Hydrodynamics of Aquatic Ecosystems: Current State, Challenges, and Prospects

The paper endorses a recently emerged interdisciplinary research subject Hydrodynamics of Aquatic Ecosystems, defined as a study of flow-organism interactions in running waters with particular focus on relevant transport processes and mutual physical impacts occurring at multiple scales from the sub-organism scale to the organism patch mosaic scale (comparable to the flow width). This new research area emerges at the interfaces between environmental fluid mechanics, biomechanics, and aquatic ecology, bridging these disciplines together and offering new promising research avenues. After a brief review of the current state, the paper focuses on the challenges that this subject area currently faces, and then outlines research directions to pursue for resolving the highlighted challenges. Introduction Sixteen years ago a prominent aquatic ecologist B. Statzner stressed that “a broader incorporation of aspects of fluid dynamics into studies of various ecosystems will advance general ecological theory faster than past or current research routes, which largely ignore(d) the physical principles of moving air or water” [26]. Since then, the situation has not changed much [27], reflecting a slow progress in the implementation of fluid mechanical concepts into ecological theories. There are at least three reasons for such a slow progress in the current knowledge. First, measurements at the organism scales still represent great challenges and thus data related to these scales remain very limited. Second, most studies of flow-organism interactions pay little attention to the biomechanical properties of organisms, which change significantly across species, scales, and environments and are poorly understood. Third, the subject of flow-organism interactions lies at the borders between fluid mechanics, ecology, and biomechanics, i.e., at the discipline interfaces which are typically avoided by researchers. Another problem that has also to be addressed is how to integrate fluid mechanical, biomechanical, and ecological processes together and how to upscale the effects of these processes from the suborganism scale to the patch mosaic scale. Because of these knowledge gaps, the progress in studies of flow-biota interactions is slow and a solid unifying interdisciplinary platform is urgently required to accelerate it and to enhance current ecological concepts. This paper and talk are an attempt to further enhance and promote such a platform as an emerging research area at the interfaces between environmental fluid mechanics, aquatic ecology, and biomechanics. This new area, Hydrodynamics of Aquatic Ecosystems, bridges these disciplines together and can be defined as a study of flow-organism interactions in running waters with particular focus on relevant transport processes and mutual physical impacts occurring at multiple scales from the sub-organism scale to the organism patch mosaic scale comparable to the flow width [19]. Being an important part of its mother disciplines, Hydrodynamics of Aquatic Ecosystems deals with two key interconnected issues: (i) physical interactions between flow and organisms (e.g., due to an interplay between flow-induced forces and reaction forces generated by organisms); and (ii) ecologically relevant mass-transfer processes (e.g., due to molecular and turbulent diffusion). The focus of Hydrodynamics of Aquatic Ecosystems on the interfaces between fluid mechanics, ecology and biomechanics should help with elimination of existing knowledge gaps in the least studied areas. Hydrodynamics of Aquatic Ecosystems as a subject covers both marine and freshwater environments and therefore should include, as equal branches, Hydrodynamics of Freshwater Ecosystems (i.e., streams, rivers) and Hydrodynamics of Marine Ecosystems. The centre of attention of this paper is on freshwater systems (i.e., streams and rivers) where, compared to marine systems, development of Hydrodynamics of Aquatic Ecosystems is deferred and thus this talk may help in its enhancement. Key concepts of Hydrodynamics of Aquatic Ecosystems and current challenges will be outlined first and then future research directions will be listed. Scale Range of Hydrodynamics of Aquatic Ecosystems in Relation to Streams and Rivers Flow variability in streams and rivers covers wide ranges of temporal and spatial scales, from milliseconds to many years and from sub-millimetres to tens of kilometres (figure 1, [18]). The low frequency (large periods) range in the frequency spectrum is formed by intra-annual and inter-annual hydrological variability while the high-frequency (small periods) range is formed by flow turbulence (figure 1a). The low wave-number (large spatial scale) range in the wave-number spectrum is formed by morphological variability along the flow such as bars and/or meanders (figure. 1b). At smaller spatial scales (comparable to and less than the flow width) velocity fluctuations are due to turbulence. This ‘turbulence’ range of scales is most relevant to organisms as their own scales (including patchiness) typically fall within this range. In addition, turbulence is often the main mechanism controlling drag forces acting on the organisms as well as driving transport processes in biological communities. Thus, the main focus of Hydrodynamics of Aquatic Ecosystems in relation to streams and rivers is on the turbulent range of scales as sketched in figure 1 [18, 19]. Tools and Concepts of Hydrodynamics of Aquatic Ecosystems Research methods, tools and concepts of Hydrodynamics of Aquatic Ecosystems come from its mother disciplines and are still emerging. On one hand, they gradually become interconnected and adjusted to address common goals and research questions. On the other hand, they feed back to the original disciplines providing new challenges and making these disciplines conceptually richer. Hence, the methodological suit of Hydrodynamics of Aquatic Ecosystems is likely to be always strongly linked to the original mother disciplines. Fluid Mechanics contributes to Hydrodynamics of Aquatic Ecosystems with concepts of boundary layers (BL), mixing layers (ML), wakes, and jets. Depending on the specific conditions these flow types may exhibit properties of two turbulence phenomena: coherent structures (CS) and/or eddy cascades (EC). These canonical flow types and concepts can be both actively utilised and significantly altered by biological communities. Furthermore, these communities, in principle, may also create unconventional flow configurations which are still unidentified in Fluid Mechanics. In a similar way, biomechanics, which stems from mechanics of materials and structural mechanics, provides a range of methods and concepts that help in describing biological communities at multiple scales and linking them to their fluid environments. Indeed, reactions of organisms to physical forces imposed by flow patterns described above largely depend on their biomechanical properties. This dependence has been recognised long ago and some progress has been made for terrestrial ecosystems (e.g., trees, vertebrates, and insects) and partially for marine ecosystems [2, 3, 14, 20, 22, 30, 31] while biomechanics of freshwater organisms is still largely unknown and is represented by very few studies (e.g., [10, 32]). Combined consideration of flow-induced, organism-induced, and organismreaction forces leads to a set of similarity numbers that describe interplay between gravity, buoyancy, drag, and elastic forces. The flow-organism similarity numbers may be helpful in data interpretation and in identifying specific regimes of flow-biota interactions. They can also be introduced in a more formal way considering coupled flow-organism equations of motion, where the flow equations provide drag terms that are used in organism motion equations as external forcing. A wide expansion of this approach, however, is slowed down by very limited information on organism material parameters and their variability across species, scales, and environments. Combined together, fluid mechanical and biomechanical concepts should explain a number of ecological issues related to design, spatial and temporal patterns, and performance of aquatic organisms and their communities. Recent ecological studies suggest that organism functioning, morphology, and the role in aquatic ecosystems are largely driven by transport processes and mutual physical impacts and their interactions, i.e., remits of Hydrodynamics of Aquatic Ecosystems) [7, 9, 11, 15, 16, 25, 28, 30]. The current studies of flow-biota interactions in streams and rivers encounter a number of challenges related to fluid mechanical, biological, and ecological aspects which slows down the knowledge advancement in this area. These challenges, discipline-grouped for convenience, are highlighted below. Research Challenges Fluid mechanical challenges In aquatic ecosystems, the canonical flow types (BL, ML, wakes, jets) are fundamental for characterisation of both (1) hydraulic habitats, as most aquatic communities live within BLs, MLs, etc; and (2) flow patterns around individual organisms that are often surrounded by BLs, MLs, or wakes generated at organism surfaces or within/around organism communities. The occurrence of these flow types in aquatic ecosystems, however, often deviates from their canonical forms, thus leading to Challenge #1: What are the manifestations of the canonical flow types in aquatic ecosystems? Figure 2 may illustrate this challenge for the case of aquatic plants which typically span a wide range of scales from a leaf scale to individual plant to the plant patch mosaic (i.e., an assemblage of plant patches of different shapes and sizes). As an example, biological communities quite often are embedded in a superposition of interacting multi-scale BLs generated by a variety of boundaries including those introduced by the organisms themselves (e.g., flow-depth BL and leaf/stem BLs in figure 2a). As a rule, these BLs have limited thicknesses and small relative submergence of roughness elements, being often organised as a cascade of internal boundary layers (e.g., [11, 15]). As a result, the conventional concepts and descriptions may not be always applicable and may require refinements (e.g., applicability of the log velocity profile in low-submergence BLs is questionable). The flow patterns, schematically summarised in figure 2 for the case of aquatic plants, include: (1) ‘conventional’ depth-scale shear-generated turbulence which may be significantly altered by the vegetation; (2) canopy-height-scale turbulence resulting from the Kelvin-Helmholtz instability (KHI) at the upper boundary of the vegetation canopy (known as the mixing-layer analogy [23]); (3) generation of small-scale turbulence associated with flow separation from stems (i.e., von Karman vortices); (4) generation of small-scale turbulence within local boundary layers attached to leaf/stem surfaces; (5) generation of small-scale turbulence behind plant leaves serving as small ‘splitter plates’ that generate local leeward mixing layers, with subsequent turbulence production through the Kelvin-Helmholtz instability (most likely occurring when the leaf surface roughness differs between sides); (6) turbulence generation due to plant waviness at a range of scales (if biomechanical properties allow this); (7) generation of largescale 3D and 2D turbulence associated with wakes and flow separation at a patch scale; (8) generation of 3D and 2D boundary layer and mixing layer turbulence at patch sides aligned with the flow; and (9) generation of interacting vertical and horizontal internal boundary layers at the patch mosaic scale (figure 2). Among these patterns, only studies of patterns 1, 2, and 3 have been carried out [6, 12, 13, 21] while other patterns are hypothesised in figure 2 based on conceptual consideration and our preliminary results from laboratory and field studies ([1, 15, 16, 19], figure 3). The identification and quantification of interrelationships between these patterns, as well as detection of their individual and combined roles in transport processes and drag generation for a range of biomechanical parameters Figure 1. Schematised velocity spectra in rivers: (a) frequency spectrum; and (b) wave-number spectrum (Wo and W = river valley and river channel widths, H = depth, Z = distance from the bed,  = roughness height, U = flow velocity, = turbulence micro-scale [18]. T i m e s c a l e Ve lo ci ty Fr eq ue nc y Sp ec tru m T u r b u l e n c e