Understanding the influence of suspended solids on water quality and aquatic biota.

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 recommended 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 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.