ABSTRACT1. Improved linkage between physical characteristics of rivers and biological performance or potential is a recurrent theme in contemporary river survey, management and design. This paper examines the degree to which flow biotopes and functional habitats may be differentiated with respect to physical habitat delimiters, i.e. flow depth, velocity and Froude number.2. Re-examination of published data demonstrates only very broad associations between biotopes, functional habitats and 'low', 'medium' and 'high' bands in the chosen physical habitat delimiter. The associations are also not consistent between different delimiters.3. Re-analysis is complicated by considerations of research design and methodology. Further studies are required with greater control on the circumstances of observation (particularly flow stage and seasonality), which explicitly control for variance within and between different river cases, and which adopt more consistent terminology.4. Field data for a single river reach at low and high flows indicate that use of the Froude number, in particular, requires careful interpretation. Very different velocity and depth combinations can exhibit similar Froude numbers. This may obscure important contrasts, such as those between channel margin and channel centreline environments.5. Field data also reveal that the flow characteristics of even basic (riffle or pool) units of channel morphology exhibit strong stage-dependence. There are also significant variations when channel margins are isolated from the definitions of the bedforms or analysis of the data. Current practice which seeks to simplify field survey to channel cross-section transects is therefore likely to be misleading.6. Given these difficulties, attempts to link biotopes with ecological response appear premature. Further research might, however, be directed to identifying possible associations between combinations of flow types and bedforms or functional habitats. In this sense, the biotope concept may be more profitably employed as one of several surrogate measures for potential biodiversity.
Despite increasing recognition of the potential of aquatic biota to act as ‘geomorphic agents’, key knowledge gaps exist in relation to biotic drivers of fine sediment dynamics at microscales and particularly the role of invasive species. This study explores the impacts of invasive signal crayfish on suspended sediment dynamics at the patch scale through laboratory and field study. Three hypotheses are presented and tested: (1) that signal crayfish generate pulses of fine sediment mobilisation through burrowing and movement that are detectable in the flow field; (2) that such pulses may be more frequent during nocturnal periods when signal crayfish are known to be most active; and (3) that cumulatively the pulses would be sufficient to drive an overall increase in turbidity. Laboratory mesocosm experiments were used to explore crayfish impacts on suspended sediment concentrations for two treatments: clay banks and clay bed substrate. For the field study, high frequency near‐bed and mid‐flow turbidity time series from a lowland river with known high densities of signal crayfish were examined. Laboratory data demonstrate the direct influence of signal crayfish on mobilisation of pulses of fine sediment through burrowing into banks and fine bed material, with evidence of enhanced activity levels around the mid‐point of the nocturnal period. Similar patterns of pulsed fine sediment mobilisation identified under field conditions follow a clear nocturnal trend and appear capable of driving an increase in ambient turbidity levels. The findings indicate that signal crayfish have the potential to influence suspended sediment yields, with implications for morphological change, physical habitat quality and the transfer of nutrients and contaminants. This is particularly important given the spread of signal crayfish across Europe and their presence in extremely high densities in many catchments. Further process‐based studies are required to develop a full understanding of impacts across a range of river styles. Copyright © 2013 John Wiley & Sons, Ltd.
Summary To evaluate the effects of large wood (LW) on benthic habitats and macroinvertebrates in sand‐bed lowland rivers, we compared invertebrate communities recorded on four pieces of LW (12 samples in total) and around them (60 samples) with those in four control sites in the same river (four samples). Mean flow velocity was 32% lower in the channel areas surrounding the LW than in control sites, while median sediment grain size was 50% higher, and the organic matter content of the riverbed sediments was 287% higher. At the same time, habitat conditions showed threefold to 1000‐fold increases in variance for five key abiotic habitat descriptors in the surrounding channel extending at least 60 cm upstream and 160 cm downstream of the LW. Three habitat patches typically occurred around the LW pieces: scouring pools, sand bars and accumulations of organic matter. These patches were colonised by distinctive invertebrate communities (e.g. accumulations of organic matter and gravel hosted 15 and 2 indicator taxa, respectively) that overall harboured 110% more taxa and exhibited a 168% higher diversity than control sites. The LW itself contributed only a small fraction to these increases, exhibiting a 15% increase in taxa richness and a 21% increase in species diversity compared to the control sites. The diversification of benthic invertebrate communities colonising streambed sediments around LW could be directly linked to the much more heterogeneous habitat conditions recorded there. Thus, local additions of large wood within river restoration programmes have the potential to promote the establishment of diverse invertebrate communities in extended areas of a river channel.
Abstract:Recreation or restoration of salt marsh through the deliberate removal of flood defences (managed realignment or deembankment) is a common practice across Europe and the USA, with potential to enhance delivery of ecosystem services. However, recent research suggests that physical, chemical and ecological processes may be impaired in recreated sites as a result of the modified morphology, sediment structure and hydrology associated with both the restoration process and historic land use. This paper compares physical sediment properties and subsurface water levels recorded in paired natural and de-embanked (recreated) salt marshes in SE England. Using a combination of statistical and time-series modelling, significant differences between the natural and recreated marshes are identified. Sediment properties (bulk density, moisture content and organic content) within each marsh were statistically different and imply that de-embanked sediments are compacted, which may affect subsurface water movement. Analysis of hydrological time series reveals that the de-embanked salt marsh is characterized by a damped response to tidal flooding with elevated and less variable water levels. This, combined with analysis of hydrographs and hysteresis patterns over individual tidal cycles, suggests that fast, horizontal near-surface flows enhanced by the relict land surface may play a greater role in drainage of the de-embanked salt marsh. The importance of hydrological functioning in governing many important physical and biogeochemical processes in salt marshes suggests any modifications would have significant implications for the delivery of ecosystem services.
Techniques for the rapid appraisal and documentation of water resources and habitat are increasingly utilized in a variety of legislative compliance situations and environmental management/design projects. Within river survey, the 'physical biotope' (i.e. riffles, runs, pools, glides) has been advocated as the basic unit of river habitat and an appropriate focus for stream reconnaissance and habitat assessments at the mesoscale of river systems. To date, however, much research into the hydraulic characterization of physical biotopes has focused on spatially-and temporally averaged velocity and depth measures, overlooking more detailed hydraulic parameters and variation at microscales, which may have more direct influences on the survival of individual organisms. This paper outlines the methodology, analytical techniques and initial results of a microscale exploration of the hydraulics of physical biotopes through the analysis of a range of turbulence properties. Results of the analyses offer both detailed descriptions of hydraulic habitat provided by the biotopes studied (related to various mechanisms of turbulence generation), as well as an indication of levels of 'within-biotope' heterogeneity which gives rise to a conceptual classification. A continuum of increasing habitat complexity is observed from glide, to riffle, to pool biotopes, associated with the generation of turbulence and organization of flow structures and with the level of 'internal' spatial and temporal hydraulic heterogeneity. Physical biotope characterization approached in this way may offer a more robust and transferable classification compared to mesoscale methods, particularly if levels of within-biotope heterogeneity are treated as an additional biotope characteristic.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.