of the Hyporheic Zone for the Improvement of Water Quality
Index
End User License Agreement
List of Tables
CHAPTER 05
Table 5.1 Scenarios for LGD-derived...
CHAPTER 06
Table 6.1 Overview of methods...
CHAPTER 10
Table 10.1 Summary of commonly-used...
Table 10.2 Key factors to...
CHAPTER 14
Table 14.1 List of waterborne...
Table 14.2 Methods for concentration...
Table 14.3 Methods for detection...
Table 14.4 Molecular-based methods...
CHAPTER 15
Table 15.1 Planetary boundaries identified...
List of Illustrations
CHAPTER 01
Figure 1.1 Landscape perspective of...
Figure 1.2 Conceptual model of...
Figure 1.3 Conceptual model of...
Figure 1.4 Examples for the...
Figure 1.5 Enhanced ecohydrological interface...
Figure 1.6 Variable characteristics and...
CHAPTER 02
Figure 2.1 Mechanisms by which...
CHAPTER 03
Figure 3.1 Schematic representation of...
Figure 3.2 (a) Illustration of...
Figure 3.3 Perceptions of dissolved...
Figure 3.4 Illustration of the...
Figure 3.5 Relationship between the...
Figure 3.6 Hyporheic flows.Adapted...
CHAPTER 04
Figure 4.1 Spatial and temporal...
Figure 4.2 Drivers and controls...
Figure 4.3 Inter-meander flow...
Figure 4.4 Conceptual models of...
Figure 4.5 Multi-directional interactions...
Figure 4.6 Landscape-scale organizational...
CHAPTER 05
Figure 5.1 Schematic drawing of...
Figure 5.2 Boxplot of logarithmic...
Figure 5.3 Boxplots of logarithmic...
Figure 5.4 Subsurface catchment of...
Figure 5.5 Maximum LGD rate...
Figure 5.6 Examples of transects...
Figure 5.7 (a) Soluble reactive...
CHAPTER 06
Figure 6.1 Scheme compiling the...
Figure 6.2 In situ application...
Figure 6.3 Compilation of spatial...
Figure 6.4 Acoustic transect AT2...
Figure 6.5 The study area...
Figure 6.6 Horizontal and vertical...
Figure 6.8 (A) Cross-section...
Figure 6.7 Covariation of selected...
Figure 6.9 Distribution of published...
CHAPTER 08
Figure 8.1 Sketch of a...
Figure 8.2 Example of a...
Figure 8.3 Working principle of...
Figure 8.4 Measurement set-up...
Figure 8.5 FO-DTS monitoring...
Figure 8.6 Groundwater exfiltration and...
Figure 8.7 The total radiation...
CHAPTER 09
Figure 9.1 Selection of design...
Figure 9.2 Cross-sectional geometries...
Figure 9.3 (A) Common design...
Figure 9.4 Two-dimensional pore...
CHAPTER 10
Figure 10.1 (from Adapted from...
Figure 10.2 (from Blaen et...
CHAPTER 11
Figure 11.1 Conceptual classification of...
CHAPTER 12
Figure 12.1 Images of different...
CHAPTER 13
Figure 13.1 Major processes affecting...
Figure 13.2 (A) Primary productivity...
Figure 13.3 Estimating removal rates...
Figure 13.4 Example plot summarizing...
CHAPTER 15
Figure 15.1 Thresholds in damage...
Figure 15.2 The impact of...
Figure 15.3 Conventional approaches to...
Figure 15.4 The impact of...
CHAPTER 16
Figure 16.1 Hyporheic exchange through...
Figure 16.2(i) Schematics representing...
Figure 16.2(ii) (j) J...
Figure 16.3 Schematic representing scenario...
Figure 16.4 Schematic representing scenario...
Figure 16.5 Schematic representing scenario...
Figure 16.6 Schematic representing scenario...
Figure 16.7 Schematic representing scenario...
Guide
Cover
Title Page
Copyright Page
Table of Contents
Preface
List of Contributors
Begin Reading
Index
End User License Agreement
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Preface
Ecohydrological interfaces represent the dynamic transition zones at ecosystem boundaries that control the fate, transport, and transformation of water, matter, energy, and organisms between adjacent systems. In contrast to more stationary concepts of boundaries (as separators of different ecosystems or subsystems) or ecotones (boundaries that have a defined thickness and share characteristics with each of the systems they separate), ecohydrological interfaces dynamically evolve at ecosystem boundaries in space and time where they can emerge, disappear, expand, or contract. Ecohydrological interfaces, such as groundwater–surface water interfaces including hyporheic and riparian zones, soil–atmosphere interfaces, or the dynamic interfaces between phreatic and vadose zones, are hotspots of ecohydrological and biogeochemical processes. As such, they harbour key ecosystem functions of biogeochemical cycling, water purification, and buffering of thermal extremes, and they support biodiversity and ecosystem resilience.
This state-of-the-art edited volume provides novel insights into the diverse nature of different types of ecohydrological interfaces, experimental and modelling tools to study the mechanisms controlling their functioning, and integrated approaches to restore and protect their functions. The 16 chapters of this book are therefore organized into 4 sections. Section 1 provides an introduction to the conceptual background and the role and spatial-temporal organization of ecohydrological interfaces in the environment. Section 2 discusses the principles of ecohydrological interface controls on energy, water, and solute fluxes across neighbouring ecosystems for a selection of different ecohydrological interfaces, and Section 3 provides an overview of quantitative experimental and model-based methods for characterizing the functioning of ecohydrological interfaces. Section 4 analyses the functioning of ecohydrological interfaces in a globally changing environment and discusses how the functioning of ecohydrological interfaces can be restored and protected in practice.
Section 1 introduces the concept of ecohydrological interfaces as a new way to investigate the often non-linear system behaviour stimulated by complex interactions between hydrological, biogeochemical, and ecological processes across system boundaries. Chapter 1 provides an overview of the principles of ecohydrological interfaces, their functioning and role in a landscape context. It discusses how ecohydrological interfaces are defined and delineated in space and time by their specific functioning rather than system properties, which distinguishes them from ecotones or ecosystem boundaries. Based on this introduction, Chapter 2 provides novel insights into the role of biological activity for enhancing hydrological and biogeochemical activity at ecohydrological interfaces. Reflecting on the growing body of functional analyses performed by microbial and invertebrate ecologists, this chapter specifically discusses mechanisms of how biological activity (such as of bioturbating species) is affected by hydrological conditions but can also trigger and control ecohydrological interface functioning.
Section 2 provides a detailed overview of the mechanisms by which ecohydrological interfaces control fluxes of energy, water, and solutes. These are discussed for different types of groundwater–surface water interfaces including in riparian corridors (Chapter 3), stream hyporheic zones (Chapter 4), groundwater–lake (Chapter 5), and coastal and marine interfaces with groundwater (Chapter 6).
Section 3 discusses new developments in quantitative methods for characterizing the functioning of the ecohydrological interface across a wide range of spatial and temporal scales for processes covering (i) ecohydrological interface fluxes of water, (ii) characterization of concentrations and fluxes of reactive substances and their biogeochemical cycling at ecohydrological interfaces, and (iii) biological process dynamics at ecohydrological interfaces. In addition to laboratory and field experimental methods ranging from micro-plate-reader scale to remote sensing, a special focus is given to advances in integrated approaches to quantify and predict ecohydrological interface behaviour.
Chapter 7 therefore provides an overview of point- to reach-scale methods for identifying water and energy fluxes across ecohydrological interfaces in the field, followed by Chapter 8, which introduces novel methods of using heat as a tracer for tracking and quantifying hydrological fluxes across ecohydrological interfaces. Chapter 9 provides a comprehensive overview of capabilities and limitations of active and passive in situ sampling approaches at groundwater–surface water interfaces, with Chapter 10 discussing advances in automated in situ sensing technologies for monitoring nutrient and organic matter cycling at freshwater interfaces.
Chapter 11 introduces the use of terrestrial diatoms as tracers at ecohydrological interfaces with Chapter 12 discussing the use of experimental ecosystems for quantifying metabolism at sediment–water interfaces. Using advanced data analysis techniques, Chapter 13 provides new insights into the use of diel solute signals for assessing ecohydrological and biogeochemical processing at ecohydrological interfaces, followed by Chapter 14, which explores the use of molecular methodologies for monitoring pathogenic viruses at ecohydrological interfaces.
Section 4 analyses the role of ecohydrological interfaces in globally changing river basins, providing a systematic overview of interface impacts and responses to global environmental pressures, as well as increasing evidence of facilitation of ecohydrological interface functions in environmental restoration and engineering schemes. By providing an overview of general global environmental pressures, Chapter 15 identifies threats and risks to the functioning and health of ecohydrological interfaces, whereas Chapter 16 introduces new, integrated concepts for restoring ecosystem functioning of ecohydrological interfaces.
This section concludes the discussion of the functioning of ecohydrological interfaces by developing a new paradigm of how research at ecohydrological interfaces can be used not only to improve mechanistic process understanding but also to facilitate novel approaches in interdisciplinary research.
Stefan Krause, David M. Hannah, Nancy B. Grimm
Birmingham, UK; Tempe, AZ, USA; May 2023
List of Contributors
Benjamin W. Abbott Brigham Young University Provo Utah USA
Ion Gutierrez Aguirre Department of Biotechnology and Systems Biology National Institute of Biology Ljubljana Slovenia
M. Antonelli Wageningen University Department of Environmental Sciences, Hydrology and Quantitative Water Manag-ement Group The Netherlands Luxembourg Institute of Science and Technology Department Environmental Research and Innovation Catchment and Eco-hydrology research group 41 rue du Brill L-Belvaux Luxembourg Wageningen University Department of Environmental Sciences Hydrology and Quantitative Water Manag-ement Group Droevendaalsesteeg 3a Building 100 PB Wageningen The Netherlands
Alba Argerich Oregon State University Department of Forest Engineering, Resources & Management Corvallis USA School of Natural Resources University of Missouri Columbia Missouri USA
Ian Baker Small Woods Association Station Road Coalbrookdale Telford TF8 7DR UK
Mukundh N. Balasubramanian BioSistemika LLC Ljubljana Slovenia
Viktor Baranov LMU Munich Biocenter Großhaderner Str. 2, Planegg-Martinsried Germany Ludwig Maximillian University Munich
Tom Battin Ecole Polytechnique Fédérale de Lausanne School of Architecture, Civil and Environ-mental Engineering Lausanne Switzerland
S. Bernal CSIC Blanes Spain
Susana Bernal Center for Advanced Studies of Blanes
Mike Blackmore Wessex Rivers Trust Phillips Lane Salisbury SP1 3YR UK
Phillip J Blaen School of Geography, Earth and Environ-mental Sciences University of Birmingham Edgbaston Birmingham B15 2TT UK Birmingham Institute of Forest Research (BIFoR) University of Birmingham Edgbaston Birmingham B15 2TT UK Yorkshire Water Halifax Road Bradford BD6 2SZ UK
Michael E. Böttcher Leibniz-Institute for Baltic Sea Research Seestrasse 15 D-Warnemünde Germany
Chris Bradley School of Geography, Earth and Environ-mental Sciences University of Birmingham Edgbaston Birmingham UK
Athena Chalari Silixa Watford England UK
Francesco Ciocca Silixa Watford England UK
Thibault Datry IRSTEA
Jake Diamond RiverLy INRAe Villeurbanne France
Jennifer Drummond Center for Advanced Studies of Blanes
Jan H. Fleckenstein Helmholtz-Center for Environmental Re-search (UFZ) Department of Hydrogeology Leipzig Germany
Jan Fleckenstein Department of Hydrogeology Helmholtz-Center for Environmental Re-search – UFZ Leipzig Germany
J.N. Galloway Department Ecohydrology Leibniz-Institute of Freshwater Ecology and Inland Fisheries Berlin Germany
Nancy B. Grimm Arizona State University School of Life Sciences Tempe AZ USA
David M. Hannah School of Geography, Earth and Environ-mental Sciences University of Birmingham Edgbaston Birmingham B15 2TT UK
Ben Christopher Howard School of Geography, Earth & Environ-mental Sciences University of Birmingham Birmingham B15 2TT UK and Birmingham Institute of Forest Research
Matjaž Hren BioSistemika LLC Ljubljana Slovenia
Nicholas Kettridge School of Geography, Earth & Environ-mental Sciences University of Birmingham Birmingham B15 2TT UK and Birmingham Institute of Forest Research
Kieran Khamis School of Geography, Earth and Environ-mental Sciences University of Birmingham Edgbaston Birmingham B15 2TT UK
Julian Klaus Luxembourg Institute of Science and Technology Department in Environmental Research and Innovation Belvaux Luxembourg Department of Geography University of Bonn Bonn Germany Luxembourg Institute of Science and Technology Department Environmental Research and Innovation Catchment and Eco-hydrology research group 41 rue du Brill L-4422 Belvaux Luxembourg
Julia L.A. Knapp Department of Earth Sciences Durham University Durham UK
Katarina Kovač BioSistemika LLC Ljubljana Slovenia
Stefan Krause School of Geography, Earth and Environ-mental Sciences University of Birmingham Edgbaston Birmingham B15 2TT UK Birmingham Institute of Forest Research (BIFoR) University of Birmingham Edgbaston Birmingham UK
Marie Kurz Drexel University
Marie J. Kurz Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge Tennessee USA
Scott T. Larned National Institute of Water and Atmospheric Research Christchurch New Zealand
Jörg Lewandowski Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) Ecohydrology Department Berlin Germany
Charlotte E.M. Lloyd Organic Geochemistry Unit Bristol Biogeochemistry Research Centre School of Chemistry University of Bristol Cantocks Close Bristol BS8 1TS UK
Ulf Mallast Department Monitoring- and Exploration Technology Helmholtz Centre for Environmental Research- UFZ Permoserstr. 15 Leipzig The Netherlands
Eugènia Martí Centre d’Estudis Avançats de Blanes Biogeodynamics and Biodiversity Group Blanes Spain
N. Martínez-Carreras Luxembourg Institute of Science and Technology Department Environmental Research and Innovation Catchment and Eco-hydrology research group 41 rue du Brill L-Belvaux Luxembourg
Gudrun Massmann Institute for Biology and Environmental Sciences Carl von Ossietzky University of Oldenburg Ammerländer Heerstraße 114–118 Oldenburg Germany
Karlie McDonald University of Birmingham School of Geography, Earth and Environ-mental Sciences Birmingham UK
Karin Meinikmann Julius Kühn Institute Institute for Ecological Chemistry Plant Analysis and Stored Product Protection Berlin Germany
Clara Mendoza-Lera University of Koblenz-Landau
Alexander Milner University of Birmingham
Florentina Moatar RiverLy INRAe Villeurbanne France
Nils Moosdorf Leibniz Centre for Tropical Marine Research Fahrenheitstr. 6, Bremen Germany
Mike Müller-Petke Leibniz Institute for Applied Geophysics Hannover Niedersachsen Germany
Aaron Packman Northwestern University
Laurent Pfister Luxembourg Institute of Science and Technology Department in Environmental Research and Innovation Belvaux Luxembourg
L. Pfister Luxembourg Institute of Science and Technology Department Environmental Research and Innovation Catchment and Eco-hydrology research group 41 rue du Brill L-4422 Belvaux Luxembourg
Gilles Pinay Centre national de la recherche scientifique Observatoire des Sciences de l’Univers de Rennes Université de Rennes 1 Rennes France Environnement Ville & Sociétés - UMR CNRS Lyon France
Michael O. Rivett University of Birmingham School of Geography, Earth and Environ-mental Sciences Birmingham UK University of Strathclyde Department of Civil and Environmental Engineering Glasgow G1 1XJ Center for Applied Geoscience Eberhard Karls University of Tübingen Tübingen Germany
Donald O. Rosenberry U.S. Geological Survey Sunrise Valley Drive Reston VA USA
Janine Rüegg Stream Biofilm and Ecosystem Research Laboratory École Polytechnique Fédéral de Lausanne 1015 Lausanne Switzerland Current address: Interdisciplinary Center for Mountain Research University of Lausanne Lausanne Switzerland
Francesc Sabater University of Barcelona Department of Ecology Barcelona Spain
Hanieh Sayedhashemi RiverLy INRAe Villeurbanne France
Jonas Schaper Geography Department Humbold University Berlin Germany
Jacob Schelker University of Vienna Department of Limnology & Bio-Ocean-ography Vienna Austria
Christian Schmidt Helmholtz-Center for Environmental Research (UFZ) Department of Hydrogeology Leipzig Germany
Albert Sorolla Naturalea Castellar del Vallès Spain
Valentina Turk National Institute of Biology Marine Biology Station Piran Slovenia
Sami Ullah School of Geography, Earth & Environ-mental Sciences University of Birmingham Birmingham,
B15 2TT UK and Birmingham Institute of Forest Research
Loes van Schaik Department of Environmental Sciences Wageningen University and Research The Netherlands
Jesus Gomez Velez Vanderbilt University
Adam S. Ward Indiana University Bloomington
Hannelore Waska Institute of Chemistry and Biology of the Sea Carl von Ossietzky University of Oldenburg Ammerländer Heerstraße 26129 Oldenburg Germany
Glenn Watts Environment Agency Scientific and Evidence Service Bristol UK
C.E Wetzel Luxembourg Institute of Science and Technology Department Environmental Research and Innovation Catchment and Eco-hydrology research group 41 rue du Brill L-Belvaux Luxembourg
Anne Zangerlé Ministry of Agriculture, Viticulture and Rural Development Luxembourg Luxembourg
Jay P. Zarnetzke Michigan State University
Section 1
1 Ecohydrological Interfaces as Hotspots of Ecosystem Processes
Stefan Krause1, Jörg Lewandowski2, Nancy B. Grimm3, David M. Hannah1, Gilles Pinay4, Karlie McDonald1, Eugènia Martí5, Alba Argerich6, Laurent Pfister7, Julian Klaus7, Tom Battin8, Scott T. Larned9, Jacob Schelker10, Jan Fleckenstein11, Christian Schmidt11, Michael O Rivett1,16, Glenn Watts12, Francesc Sabater13, Albert Sorolla14 and Valentina Turk15
1 University of Birmingham, School of Geography, Earth and Environmental Sciences, Birmingham, UK 2 Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Ecohydrology Department, Berlin, Germany 3 Arizona State University, School of Life Sciences, Tempe, AZ, USA 4 Centre national de la recherche scientifique, Observatoire des Sciences de l’Univers de Rennes, Université de Rennes 1, Rennes, France 5 Centre d’Estudis Avançats de Blanes, Biogeodynamics and Biodiversity Group, Blanes, Spain 6 Oregon State University, Department of Forest Engineering, Resources & Management, Corvallis, USA 7 Luxembourg Institute of Science and Technology, Department in Environmental Research and Innovation, Belvaux, Luxembourg 8 Ecole Polytechnique Fédérale de Lausanne, School of Architecture, Civil and Environmental Engineering Lausanne, Switzerland 9 National Institute of Water and Atmospheric Research, Christchurch, New Zealand 10 University of Vienna, Department of Limnology & Bio-Oceanography, Vienna, Austria 11 Helmholtz-Center for Environmental Research (UFZ), Department of Hydrogeology, Leipzig, Germany 12 Environment Agency, Scientific and Evidence Services, Bristol, United Kingdom 13 University of Barcelona, Department of Ecology, Barcelona, Spain 14 Naturalea, Castellar del Vallès, Spain 15 National Institute of Biology, Marine Biology Station, Piran, Slovenia 16 University of Strathclyde, Department of Civil and Environmental Engineering, Glasgow G1 1XJ, UK
1.1 Introduction
The study of system boundaries has been a mainstay in ecological and hydrological research (Cadenasso et al. 2003; Strayer et al. 2003; Yarrow and Marin 2007). Interdisciplinary research has highlighted the importance of ecosystem boundaries, many of which are “hotspots” of ecological, biogeochemical, or hydrological processes (Caraco et al. 2006; McClain et al. 2003; Peipoch et al. 2016; Pinay et al. 2015).
We introduce ecohydrological interfaces as a new concept to support the quantitative analysis of non-linear system behaviour stimulated by the complex and multi-faceted interactions of hydrological, biogeochemical, and ecological processes across system boundaries. Ecohydrological interfaces represent the dynamic transition zones that may develop at ecosystem (or subsystem) boundaries and control the movement and transformation of organisms, water, matter, and energy between adjacent systems (referred to by Hedin et al. (1998) as “control points”). In contrast to stationary boundaries (separators of different ecosystems or subsystems) or ecotones (boundaries that have a defined thickness and share characteristics with each of the systems they separate), ecohydrological interfaces are non-stationary, emerging for a limited time and then disappearing, expanding, and contracting, or moving around within a boundary or ecotone. Differing from boundaries and ecotones, which are delineated primarily based on system properties (Cadenasso et al. 2003; Strayer et al. 2003; Yarrow and Marin 2007), ecohydrological interfaces are defined by their specific functioning (for example, the dynamic extent of surface water mixing in streambed environments forming hyporheic zones as ecohydrological interfaces with distinct redox environments and ecological niche functions and behaviour).
Ecohydrological interfaces are manifold, including (1) soil–atmosphere interfaces, (2) capillary fringes as interfaces between phreatic and vadose zones, (3) interfaces between terrestrial upland and lowland aquatic ecosystems, (4) groundwater–surface water interfaces, including those associated with riparian or hyporheic zones, biofilms, and surface water–benthic zone interfaces (Figure 1.1). Ecohydrological interfaces provide key ecosystem functions and services (Belnap et al. 2003), including water purification, thermal regulation, and maintenance of biodiversity (Freitas et al. 2015; Krause et al. 2011a; Perelo 2010). They increase ecological resilience by providing refuge for organisms during extreme events or source areas for recolonization after disturbances (Clinton et al. 1996; Crump et al. 2012; Kumar et al. 2011; Stubbington 2012).
Figure 1.1 Landscape perspective of different types of ecohydrological interfaces with (1) atmosphere–soil interfaces, (2) unsaturated–saturated soil interfaces, (3) riparian–stream interfaces, and (4) hyporheic zone interfaces and characteristic profiles of water fluxes, mixing, gas exchange, and redox conditions (Eh).
In this chapter we aim to uncover the organizational principles – the main drivers and controls, and their interactions and feedbacks – that determine the development and capacity of ecohydrological interfaces to transform the flow of energy, water, and matter between adjacent ecosystems. We compare the characteristics of transformation processes at different ecohydrological interfaces in freshwater ecosystems, including groundwater–surface water, groundwater–vadose zone, and benthic–pelagic interfaces, to determine common or unique features of their non-linear process dynamics. Based on a comparison of the organizational principles of different ecohydrological interfaces, we propose a roadmap for the development of multi-scale conceptual models of ecohydrological interface processes and their interactions that can be expanded to other types of ecohydrological interfaces not covered here.
1.2 Transformation of Energy, Water, and Matter Fluxes Across Ecohydrological Interfaces
Ecohydrological interfaces developing in aquatic ecosystems (e.g. between groundwater and surface water or groundwater and the vadose zone) extend from the micro-scale (e.g. interfaces at microbial biofilms) to kilometre scale (e.g. aquifer–river interfaces). Despite their varied dimensions, these interfaces share common properties: (1) abrupt changes in aggregate state (e.g. solid, liquid, or gas phase), and (2) steep gradients in physical and biogeochemical conditions (Naiman 1988; Naiman and Decamps 1997). The steep physical, chemical, and biological gradients in ecohydrological interfaces often correspond to distinct types and enhanced rates of biogeochemical processes (McClain et al. 2003; Yarrow and Marin 2007), and have significant impacts on ecosystem responses and resilience to environmental change (Brunke and Gonser 1997). Examples of specific conditions at ecohydrological interfaces that facilitate transformative processes include:
Steep redox
gradients
across groundwater–surface water interfaces as a result of enhanced biogeochemical activity (Krause et al. 2013; Lautz and Fanelli 2008; Lewandowski et al. 2007; Trauth et al. 2015).
Step changes
in microbial metabolic activity
(Argerich et al. 2011; Haggerty et al. 2009) and high concentrations of bioavailable organic carbon, nitrogen, or phosphorus at riparian–wetland interfaces (Schelker et al. 2013), at groundwater–surface water interfaces (Zarnetske et al. 2011a, 2011b), and in biofilms (Battin et al. 2003, 2007, 2016), resulting in micro-zonation of denitrification (Briggs et al. 2015) and enhanced interface denitrification rates where microbial denitrifiers are concentrated (Harvey et al. 2013).
Co-existence of multiple aggregate states (solid, liquid, gas phase)
,
across which energy and matter are transferred, such as between the atmosphere and porous soil matrix (Shahraeeni et al. 2012), atmosphere–water interfaces (Assouline et al. 2010), unsaturated and saturated soil compartments (Li and Jiao 2005), and between the soil matrix and soil water or air in soil macropores (van Schaik et al. 2014).
Shifts between physical and biological controls
of solute transport
across water–organism interfaces (Larned et al. 2004; Nishihara and Ackerman 2009; Nishizaki and Carrington 2014).
While there have been recent improvements in understanding how ecohydrological interfaces control energy and water fluxes (in particular, between groundwater and surface water (Boano et al. 2014; Cardenas 2015; Krause et al. 2011a)), critical knowledge gaps remain with respect to how they affect reactive transport, solute mixing, and biogeochemical cycling across system boundaries (Krause et al. 2011a; Puth and Wilson 2001). Our understanding of the spatial and temporal organization of driving forces (e.g. hydrostatic pressure distribution, concentration gradients, turbulence intensity) and controls (e.g. interface transmissivity, roughness) of ecohydrological interface fluxes and reactivity are at an early stage (Gomez-Velez et al. 2012, 2014; González-Pinzón et al. 2015; Zhang et al. 2015).
Many ecohydrological interfaces are spatially heterogeneous and temporally dynamic (Kennedy et al. 2009; Roskosch et al. 2012). While the physical (structural) boundaries between adjacent and interacting systems (e.g. between groundwater and surface water) are usually clearly defined and stationary, dynamically developing ecohydrological interfaces (e.g. hyporheic zones) are defined by their functioning and may change in time with regard to their spatial extent and activity (Boano et al. 2010, 2014; Cardenas and Wilson 2006, 2007; Gomez et al. 2014; Stubbington 2012; Trauth et al. 2015). However, some structural boundaries around which ecohydrological interfaces evolve can themselves be dynamic, such as migrating bedforms and flexible and compressible benthic organisms (Harvey et al. 2012; Huang et al. 2011; Larned et al. 2011; Ren and Packman 2004), further complicating the identification and delineation of ecohydrological interfaces.
Patterns and dynamics of ecohydrological interface activity include the development of hotspots (zones of enhanced activity: Frei et al. 2012; Krause et al. 2013; Lautz and Fanelli 2008; McClain et al. 2003) and hot moments (periods of increased activity: Battin et al. 2003; Harms and Grimm 2008; McClain et al. 2003) that disproportionately alter the fluxes of water, energy, and matter. Hotspots or “control points” (Bernhardt et al. 2017) have captured the attention of many researchers, who study how they affect nutrient turnover (Lewandowski et al. 2007; Moslemi et al. 2012), ecosystem productivity (Poungparn et al. 2012), pesticide degradation (Klaus et al. 2014), and the bioavailability of metals, such as mercury, to organisms at higher trophic levels (Sizmur et al. 2013). Yet, when and under what conditions ecohydrological interfaces represent hotspots or control points, or what makes them behave as such, has not always been clearly determined.
We have, for instance, only just begun to understand how biological activity (e.g. earthworm and chironomid burrowing, stream periphyton growth, or riparian plant root growth) can create small-scale ecohydrological interfaces that are hotspots of microbial and biogeochemical activity (Baranov et al. 2016; Hölker et al. 2015). Furthermore, the concept of hot moments entails long periods of relatively low activity punctuated by pulses of rapid activity. These temporal dynamics suggest that some ecohydrological interfaces can be ephemeral. We now turn to these and other gaps in our understanding of ecohydrological interfaces.
1.3 Critical Gaps in Understanding Ecohydrological Interfaces
We currently lack an overarching framework that integrates the factors that drive and control transformation processes at ecohydrological interfaces. Perceptions and conceptualizations of boundaries, and with that ecohydrological interfaces, are often scale-dependent (Cadenasso et al. 2003; Strayer et al. 2003). At large scales, some ecohydrological interfaces (e.g. between aquifers and rivers) may be conceptualized as discrete boundaries, causing abrupt transitions with step changes in processes across the boundary (Figure 1.2A). However, down-scaling reveals three-dimensional gradients within interfaces (e.g. in hyporheic zones), and transient or gradual changes of physical or biogeochemical properties (Figure 1.2B). Acknowledgement of the context and scale-dependent view of ecohydrological interfaces is important because the scale in which ecohydrological interfaces are investigated can preclude the detection and quantification of physical, chemical, and biological activity at other scales (Atkinson and Vaughn 2015). Further, temporal variation in the shape or spatial extent of interfaces and the steepness of gradients within them suggests that our conceptualizations of interfaces vary over temporal as well as spatial scales – as, for instance, shown for transient behaviour of hyporheic zones in response to hydrological forcing (Malzone et al. 2016).
Figure 1.2 Conceptual model of ecohydrological interfaces connecting two adjacent contrasting environments (Component 1 and Component 2) with scale-dependent representation of gradients of chemical, physical, and biological properties (solid black line). (A) Large scales exhibit step functions in interface properties, where interfaces appear as two-dimensional layers of zero depth; (B) zoomed into smaller scales with steep gradient of chemical, physical, and biological properties and a three-dimensional interface zone with some depth dimension; (C) difficulties are frequently encountered in determining the upper and lower boundary and depth of the interface zone, especially where property distributions blend into background properties due to their non-linearity.
Clear delineations of the spatial and temporal extent of ecohydrological interfaces are further complicated by discipline-specific perspectives on interface properties, processes, and functions (Figure 1.2C; Harvey et al. 2013; Yarrow and Marin 2007). Based on discipline-specific perceptions, hyporheic zones, for instance, are defined by the spatial extent of groundwater and surface water mixing (hydrology), the extent of steep chemical gradients (biogeochemistry), or the abundance of benthic and hypogean taxa (ecology), resulting in significantly different perceptions of their extent (Krause et al. 2011b, 2014b). Recent studies of benthic systems have focused on the dynamics and ecological effects of multi-layered interfaces (e.g. small-scale diffusive boundary layers nested within larger-scale roughness layers, within larger benthic boundary layers; Larned et al. 2004; Nikora 2010) and on micro-zonation of biogeochemical processes, e.g. redox micro-zones (Briggs et al. 2015). Views of the capillary boundary at the groundwater–vadose zone interface differ between ecologists focusing on matric potential effects on plant available water and water uptake, (bio)geochemists interested in redox chemistry differences between pore water and adsorption to mineral surfaces (Alexander and Scow 1989; Baham and Sposito 1994), and groundwater hydrologists and hydrogeological engineers concerned with water table depths. Such discipline-specific perceptions of ecohydrological interfaces can limit the transferability of process understanding and the exchange of data and knowledge across disciplinary boundaries.
Detailed understanding of the drivers and controls of enhanced interface activity is critical for evaluating the functional significance of ecohydrological interfaces. Examples include the shift from aerobic to anaerobic respiration in hyporheic zones, which is controlled by residence time of hyporheic water and nutrients in the streambed (Briggs et al. 2014; Zarnetske et al. 2011a), or temperature thresholds triggering bacterial activity (Bourg and Bertin 1994). Here we pose four critical questions (spanning scales and crossing disciplinary boundaries) that must be answered to understand the role of ecohydrological interfaces in ecosystem functioning:
What environmental conditions determine the capacity of ecohydrological interfaces to transform the flow of energy, water, and matter between adjacent ecosystems?
How are ecohydrological interfaces organized and how do they evolve in space and time?
What mechanisms (drivers and controls) determine the spatio-temporal organization of ecohydrological interfaces?
How do the impacts of hotspots and hot moments at ecohydrological interfaces upscale to ecosystem ecohydrological, biogeochemical, and ecological processes?
1.3.1 What Environmental Conditions Determine the Capacity of Ecohydrological Interfaces to Transform the Flow of Energy, Water, and Matter between Adjacent Ecosystems?
Ecohydrological interfaces have been described as intensive modifiers of energy, water, and solute fluxes and biogeochemical cycling (Harvey and Fuller 1998), that exhibit hotspot characteristics (Krause et al. 2013; Lautz and Fanelli 2008; McClain et al. 2003) and non-linear behaviour (Briggs et al. 2014; Zarnetske et al. 2011a, 2011b). To understand why ecohydrological, biogeochemical, and ecological transformation processes in ecohydrological interfaces often differ from their neighbouring ecosystems, it is necessary to review the physical, chemical, and ecological interactions that characterize them.
1.3.1.1 Physical Properties
Contrasts in interface material properties from the adjacent environmental systems (sometimes coinciding with aggregate state boundaries such as between liquid and gas phase, or with changes in transmissivity) affect velocity and direction of exchange fluxes (Figure 1.3). Impacts of ecohydrological interfaces on exchange fluxes can vary from complete cessation, if the interface is impermeable (Figure 1.3A), to unaffected (Figure 1.3B) or even accelerated exchange. The geometry of property distributions at ecohydrological interfaces (such as hydraulic conductivities at groundwater–surface water interfaces) may cause hysteretic behaviour that is dependent on exchange-flow direction (Figure 1.3C). For example, surface water flow velocities decrease when infiltrating into the streambed, while groundwater up-welling through the streambed may accelerate towards the interface with surface water. Reduced flow velocities and increased residence times that have been observed at many ecohydrological interfaces (Figure 1.3D) can substantially enhance biogeochemical processing (Briggs et al. 2014; Zarnetske et al. 2011b). Quantifying the spatio-temporal variability of biogeochemical processing in heterogeneous interface zones of variable activity will require a shift from the current focus on mean residence times to residence-time distributions that are dynamic (Botter et al. 2011; Pinay et al. 2015).
Figure 1.3 Conceptual model of the scale-dependent complexity of ecohydrological interface exchange-fluxes in systems with interfaces representing thresholds with infinitesimal thickness (left), steep gradients with abrupt property changes (centre), or variable (transient and abrupt) property changes between interface and adjacent environments (right), with one-directional flow ceasing at (in) the interface zone (A) or crossing the interface (B), bi-directional exchange fluxes across the interface (C), flow reduction across the interface pathway (D), and the advective mixing of interface exchange fluxes with intra-compartmental fluxes (E).
In many cases, exchange fluxes at ecohydrological interfaces interact with larger flow systems in the adjacent ecosystem (Figure 1.3E). At aquifer–river interfaces, for instance, exchange fluxes interact across multiple scales. Hyporheic exchange here can be affected by regional groundwater flow, causing complex and nested patterns of exchange fluxes (Gomez-Velez et al. 2014; Trauth et al. 2015) and thus, spatially complex end temporally dynamic ecohydrological interfaces. In this context, we have only begun to understand the impacts of interacting drivers and controls of interfaces exchange. Following the previous example, this includes how streambed transmissivity (Krause et al. 2013) and pressure variations caused by interface topography, such as riparian micro-topography (Frei et al. 2012), bedforms (Cardenas et al. 2004), or meanders (Boano et al. 2010), overlap in their impacts on hyporheic exchange fluxes (Boano et al. 2010, 2014; Gomez-Velez et al. 2014) and dynamically evolve in time due to variability in atmospheric and hydrodynamic forcing (Malzone et al. 2016).
1.3.1.2 Ecological (Including Microbiological) Processes
Ecohydrological interfaces between groundwater, surface water, and vadose zones can have large effects on ecological conditions in the adjacent systems (Cadenasso et al. 2003; Pinay et al. 2015). Thermodynamically controlled microbial processes drive biogeochemical transformations in these subsurface systems, and in turn, biota respond to the chemical gradients that result from their activity. A classic example is aerobic respiration, which in subsurface zones is largely carried out by microorganisms. As they consume oxygen and organic carbon, microbes create conditions that favour transition to anaerobic metabolism. Although some microorganisms are facultative anaerobes, others are excluded once oxygen concentration drops below a threshold. In fact, a sequence of terminal electron accepting processes, each with their suite of microbial specialists, ensues along redox gradients that characterize anoxic environments (Morrice et al. 2000). Aquatic macrophytes, benthic biofilms, and riparian vegetation may exude or release organic matter during metabolism or upon death or decomposition, which provides an energy source for microbial metabolism. Community structure and elemental composition of primary producers may influence biogeochemical turnover and location of biogeochemical hotspots at ecohydrological interfaces, as they are likely to release organic matter at different rates and with different chemical composition. Hence, in addition to altering nutrient availability and stoichiometry, aquatic macrophytes, benthic algae, and pelagic phytoplankton colonies may induce hotspots of microbial metabolism (de Moraes et al. 2014).
Aquatic and wetland plants influence the saturated substrate where fine-scale microenvironments develop around their root systems, altering the oxygen concentrations, nutrient uptake, sediment structure, and microbial activity of riparian and hyporheic zones. For example, exudates from the roots of a wetland shrub, Baccharis sp., fuel microbial respiration, including denitrification, in streamside sediments and riparian zones (Harms and Grimm 2008; Schade et al. 2001). The size of the ecohydrological interface zone in which these root exudates drive microbial metabolism tends to be restricted to a few centimetres around the root zone (Schade et al. 2001). Vascular plants influence not only the interstitial water of the sediment but also the water column, through mutualistic interactions with phytoplankton and bacterial communities (Brodersen et al. 2014), and the atmosphere, by respiration and gas exchange (Xing et al. 2006). Ecological impacts on ecohydrological interface functioning are not restricted to living organisms. Large woody debris alters streambed topography and enhances groundwater–surface water interactions and supply of organic carbon, thus supporting habitat complexity and biotic activity (Krause et al. 2014a; Warren et al. 2013). The nutrients and pollutants that had previously been absorbed by biota are now released during decomposition and can stimulate localized hotspots of increased resource availability (Krause et al. 2014a), or invertebrates can induce the development of biogeochemical hotspots through the regeneration of nutrients (Grimm 1988a).
The morphology, physiology, and productivity of benthic autotrophs (e.g. algal and cyanobacterial mats, seagrasses, corals growing on the bottom of streams, lakes, and coastal marine ecosystems) are strongly influenced by the hydrodynamic and chemical conditions in surface water–benthic interfaces. Mass transport across these interfaces is often the rate-limiting step for nutrient acquisition and gas exchange by the organisms (Jumars et al. 2001; Larned et al. 2004), and hydrodynamic forces imposed by these interfaces affect the organism stature and biomechanical properties (Albayrak et al. 2014; Statzner et al. 2006). While interface conditions clearly affect benthic autotrophs, the opposite is also true. Benthic autotrophs function as roughness elements that modify flow structure and as biogeochemical reactors that alter water chemistry (Folkard 2005; Larned et al. 2011; Reidenbach et al. 2006). The picture that is emerging from recent studies of surface water–benthic interfaces is a flow–organism feedback system consisting of responses by organisms to flow conditions, flow modifications induced by the organisms, subsequent responses by the organisms to the modified flow, and so forth (Dijkstra and Uittenbogaard 2010; Larned et al. 2011; Nikora 2010). Similar feedback systems should apply to the heterotrophic organisms in sedimentary systems, as described later. Such feedbacks are an important source of non-linearity in process rates at ecohydrological interfaces.
Recently, ecohydrological research has considered biota at higher trophic levels, such as macroinvertebrates and aquatic vertebrates, and their capacity to alter the physical-chemical characteristics that regulate the rate of activity and ecosystem functioning at ecohydrological interfaces (Coco et al. 2006; Layman et al. 2013; Patrick 2014). Lewandowski et al. (2007), Roskosch et al. (2012), and Baranov et al. (2016), for instance, describe a system of interactions and feedbacks between chironomids and aquifer–lake ecohydrological interfaces. In these studies, chironomid activity had direct impacts on hydrodynamics and biogeochemistry, while physical-chemical conditions, such as temperature, affected chironomid pumping behaviour (Roskosch et al. 2012) and hence, subsurface flow pattern and biogeochemical processing rates. Similarly, vertebrates may alter streambed topography, for example, through nest-building activities, which lead to changes in connectivity and fluxes across the surface water–sediment interface (Collins et al. 2014), through their movement (Hippopotamus) or beaver dam construction (Naiman et al. 1994). Additionally, fish induce biogeochemical hotspots by excretion (Grimm 1988b; Vanni 2002) and nutrient release following their death and decomposition (Levi and Tank 2013).
1.3.1.3 Thermodynamics and Biogeochemistry
At stationary boundaries, matter and energy fluxes may be absorbed, transmitted, reflected, transformed, amplified, or unaffected. Boundaries can be highly permeable to some substances, and represent reactive filters for others (Belnap et al. 2003; Cadenasso et al. 2003; Strayer et al. 2003). We propose that these concepts of flux behaviour at boundaries can be extended to non-stationary ecohydrological interfaces, which develop dynamically in space and time. Processing rates at ecohydrological interfaces are controlled by both mass transport and reaction kinetics, with transport-limited conditions arising when reaction rates are faster than mass-transport rates (Cornelisen and Thomas 2009; Larned et al. 2004; Sanford and Crawford 2000). Conversely, process rates tend to be kinetically controlled (reaction limited) when mass-transport rates are faster than reaction rates (Argerich et al. 2011; Nishihara and Ackerman 2009; Sanford and Crawford 2000). Increased biogeochemical activity is often attributed to the spatial and temporal coincidence of reactants in a mixing zone (Figure 1.4A) (McClain et al. 2003); however, enhanced turnover may also be controlled by high reactivity in interfaces (Figure 1.4B), resulting directly from the chemical gradients at the interface (Krause et al. 2013; Trauth et al. 2015). It has yet to be established how the mixing of reactants at ecohydrological interfaces influences interface redox conditions and controls residence-time distributions of different reactants, and hence, biogeochemical processing rates. Possible approaches to achieve this involve combinations of residence-time distributions and dimensionless numbers used to describe the transport vs. reaction relationships of flow systems, such as the Damköhler number or Péclet number describing diffusion/advection ratios (Pinay et al. 2015). Furthermore, the reaction significance factor (RSF) approach has been applied for quantifying reaction vs. transport limitation in single hyporheic flow paths within basin-scale assessments of the number of river excursions through the hyporheic zone (Gomez-Velez et al. 2015; Harvey et al. 2013). Despite these advances, predictions of biogeochemical processing at ecohydrological interfaces remain challenging, since not only can biogeochemical turnover be enhanced, but also the type of processes and chemical reactions may differ distinctively from adjacent ecosystems (Naiman et al. 1988).
Figure 1.4 Examples for the development of biogeochemical hotspots at ecohydrological interfaces, hosting distinctly different reaction properties and hence biogeochemical processes than its adjacent environments: (A) Enhanced reactivity directly resulting from the interaction of interface exchange fluxes such as the precipitation of a reactant at the ecohydrological interface due to exceeding its solubility product. (B) Enhanced reactivity as an intrinsic property of the interface environment, such as anoxic areas in hyporheic or riparian zones.
1.3.2 How are Ecohydrological Interfaces Organized in Space and Time?
Complex microhabitat structure and biological activity create ecohydrological interface heterogeneity (Hanzel et al. 2013; Lewandowski et al. 2007), with interface processes often varying over a wide range of spatial and temporal scales (Belnap et al. 2003). Hyporheic exchange flows, for instance, include sinuosity-driven flows in meandering streams (Boano et al. 2010; Gomez-Velez et al. 2012) and bedform-driven flows caused by streambed features such as riffles and pools (Käser et al. 2013; Thibodeaux and Boyle 1987; Tonina and Buffington 2007), small-scale ripples and dunes (Cardenas and Wilson 2007), and flow obstacles such as dams or wood (Briggs et al. 2012; Krause et al. 2014a; Sawyer et al. 2011).
Scale-dependent drivers of the spatial and temporal organization of ecohydrological interface properties are complex. Mixing of chemical reactants in ecohydrological interfaces may involve the transport of multiple reactants from source areas to the interface (Figure 1.5A; e.g. Zarnetske et al. 2011a, 2011b), or the mixing of a reactant already present at the interface with another reactant that is transported into it (Figure 1.5C; e.g. Krause et al. 2013; Lewandowski et al. 2007). In many cases, just a fraction of the mass flux crosses the ecohydrological interface. Often mass fluxes return to their original compartments (Figure 1.5B, D); e.g. surface water infiltrates into the hyporheic zone and exfiltrates back into the stream after passage through the bed.
Figure 1.5 Enhanced ecohydrological interface reactivity as a function of exchange flow patterns at/in/across the interface with fluxes carrying reactant R and S to meet and mix at the interface (A) with not all but just a fraction of the reactants mixing at the interface due to tangential fluxes (B) and transport of reactant S into the ecohydrological interface already containing autochthonous reactant R, results in the processing of S and R to product P (C) with some of the external reactant (S) returning to the compartment it originated from (D).
Ecohydrological interfaces are frequently characterized by non-linear temporal dynamics, including tipping points, caused by rapid changes in thermodynamic or biogeochemical characteristics at the interface, such as the shift from aerobic to anaerobic metabolism (Briggs et al. 2014, 2015; Harvey et al. 2012; McClain et al. 2003; Zarnetske et al. 2011a, 2011b) or biogeochemical responses to fast changes in soil water content (Fromin et al. 2010). Also, rainfall pulses in dryland environments can result in rapid and non-linear increases in microbial respiration at the soil–air interface (Collins et al. 2014) or in the vadose zone–groundwater interface of riparian zones during dry seasons (Baker et al. 2000; Harms and Grimm 2008). In both of these examples, ecohydrological interfaces come into existence when water is added (i.e. rainfall impinges on the soil surface, or the groundwater table rises into previously dry riparian soil), such that biogeochemical processes are stimulated rapidly. However, the cumulative long-term effects of such hot moments on ecohydrological interfaces, as well as their subsequent contribution to system behaviour at a global scale (Kreyling 2014) still need to be investigated in detail.
1.3.3 What Mechanisms (Drivers and Controls) Determine the Spatio-temporal Organization of Ecohydrological Interfaces?
Spatial patterning in the properties of an ecohydrological interface can result directly from interface processes and thus, may partly be explained by the functioning of the ecohydrological interface. Examples include redox patterns in hyporheic zones resulting from oxygen depletion by hyporheic biogeochemical processing (Krause et al. 2013; Zarnetske et al. 2011a, 2011b). In other cases, the origin of spatial variability is independent of actual interface processes (e.g. spatial variability in hydraulic conductivity can control patterns of exchange fluxes in ecohydrological interfaces). Spatial patterns of solute concentration in ecohydrological interfaces may be controlled partially by the spatial organization of properties in the adjacent ecosystems (Figure 1.6). For example, spatially homogeneous physical properties in hyporheic zones (Figure 1.6A, B, C) or around chironomid burrows (Figure 1.6E) will facilitate ecohydrological interface activity that is controlled primarily by interface exchange fluxes and mean residence times (e.g. Zarnetske et al. 2011a, 2011b). In contrast, a heterogeneous matrix in surrounding ecosystems (Figure 1.6D, G) will add further complexity, making it important to quantify not only exchange fluxes and residence times but also their distributions (Gomez-Velez et al. 2014). In addition to spatial heterogeneity, patterns may evolve with time as interface processes progress. For instance, chironomid pumping can affect property distributions at the sediment/burrow wall interface (Figure 1.6F, H), where they have been shown to induce gradients of decreasing oxygen concentration with increasing distance from the tube (Figure 1.6F) or increasing concentration of soluble reactive phosphorus with increasing distance from the tube walls into the adjacent sediment (Figure 1.6H) (Baranov et al. 2016; Lewandowski et al. 2007).
Figure 1.6 Variable characteristics and heterogeneity of ecohydrological interfaces as result of differences in passive or active organizational mechanisms structuring interface properties, with: Example I, passive controls – streambed properties controlling hyporheic zone reactivity: Homogeneously low or high ecohydrological interface reactivity (concentrations) at hyporheic zones resulting from continuously low streambed reactivity (A), or depth decreasing (B), or increasing (C) streambed reactivities, in contrast to spatially heterogeneous streambed properties, subsequently causing spatial variability at the interface (D). Example II, active controls – chironomids as engineers of interface complexity: No effect of chironomids and homogenous (E) and heterogeneous (G) distribution of biological, chemical, and physical properties within the sediment matrix and at the burrow wall interface; chironomid pumping induced gradients of decreasing oxygen concentration from the tube into the adjacent sediment (F), and increasing soluble reactive phosphorus concentration from the tube into the adjacent sediment (H).
Disentangling the impacts of different drivers and controls on processes in ecohydrological interfaces remains a challenge, partly due to combined effects and feedbacks between hydrological, biogeochemical, and biological processes that may be additive, synergistic, antagonistic, or undetectable. To use freshwater microbial biofilms as an example, biogeochemical turnover in biofilms is related to their biomass (Haggerty et al. 2014; Singer et al. 2010). Hence, biofilm growth causes biogeochemical turnover rates to increase. At the same time, increased biofilm thickness changes its permeability and has the potential to cause significant clogging, increasing contact area and residence and reaction times at the biofilm surface, which in some cases has been shown to accelerate biogeochemical turnover (Battin et al. 2007), or even change the type of chemical reactions, inducing shifts from aerobic to anaerobic conditions or limiting biogeochemical processing at the interface (Treese et al. 2009).
Improving the understanding of the functioning of ecohydrological interfaces across spatial and temporal scales will require the acknowledgement that traditional hierarchical classification schemes where physical boundary conditions and hydrological behaviour control thermodynamic processes and biogeochemistry, which then define the biological template or ecological niche, are not suitable to adequately describe the complex interactions between biological, biogeochemical, and hydrological processes at ecohydrological interfaces. As previously discussed, biological activity can be a major driver of the spatial and temporal organization of ecohydrological interface functions and often actually shape the physicochemical template. It is essential to fully acknowledge this complexity of multi-directional interactions also in experimental and conceptual model designs as oversimplification of cause–impact relationships will not yield the required understanding of what drives the organizational principles of ecohydrological interface functions.
1.3.4 How do the Impacts of Hotspots and Hot Moments at Ecohydrological Interfaces Upscale to Ecosystem Ecohydrological, Biogeochemical, and Ecological Processes?
Our capacity to quantify and predict the large-scale and long-term importance of hotspots and hot moments at ecohydrological interfaces is hampered by our limited understanding of how mechanisms structuring ecohydrological interfaces and their processes scale in space and time (Krause et al. 2011b; Pinay et al. 2015). The effects of interface hotspot activity have been observed at scales ranging from micro-scales, such as biofilms, to intermediate scales of stream reaches (Lautz and Fanelli 2008; Trauth et al. 2015), and conceptual frameworks have been developed to explain interface process dynamics (Fisher et al. 1998; Harms and Grimm 2008; McClain et al. 2003; Pinay et al. 2015). For example, there is evidence that hyporheic zone processes can have implications for the whole stream network (Gomez-Velez et al. 2015; Harvey and Gooseff 2015; Kiel and Cardenas 2014; Zarnetske et al. 2015), with hyporheic nitrification and denitrification in headwater streams altering the nitrogen load in rivers (Alexander et al. 2007). Although hotspot activity has been shown to be at least temporarily significant at small local scales, its larger-scale importance for energy transfer or biogeochemical turnover in entire river networks or catchments is still widely debated. This partly results from the fact that processes specific to ecohydrological interfaces have often been studied by coupling conceptual models of different ecosystem types (e.g. coupling groundwater and surface water models: Markstrom et al. 2008; Yuan et al. 2011) or land–surface schemes and atmospheric models with hydrological models (e.g. Maxwell and Miller 2005). In both cases, ecohydrological interface conditions are at least partly defined as boundary conditions instead of integrating ecohydrological interface conditions and behaviour implicitly, a practice that restricts the way dynamic interface processes can be analysed across scales.
1.4 A Vision for Integrated Research at Ecohydrological Interfaces
The pressing challenges of global environmental change, such as increasing frequencies and magnitude of extreme events (Blöschl et al. 2015; Hall et al. 2014), call for improved understanding of their impacts from plot to regional scales, across ecosystem types, and beyond disciplinary boundaries. This will require advanced methods for multi-scale monitoring of highly dynamic ecosystem behaviour (Abbott et al. 2016; Blaen et al. 2016) in order to enhance the current understanding of quantitative implications and dynamic behaviour of ecohydrological interface processes for coupled water, matter, and energy fluxes and biogeochemical turnover. The most critical knowledge gaps outlined in this chapter include:
Inadequate conceptual frameworks for understanding how processes occurring at ecohydrological interfaces vary with scale and how and whether small-scale interface processes are manifested at large scales across complex landscapes; and
Failure to transfer and integrate scale-dependent methods and knowledge of mechanisms controlling ecohydrological interface processes across disciplinary and ecosystem boundaries (Abbott et al. 2016; Hannah et al. 2007; Krause et al. 2014b, 2011b).
Interdisciplinary research strategies will need to move the research of ecohydrological interfaces from a descriptive to a mechanistic and predictive stage, extending the scope to a wider range of ecohydrological interfaces than explored in this chapter. Ecohydrological interfaces not only connect different environmental domains but also represent a research topic that requires and fosters novel linkage between traditionally distinct disciplines. The development of multi-scale conceptual models of ecohydrological interface functioning requires interdisciplinary thinking and integration of discipline-specific methods. Following this rationale, we propose the following “roadmap” to catalyse research advances.
Roadmap for Ecohydrological Interface Research:
I) Enhance capacities for multi-scale monitoring and modelling
