Abstract
Suspended sediment (SS), ranging from nano-scale particles to sand-sized sediments, is one of the most common contributors to water quality impairment globally. However, there is currently little scientific evidence as to what should be regarded as an appropriate SS regime for different freshwater ecosystems. In this article, we compare the SS regimes of ten systematically-selected contrasting reference-condition temperate river ecosystems that were observed through high-resolution monitoring between 2011 and 2013. The results indicate that mean SS concentrations vary spatially, between 3 and 29mgL−1. The observed mean SS concentrations were compared to predicted mean SS concentrations based on a model developed by Bilotta et al. (2012). Predictions were in the form of probability of membership to one of the five SS concentration ranges, predicted as a function of a number of the natural environmental characteristics associated with each river's catchment. This model predicted the correct or next closest SS range for all of the sites. Mean annual SS concentrations varied temporally in each river, by up to three-fold between a relatively dry year (2011–2012) and a relatively wet year (2012–2013). This inter-annual variability could be predicted reasonably well for all the sites except the River Rother, using the model described above, but with modified input data to take into account the mean annual temperature (°C) and total annual precipitation (mm) in the year for which the mean SS prediction is to be made. The findings highlight the need for water quality guidelines for SS to recognise natural spatial and temporal variations in SS within rivers. The findings also demonstrate the importance of the temporal resolution of SS sampling in determining assessments of compliance against water quality guidelines.
Highlights
Managing global water resources is one of the greatest challenges of the 21st century (Garrido and Dinar, 2009; Poff, 2009; Staddon, 2012)
The largest difference in mean annual Suspended sediment (SS) concentration was observed in the River Great Ouse, which experienced a three-fold increase between year one (14 mg L−1) and year two (42 mg L−1) concomitant with roughly three-fold increases in mean annual discharge observed at this site from year one (6.37 m3 s−1) to year two (19.56 m3 s−1) (Table 4)
The degree of spatial variation in estimated mean SS concentrations (3–29 mg L−1) and observed mean SS concentrations (1–17 mg L−1), supports earlier research by Bilotta et al (2012) that analysed the mean SS concentrations derived from lowresolution monitoring programmes in 638 reference-condition sites in the UK
Summary
Managing global water resources is one of the greatest challenges of the 21st century (Garrido and Dinar, 2009; Poff, 2009; Staddon, 2012). In recognition of the potential for SS to cause aquatic degradation, and in an effort to minimise this degradation, government-led environmental organisations from around the world have established water quality guidelines and standards, which state recommended targets for SS (sometimes referred to as suspended solids, and occasionally assessed through proxy measurements such as turbidity) (Bilotta and Brazier, 2008; Collins et al, 2011) At present these guidelines are often blanket values that do not recognise the natural spatial and temporal variations of SS in streams/rivers, and are not well-linked to the biological/ecological impact evidence; these guidelines may not reflect the specific requirements of the biological communities that they are designed to protect (Bilotta et al, 2012; Collins et al, 2011; Schwartz et al, 2008, 2011). This Directive has been repealed and replaced by the Water Framework Directive (2000/60/ EC) (2008/105/EC), but there are currently no new guidelines for SS under this Directive
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