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

Abstract

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.

Introduction

The term suspended solids (SS) refers to the mass (mg) or concentration (mg L⁻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 μm (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 Table 1, Table 2, Table 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 water quality problems. Lastly, Section 4 of the paper suggests ways in which the techniques for measurement can be improved and how this should, in-turn, feed into more environment-specific water quality guidelines to alleviate the impacts of SS on water quality.

Phytoplankton (algae suspended in the water column), periphyton (algae attached to stream substrates) and macrophytes (visible plants that are either rooted in the substrate in the case of emergent and floating-leaved macrophytes, floating beneath the surface in the case of submersed macrophytes, or floating on the surface, in the case of free-floating macrophytes), are important sources of food and producers of oxygen in the aquatic environment (Bronmark, 2005; Brown, 1987). SS can influence macrophytes and algae, primarily through affecting the amount of light penetrating through the water column. The reduction in light penetration through the water column will restrict the rate at which periphyton and emergent and submersed macrophytes can assimilate energy through photosynthesis, which will impact directly on primary consumers. However, it is worth noting that this mechanism is not so important for the planktonic species including surface phytoplankton, and floating-leaved or free-floating macrophytes. Furthermore, the importance of in-stream primary producers within food chains varies amongst different stream communities. For example, the small forested streams studied by Cowie, 1983, Cowie, 1985 in New Zealand, obtain a considerable proportion of their energy inputs from allochthonous sources such as decaying leaf matter. Under these circumstances the SS entering the waterbody are an important part of the ecosystem.

Periphyton abundance can also be influenced by SS through mechanisms other than reduced light penetration; (1) High levels of SS in transport by fast flow rates can act to scour these organisms away from streambed substrates as well as being abrasive and damaging to the photosynthetic structures of organisms (Alabaster and Lloyd, 1982; Steinman and McIntire, 1990). (2) SS can indirectly affect the abundance of phytoplankton, periphyton and macrophytes through acting as a vector of nutrients such as phosphorus (Heathwaite, 1994), and toxic compounds such as pesticides and herbicides from the land surface to the waterbody (Kronvang et al., 2003).

Invertebrates can be divided into those that remain suspended in the water column (i.e. zooplankton), and those that inhabit the zone surrounding the streambed (i.e. benthic invertebrates). Benthic invertebrates include numerous species of insects, molluscs and crustaceans. SS can affect benthic invertebrates by subjecting them to abrasion and scouring as SS being carried in the flow move over the channel bed. This can damage exposed respiratory organs or make the organism more susceptible to predation through dislodgement (Langer, 1980). A number of studies have shown that increased SS are associated with an increase in invertebrate drift (down- or up-channel migration of organism). For example, Gammon (1970) showed that increases in SS of 40–80 mg L⁻¹ above background levels caused an increase in invertebrate drift of 25–90%. Ryder (1989) showed that a sudden increase in the drift densities of stream insects occurred when suspended sediment was introduced into a natural stream. Ryder (1989) noted that in the normal course of events there would be a compensating drift from up-stream, however, the introduction of fine material to the substrate and the associated turbidity may inhibit re-attachment to the stream bed and encourage fauna to continue drifting. For grazing invertebrates, Graham (1990) demonstrated that suspensions of clay-sized particulates can be trapped by epilithic periphyton and reduce its attractiveness for grazing. For filter-feeding invertebrates, high levels of SS can clog feeding structures, reducing feeding efficiency and therefore reducing growth rates, stressing and even killing these organisms (Hynes, 1970). For those invertebrates that graze periphyton for their energy and nutritional requirements, any changes in SS concentrations that adversely affect algal growth, biomass, or species composition can adversely affect populations of these types of invertebrates (Newcombe and MacDonald, 1991). Changes in the abundance of invertebrates have knock-on effects higher up in the food chain as discussed below.

As well as being important members of the aquatic food chain, salmonids, including trout, grayling, whitefish and salmon, are valuable game fish and an important economic and nutritional resource for humans (Cordone and Kelley, 1961; Ryan, 1991). As such, there has been a large amount of research into the effects of SS on salmonid fish (e.g. Alabaster and Lloyd, 1982; Cordone and Kelley, 1961; Greig et al., 2005; Harrod and Theurer, 2002; Lloyd, 1987; Newcombe and MacDonald, 1991; Redding et al., 1987). Salmonid fish can be affected by SS in several ways. The most intensively studied of these mechanisms involves the deposition and settling of SS in gravel-bed rivers. This has been recognised as a major cause for the reduced development and survival of salmonid eggs and larvae within salmonid redds (Harrod and Theurer, 2002). This is because the deposited material blocks the pores in the gravel-redd structure, preventing the sufficient exchange of dissolved oxygen and carbon dioxide between the respiring eggs/larvae and the flowing water (Greig et al., 2005; Walling et al., 2003). The presence of SS can also act directly on the free-living fish by clogging and being abrasive to their delicate gill structures (Cordone and Kelley, 1961; Ellis, 1944; Kemp, 1949), and/or stressing the fish and suppressing their immune system, leading to increased susceptibility to disease and osmotic dysfunction (Ellis, 1981; Redding and Schreck, 1983; Redding et al., 1987). Salmonid fish can also be affected by SS interfering with their natural movement and migration (Bisson and Bilby, 1982; Whitman et al., 1982).

It is worth noting, whilst salmonid fish are regarded as being particularly sensitive to SS, cyprinid fish (including carp, barbell, tench, rudd) on the other hand, are somewhat more tolerant to higher levels of SS (Alabaster, 1972; Cordone and Kelley, 1961). It is also worth noting that whilst fish are known to respond to SS fluxes, the fish themselves can also cause fluxes of SS through activities such as bioturbation whilst foraging and through excretion of waste products. For example, Matsuzaki et al. (2007) demonstrated that common carp (Cyprinus carpio, L.) could have a dramatic influence on sediment and nutrient dynamics resulting in a modification of the littoral community structure and triggering a shift from a clear water state dominated by submerged macrophytes, to a turbid water state dominated by phytoplankton. Even salmonids that prefer relatively clear waters carry out activities that may raise in-stream SS concentrations. For example, semelparous species such as the Pacific salmon invest all of their reproductive energy into one season which culminates in them digging redds in which to deposit and fertilise their eggs and then they die in the vicinity of their redd (Petticrew, 2006). The redd-digging procedure mobilises fine material previously stored in the gravel matrices and re-suspends this material in the water column (Chapman, 1998). More importantly, the decomposition of the post-reproductive salmon carcass releases organic material and nutrients into the waterbody (Ben-David et al., 1998; Bilby et al., 1996; Johnston et al., 2004; McConnachie and Petticrew, 2006).