24 Movements of a macroinvertebrate (Potamophylax latipennis) across a gravel-bed substrate: effects of local hydraulics and micro-topography under increasing discharge

Abstract

Flow refugia provide a mechanism that can explain the persistence of macroinvertebrate communities in flood-prone, gravel-bed rivers. The movement behaviour of macroinvertebrates is a key element of the flow refugia hypothesis, but surprisingly little is known about it. In particular, little is known about how local near-bed hydraulics and bed micro-topography affect macroinvertebrate movements. We used a novel casting technique to reproduce a natural gravel-bed substrate in a large flume where we were able to observe the movement behaviour of the cased caddisfly, Potamophylax latipennis at different discharges. The crawling paths and drift events of animals were analysed from video recordings and used to classify sites on the substrate according to the type of insect movement. We used acoustic Doppler velocimeter (ADV) measurements close to the boundary to characterise hydraulic conditions at different sites and a detailed Digital Elevation Model (DEM) to characterise sites topographically. Animals made shorter more disjointed crawling journeys as discharge increased, although they tended to follow consistent paths across the substrate. As hypothesised, crawling behaviour was locally associated with low elevations, low flow velocities and low turbulent kinetic energies, while sites that insects avoided were characterised by higher elevations, velocities and turbulence. Discrimination was greater at higher discharges, indicating that movement behaviour is contingent upon flow conditions. We suppose that these relations reflect the need of animals to reduce the risk of entrainment and minimise energy expenditure by avoiding areas of high fluid drag. As discharge increased, there was a general upward shift in the frequency distributions of local velocities and turbulent kinetic energies. The animals responded to these shifts and it is clear that their different activities were not limited to fixed ranges of velocity and turbulence. We assume that the absolute hydraulic forces would become a limiting factor at some higher discharge. At the discharges examined here, which are below those required to instigate framework particle entrainment, patterns of animal movement appear to be associated with the animals’ experiences of relative rather than absolute hydraulic forces.

Introduction

How do populations of stream invertebrates persist in gravel-bed rivers subject to hydrological disturbances (floods) where hydraulic forces and associated bed material movement can result in mortality and/or loss of individuals? This is a problem of enduring interest to stream ecologists and an area for fruitful collaboration between ecologists and geomorphologists. Understanding this problem involves three linked ideas.

• In gravel-bed rivers, near-bed hydraulics are conditioned by the complex three-dimensional micro-topography of the bed materials and are therefore spatially heterogeneous. This heterogeneity persists at the highest flows and some low-stress areas are present even at high discharges. Spatially distributed flow measurements above gravel beds are rare (Lamarre and Roy, 2005) and the degree to which spatial heterogeneity is maintained as discharge varies has not been widely studied or quantified. Nevertheless, the persistence of low-stress areas across a range of discharges has been demonstrated, both at fixed locations (Lancaster and Hildrew, 1993a) and at shifting locations in association with changing stage (Rempel et al., 1999).

• The distribution of benthic invertebrates across the stream bed is also spatially and temporally heterogeneous and this patchiness is associated with the heterogeneity in near-bed hydraulics. Several studies have shown that during high flows, animal densities are higher in areas of low shear stress and low velocity (Lancaster and Hildrew, 1993b; Palmer et al., 1996; Rempel et al., 1999). Additional abiotic and biotic factors (substrate, food availability, predation, competition) contribute to patchy invertebrate organisation, but a wealth of evidence suggests that flow is a primary consideration (e.g., Hart and Finelli, 1999) by direct influence on entrainment and by indirect effects, such as the availability of particulate organic foodstuffs (Bouekaert and Davis, 1998).

• During floods, parts of the stream bed that experience low hydraulic stresses may act as flow refugia, such that invertebrates that happen to be in, or move into, these areas avoid entrainment. Local population loss is thereby minimised and a group of survivors is available to re-colonise the bed. Flow refugia have been associated with single stable stones (Matthaei et al., 2000), microform bed clusters (Biggs et al., 1997), the hyporheos (Dole-Olivier et al., 1997), bar edges (Rempel et al., 1999), inundated floodplains (Badri et al., 1987) and woody debris (Palmer et al., 1996), as well as undifferentiated in-channel zones of relatively low velocity (Lancaster and Hildrew, 1993a). The relative importance of these refugia is contested (Palmer et al., 1992; Robertson et al., 1997; Matthaei and Townsend, 2000; Matthaei and Huber, 2002), and it seems most likely that they serve different animals at different times, depending on their availability and the life stage and traits of the animals.

In all cases, however, the efficiency of the flow refugia mechanism relies upon the passive or active movement of animals into and, perhaps, out of the protected areas. Thus, important keys to understanding the basis of how flow refugia can facilitate population persistence are: an understanding of the movement behaviour of invertebrates across the stream bed; how this behaviour is influenced by local hydraulic conditions; and the net effect of those movements at the population level (Lancaster and Belyea, 1997). Surprisingly little is known about the movement of benthic macroinvertebrates in natural settings; for example, there is almost no information about the velocity at which insect larvae are able to move across a gravel substrate, their preferred pathways of movement in relation to bed micro-topography and whether movement behaviour changes in response to changes in the general flow characteristics. The effects of stream flow on invertebrate drift have received some attention (e.g., recent review in Hart and Finelli, 1999), but detailed studies of movements in association with the substrate (crawling, walking) are scarce. There is indirect empirical evidence of the role hydraulics plays in the dynamic spatial distribution of invertebrates from field surveys (Lancaster and Hildrew, 1993b; Palmer et al., 1996; Rempel et al., 1999; Lancaster and Belyea, 2006) and some manipulative field experiments (Winterbottom et al., 1997; Lancaster, 2000), but there is a lack of direct observations of invertebrate movement in realistic environments. This is the general focus of our work.

In this paper, the interaction between insect movement, local micro-topography and hydraulics at the stream bed are investigated. We map the paths taken by insects as they move across a realistic facsimile of a gravel surface above which near-bed flow is spatially heterogeneous. We then examine the differences in hydraulics and elevation at locations where different types of movement and different levels of mobility are observed. In another paper, we examine how these interactions might have higher order implications for population-level processes, e.g., net displacement, spatial dispersion, etc. (Lancaster et al., 2006).