Hydraulic Stream Ecology: Observed Patterns and Potential Applications

Although it is well known that metabolism, feeding, and behaviour of lotic organisms is influenced by various flow characteristics, hydraulic variables usually are not accurately measured in lotic ecology studies. Using an approach we call "hydraulic stream ecology", we link organismic responses to a more comprehensive treatment of the physical environment. Since a unified analytical solution for all important hydraulic variables in running waters does not exist at the moment, we advocate a simpler view of the physical system. We demonstrate methods for estimating complex hydraulic key characteristics, like turbulence in the free flow, turbulence close to the stream bottom, and the force of flow prevailing at the bottom. Calculations of these complex key characteristics require measurement of simple hydraulic characteristics like mean velocity, water surface slope, depth, bottom roughness, kinematic viscosity, and density of the water. The hydraulic environment shows characteristic patterns within whole catchments or within reaches of different types of running waters (e.g., high gradient mountain stream, lowland stream, mid-order river). Examples from lotic macroinvertebrates, in particular original data on the water bug Aphelocheirus aestivalis (Fabr.), demonstrate how organismic responses are linked to the hydraulic environment. The body shapes of many lotic zoobenthos are not well adapted to minimize forces of flow, as has been generally believed. Indeed, flow forces are rather stressful for these animals. Critical resources in swift flowing microzones can often be exploited by zoobenthos for only restricted periods, a view that is consistent with the temporal, vertical migration patterns observed for most stream invertebrates. The stress of flow and temporal exposure to it may be correlated with the distribution of lotic zoobenthos. Substratum characteristics, usually perceived as a major factor explaining the distribution of lotic macroinvertebrates, is less important than mean velocity and the complex hydraulic key characteristics. Complex hydraulic characteristics are most useful in modeling specific relationships between the distribution of zoobenthos within a stream reach and the physical habitat, which differ depending on developmental stage of the organism, season, and site. On the catchment scale, the distribution of zoobenthos, and, consequently, longitudinal zonation are also largely dependent on hydraulics. Other groups of organisms found in running waters (e.g., microorganisms and fish) show responses to hydraulics that are comparable to those of lotic macroinvertebrates. There is evidence that some species can alter the hydraulic environment for other individuals (or species). Hydraulic stream ecology provides methods to scale flow in lotic research, which will lead to an increase in replicability and predictability in studies of running water ecosystems. We suggest that researchers who have measured relevant simple hydraulic characteristics in many past studies reevaluate their data considering the role of complex hydraulic key characteristics for the distribution of organisms.