Sediment targets for informing river catchment management: international experience and prospects

Abstract

Abstract:Sediment plays a pivotal role in determining the physical, chemical and biological integrity of aquatic ecosystems. A range of factors influences the impacts of sediment pressures on aquatic biota, including concentration, duration of exposure, composition and particle size. In recognition of the need to assess environmental status for sediment and mitigate excessive sediment pressures on aquatic habitats, both water column and river substrate metrics have been proposed as river sediment targets. Water column metrics include light penetration, turbidity, sediment concentration summary statistics and sediment regimes. Substrate metrics include embeddedness, the fredle index and riffle stability. Identification of meaningful numeric targets along these lines has, however, been undermined by various issues including the uncertainty associated with toxicological dose-response profiles and the impracticalities of deploying statistically robust sampling strategies capable of supporting catchment-scale targets. Many of the thresholds reported are based on correlative relationships that fail to capture the specific mechanisms controlling sediment impacts on aquatic habitats and are stationary in nature. Temporal windows represented by the key life stages of specific species must be given greater emphasis. Given such issues and the need to support the revision of sediment targets for river catchment management, it is proposed that greater emphasis should be placed on developing generic modelling toolkits with the functionality for coupling current or future projected sediment regimes with biological response for a range of biota. Such tools should permit the identification of river catchment-specific targets within a national context, based on biological effect and incorporate sufficient flexibility for utilizing updated physical, chemical, biological and catchment attribute data. Confidence will continue to be required in compliance screening to ensure cost-effective management programmes for avoiding disproportionate investment in impacted river catchments.

Changing climate and nutrient transfers: Evidence from high temporal resolution concentration-flow dynamics in headwater catchments

“…Phytobenthos (an ecological indicator under WFD) demonstrates that these periods of lower phosphorus standards are associated with lower ecological status (Snell et al, 2014). Under the WFD, there are no accepted targets for turbidity or suspended sediment in rivers, although the need for targets which consider the relationship between sediment pressures and ecological condition (Wood and Armitage, 1997) has been identified Collins et al, 2011). The annual mean SS concentration was 9.5 mg L −1 , with a concentration of 25 mg L −1 exceeded for 6% of the year, during storm events.…”

Use of spatially distributed time-integrated sediment sampling networks and distributed fine sediment modelling to inform catchment management

“…However when elevated levels persist, sediment sensitive species are affected and ecological degradation can occur (Collins et al, 2011). The negative impacts of fine sediment, as a diffuse pollutant, are widely acknowledged (cf.…”

“…Although the omission of legally-binding suspended sediment standard(s) can, to an extent, be explained by complexities in the dose-response relationship between the composition and quantity of sediment, and the sensitivity of receiving ecological communities (e.g. Collins et al, 2011;Moss, 2008); this is exacerbated by the inability of authorities to obtain meaningful spatially distributed estimates of the variability in suspended sediment fluxes and its physical properties (Brils, 2008). These factors make it impractical for any theoretically sound, legallybinding, suspended sediment thresholds to be implemented given the operational protocols currently available to competent authorities.…”

“…Investment in automated sampling systems can generate better estimates through flow-proportional, probability and stratified sampling methods (e.g. Braskerud, 2001;Thomas and Lewis, 1995), whilst surrogate technologies may generate meaningful high-resolution datasets (Collins et al, 2011;Owen et al, 2012). However, the application of such technology across catchments is currently unfeasible, or restricted, due to authorities seeking to reduce the cost of non-essential monitoring where possible (Skarbøvik et al, 2012).…”

Assessing the applicability of the Revised Universal Soil Loss Equation (RUSLE) to Irish Catchments

“…The annual average suspended sediment concentration (SSC) limit of 25 mg L -1 under the EU Freshwater Fish Directive (2006/44/EC) does not consider high temporal and spatial variations of sediment transport across a range of catchment characteristics (e.g. Lawler et al 2006), or consider complex and variable biological response to fine sediments in fluvial systems (Bilotta & Brazier, 2008;Collins et al 2011;Kemp et al 2011).…”