Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Review Understanding the influence of suspended solids on water quality and aquatic biota G.S. Bilottaa,b, , R.E. Braziera aDepartment of Geography, University of Exeter, Amory Building, Rennes Drive, Exeter, Devon EX4 4RJ, UK b North Wyke Research, North Wyke, Okehampton, Devon EX20 2SB, UK a r t i c l e i n f o Article history: Received 2 November 2007 Received in revised form 18 March 2008 Accepted 29 March 2008 Available online 7 April 2008 Keywords: Suspended solids Turbidity Impacts Salmonids Invertebrates a b s t r a c t Over the last 50 years the effects of suspended solids (SS) on fish and aquatic life have been studied intensively throughout the world. It is now accepted that SS are an extremely important cause of water quality deterioration leading to aesthetic issues, higher costs of water treatment, a decline in the fisheries resource, and serious ecological degradation of aquatic environments. As such, government-led environmental bodies have set recom- mended water quality guidelines for concentrations of SS in freshwater systems. However, these reference values are often spurious or based on the concept of turbidity as a surrogate measure of the concentration of SS. The appropriateness of these recommended water quality values is evaluated given: (1) the large variability and uncertainty in data available from research describing the effects of SS on aquatic environments, (2) the diversity of environments that these values are expected to relate to, and (3) the range of conditions experienced within these environments. Furthermore, we suggest that reliance solely upon turbidity data as a surrogate for SS must be treated with caution, as turbidity readings respond to factors other than just concentrations of SS, as well as being influenced by the particle-size distribution and shape of SS particles. In addition, turbidity is a measure of only one of the many detrimental effects, reviewed in this paper, which high levels of SS can have in waterbodies. In order to improve the understanding of the effects of SS on aquatic organisms, this review suggests that: First, high-resolution turbidity monitoring should be supplemented with direct, measurements of SS (albeit at lower resolution due to resource issues). This would allow the turbidity record to be checked and calibrated against SS, effectively building a rating-relationship between SS and turbidity, which would in-turn provide a clearer picture of the exact magnitude of the SS problem. Second, SS should also be characterised in terms of their particle-size distribution and chemical composition. This would provide information to develop a more comprehensive understanding of the observed variable effects of a given concentration of SS in aquatic habitats. These two suggested improvements, combined with lower-resolution concurrent measures of aquatic ecological status, would improve our understanding of the effects of SS in aquatic ARTICLE IN PRESS 0043-1354/$ -see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.03.018 Corresponding author at: North WykeResearch, North Wyke, Okehampton, Devon EX20 2SB, UK. Tel.: +4401837 883544 fax: +44 0183782139. E-mail address: garybilotta@yahoo.co.uk (G.S. Bilotta). WAT E R R E S E A R C H 42 (2008) 2849 ��� 2861

environments and together with a more detailed classification of aquatic environments, would provide an environment-specific evidence base for the establishment of effective water quality guidelines for SS. & 2008 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2850 1.1. The effects of SS on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2852 1.1.1. Phytoplankton, periphyton and macrophytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2852 1.1.2. Aquatic invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2852 1.1.3. Salmonid fish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2853 2. Factors determining the effect of SS on aquatic biota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2853 2.1. Concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2853 2.2. Duration of exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2854 2.3. Geochemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2854 2.4. Particle-size distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2854 3. SS and international water quality guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2856 3.1. Variability and uncertainty in data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2856 3.2. Diversity of environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2856 3.3. Range of conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2857 3.4. Turbidity as a surrogate measure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2857 4. Developing more advanced water quality guidelines for SS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2858 5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2859 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2859 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2859 1. Introduction The term suspended solids (SS) refers to the mass (mg) or concentration (mgL 1 ) of inorganic and organic matter, which is held in the water column of a stream, river, lake or reservoir by turbulence. SS are typically comprised of fine particulate matter with a diameter of less than 62 mm (Waters, 1995), though for the majority of cohesive solids, research has demonstrated that transport frequently occurs in the form of larger aggregated flocs (Droppo, 2001 Droppo et al., 1997 Phillips and Walling, 1995). All streams carry some SS under natural conditions (Ryan, 1991). However, if concentrations are enhanced through, for example, anthropogenic perturbations, this can lead to alterations to the physical, chemical and biological properties of the waterbody. Physical alterations caused by SS include reduced penetration of light, temperature changes, and infilling of channels and reservoirs when solids are deposited. These physical alterations are associated with undesirable aesthetic effects (Lloyd et al., 1987), higher costs of water treatment (Ryan, 1991), reduced navigability of channels and decreased longevity of dams and reservoirs (Butcher et al., 1993 Verstraeten and Poesen, 2000). Chemical alterations caused by SS include the release of contaminants, such as heavy metals and pesticides (Dawson and Macklin, 1998 Kronvang et al., 2003 Miller, 1997), and nutrients such as phosphorus (Harrod and Theurer, 2002 Haygarth et al., 2006 Russell et al., 1998), into the water body from adsorption sites on the sediment. Furthermore, where the SS have a high organic content, their in-situ decomposition can deplete levels of dissolved oxygen in the water, producing a critical oxygen shortage which can lead to fish kills during low-flow conditions (Ryan, 1991). The biological effects of high levels of SS on different groups of organisms are discussed below and are summarised in Tables 1���3. The effects of SS on various aquatic biota have been reviewed in the past (see Alabaster and Lloyd, 1982 Cordone and Kelley, 1961 Gammon, 1970 Newcombe and MacDonald, 1991 Owens et al., 2005 Petticord, 1980 Ryan, 1991 Wood and Armitage, 1997). In this review paper, we first provide an in-depth overview of the different mechanisms by which SS can affect different types of aquatic biota (Section 1). This section of the paper presents essential knowledge that underpins why we cannot rely solely on turbidity data to monitor and assess the effects of SS in aquatic environments. Section 2 identifies and reviews the key factors that determine the effect of SS on water quality and aquatic biota. This section of the paper demonstrates the complexities involved behind understanding the effects of a given concentration of SS on aquatic biota. Section 3 of the review paper discusses the various conventional methods applied in environmental monitoring of SS, highlighting, with reference to international water quality guidelines, several key issues and deficiencies with the existing measurement techniques for both turbidity and SS. This section of the paper also examines how these flaws may limit our understanding of the effects of SS in waterbodies and will inhibit attempts to mitigate SS-related ARTICLE IN PRESS WAT E R R E S E A R C H 42 (2008) 2849 ��� 2861 2850