24 Movements of a macroinvertebrate (Potamophylax latipennis) across a gravel-bed substrate: effects of local hydraulics and micro-topography under increasing discharge
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.
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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).
- (2)
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).
- (3)
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).
Section snippets
Rationale, aims and approach
Our basic assumptions are that animals seek to minimise the energy costs of movement (Vogel, 1981; Huryn and Denny, 1997) and minimise the risk of long-distance, downstream displacement through entrainment and drift. Drift is often regarded as a surrogate measure of mortality because it may result in increased predation risk, reduced feeding opportunities, physical damage and/or transport to unsuitable habitat (Palmer et al., 1996, Palmer et al., 1992). A few individuals may survive long
Experimental arrangement
A precise replica of a natural cobble-gravel substrate, 1.0 by 2.0 m, was made using the casting technique described by Buffin-Bélanger et al. (2003). The cast was obtained from an exposed gravel bar in the River Manifold, UK, and reproduces the true three-dimensional complexity of a natural, water-worked unit. The cast retains significant small-scale details including the texture of mosses and sands and most of the surface interstices. Orthophotographs and a digital elevation model (DEM) of the
Path Maps
The path maps provide a clear illustration of how insect movements were affected by flow. As discharge increased (Fig. 24.2, panels a–c), crawling paths became shorter and increasingly disjointed. The average straight-line displacement resulting from individual crawling events declined significantly, from 67 to 30 mm and then to 27 mm under flows 1, 2 and 3, respectively (F2,15=18.1, p<0.001, Flow effect in split-plot ANOVA). In contrast, the frequency of entrainment events increased 23-fold from
Discussion
Crawling paths and entrainment maps (Figure 24.2, Figure 24.3) highlight the patchy distribution of P. latipennis larvae as they crawl across a rough, gravelly substrate. Individual paths do not criss-cross the substrate at random but exhibit a degree of co-location which suggests that there are preferred crawling tracks. Analysis of the hydraulics and elevation at sites where different activities occur provides some insights into these insect preferences that, it is assumed, reflect a
Conclusion
The substrate of a gravel-bed river is a dangerous place to live, subject to large hydraulic forces and prone to instability during large floods, yet benthic fauna are typically diverse and abundant. Flow refugia mechanisms help to explain the persistence of macroinvertebrate communities in gravel-bed streams. The movement of insects is central to the refugia idea, but little is known about the nature of insect movements on rough substrates and the effect of increases in flow or differences in
Acknowledgements
The project was funded by NERC Grant NER/B/S/2000/00697 to Rice, Reid and Lancaster. We are grateful to Natasha Todd-Burley, Mick Barker, David Graham and Stuart Ashby for their help with the flume experiments, which were conducted in the Department of Civil & Building Engineering at Loughborough University. Ian Atkins and Adam Evans helped with video analysis of insect movements and Jim Chandler provided photogrammetric expertise. We are grateful to two anonymous reviewers for their useful
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