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What are Suspended and Bedded Sediments (SABS)?

The following discussion is excerpted from USEPA 2003. "Developing Water Quality Criteria for Suspended and Bedded Sediments (SABS): Potential Approaches (Draft, August 2003)."

Suspended and bedded sediments (SABS) are defined by EPA as particulate organic and inorganic matter that suspend in or are carried by the water, and/or accumulate in a loose, unconsolidated form on the bottom of natural water bodies. This includes the frequently used terms of clean sediment, suspended sediment, total suspended solids, bedload, turbidity, or in common terms, dirt, soils or eroded materials.

EPA's definition of SABS also includes organic solids such as algal material, particulate leaf detritus and other organic material. This initiative on SABS criteria intentionally does not look at contamination in sediments, another significant environmental issue, Rather, EPA has dealt directly with the toxicity of chemicals in sediments through its work on Equilibrium Partitioning-Derived Sediment Benchmarks. EPA does recognize however, that managing SABS in the aquatic environment will have either direct or indirect consequences on the amount of contaminated sediments and may need to further examine these relationships in future efforts.

SABS can be further defined in regards to particle size which are related to the mode of action in the aquatic environment. SABS can be broken into two fractions based on size – fine sediment and coarse sediment. Fine sediment is typically considered to consist mostly of particles smaller than 0.85 mm and coarse sediment is defined as greater than 9.5 mm. Particles less than 0.063 mm (silt and clay) remain suspended in flowing water and are largely the cause of turbidity (IDEQ, 2003).

What are the Impacts of SABS?

SABS are a unique water quality problem when compared to toxic chemicals, in that suspended solids and bedded sediments (including the organic fraction) occur naturally in water bodies in natural or background amounts and are essential to the ecological function of a water body. Suspended solids and sediments transport nutrients, detritus, and other organic matter in natural amounts which are critical to the health of a water body. Suspended solids and sediment in natural quantities also replenish sediment bedloads and create valuable micro-habitats, such as pools and sand bars. Therefore, a basic premise for managing suspended and bedded sediments in water bodies to protect aquatic life uses may be the need to maintain natural or background levels of SABS in water bodies.

However, SABS in excessive amounts constitute a major ecosystem stressor. According to the EPA National Water Quality Inventory - 2000 Report, excessive sediment was the leading cause of impairment of the Nation's waters. The highest frequency of impairment was reported for rivers and streams, followed by lakes, reservoirs, ponds, and estuaries. In 1998, approximately 40% of assessed river miles in the U.S. were impaired or threatened from excessive SABS.

Suspended and bedded sediments have two major avenues of effect in aquatic systems; 1) direct effects on biota, and 2) direct effects on physical habitat, which result in effects on biota. In considering impacts, suspended sediment is the portion of SABS that exert a negative impact via suspension in the water column, such as shading of submerged macrophytes. Bedded sediments are those sediments that have a negative impact when they settle out on the bottom of the water body and smother spawning beds and other habitats. (The following discussion is excerpted from Jha, 2003.)

In streams and rivers, fine inorganic sediments, especially silts and clays, affect the habitat for macroinvertebrates and fish spawning, as well as fish rearing and feeding behavior. Larger sands and gravels can scour diatoms and bury invertebrates, whereas suspended sediment affects plant photosynthesis light availability and visual capacity of animals. A potential problem with suspended sediment in reservoirs, coastal wetlands, estuaries, and near-shore zones is decreased light penetration, which often causes aquatic macrophytes to be replaced with algal communities, with resulting changes in both the invertebrate and fish communities. Increased sedimentation also may functionally shift the fish community from generalist feeding and spawning guilds to more bottom-oriented, silt tolerant fishes.

Sediment starvation caused by structures such as dams and levees is also a problem in some ecosystems, ranging from the loss of native fish species and native riparian ecosystem structure in many dammed Western rivers (e.g., Colorado River, Platte River, Missouri River), to the subsidence and loss of wetlands (e.g., Mississippi Delta in Louisiana). Effects of excess suspended and bedded sediments on habitat structure include changes in refugia for biota (e.g., changes in macrophyte communities), increased fines (and embeddedness) and scouring in streams, aggradation and destabilization of stream channels, and filling in of wetlands and other receiving waters. Sediment starvation can lead to scouring and removal of riparian and pool habitat, and subsidence and disappearance of wetlands and lowering of the water table.

Increased turbidity and concomitant changes in light regime may be considered to be aspects of altered habitat. Indirect effects on biota will occur as the fish, invertebrates, algae, amphibians, and birds that rely upon aquatic habitat for reproduction, feeding, and cover are adversely affected by habitat loss or degradation. Sea grasses and other submerged aquatic vegetation (SAV) are considered "keystone" species in temperate and tropical estuaries and coastal areas. These flora have a variety of beneficial attributes including providing food and shelter for many aquatic and terrestrial species. There has been a worldwide decline in sea grasses including dramatic regional losses in the Gulf of Mexico. When studied in detail, sea grass declines have always been linked to nutrient enrichment as the most important cause, but suspended sediment remains a suspected secondary cause in several cases.

SABS also affect fish populations. Three major effects of SABS on fishes include: 1) behavioral effects, such as inability to see prey or feed normally; 2) physiological effects, such as gill clogging; and 3) effects due to sediment deposition, such as burial and suffocation of eggs and larvae. Physiological effects of sedimentation can result in impaired growth, histological changes to gill tissue, alterations in blood chemistry, and an overall decrease in health and resistance to parasitism and disease. Lower doses or shorter duration of SABS will have transitory effects, while higher doses for longer periods can result in more lasting and severe effects.

Fish can also swallow large quantities of sediment, causing illness, reduced growth and eventual death, depending on other contaminants that may be adsorbed to the sediment. Some other physiological changes include; release of stress hormones (i.e., cortisol and epinephrine), a compensatory response to a decrease in gill function, and clogging gill mucus causing asphyxiation and traumatization of gill tissue. The severity of damage appears to be related to the dose of exposure as well as the size and angularity of the particles involved.

Certain fish populations may be severely impacted in their ability to feed by even small increases in SABS concentrations because of increased turbidity. Fish that need to see their prey to feed suffer from reduced visibility in turbid water and may be restricted from otherwise satisfactory habitat. Some fishes are able to hunt better as SABS concentrations increase up to a point because of increased contrast between the prey and the surrounding water.

Many species of fish may relocate when sediment load is increased, because fish can readily disperse. Other behavior responses include an increased frequency of the cough reflex and temporary disruption of territoriality. The severity of the behavioral response is associated with the timing of disturbance, the level of stress, decreased energy reserves, phagocytes, metabolic depletion, seasonal variation, and alteration of the habitat.

Severity of effect caused by suspended sediments is a function of many factors, which, in addition to sediment concentration, duration, particle size, and life history stage, may include temperature, physical and chemical characteristics of the particles, associated toxicants, acclimatization, other stressors, and interactions of these factors. Suspended sedimentation effects have been scored on a qualitative scale as "severity of ill effect" (SEV), that include everything from "no behavioral effects" (lowest on the scale) to behavioral effects (low on the scale); to sublethal effects (higher on the scale); to lethal effects (highest on the scale). According to Griffiths and Walton (1978), the upper tolerance level for suspended sediment is between 80-100 mg/l for fish, and as low as 10-15 mg/l for bottom invertebrates.

Many species of fish and macroinvertebrates use the interstitial spaces at the bottom of streams to lay their eggs. Reproductive success is severely affected by sediment deposition particularly in benthic spawning fishes. The primary mechanisms of action are through increased egg mortality, reduced egg hatch and a reduction in the successful emergence of larvae . The cause of egg survival rates and egg death are due to reduced permeability of the streambed and from burial by settled particles. Thin coverings (a few mm) of fine particles are believed to disrupt the normal exchange of gases and metabolic wastes between the egg and water. Sediment deposition has caused a 94% reduction in numbers and standing crop biomass in large game fish, because of increased vulnerability of their eggs to predation in gravel and small rubble, reduction in oxygen supply to eggs, and increased embryo mortality. It can also cause reduced larval survival because of armoring (crusting) of the sediment surface, which traps the larvae. Differences in sensitivity, egg mortality effects, early life stages (i.e., eggs, larvae) and magnitude of impact upon fish population are associated with amount of elevated sediment loads, size of the sediment particles involved, seasonal variation, and rates of sediment deposition. Even if intergravel flow is adequate for embryo development, sand that plugs the interstitial areas near the surface of the stream bed can prevent alevins from emerging from the gravel. For example, emergence success of cutthroat trout was reduced from 76% to 4% when fine sediment was added to redds (Weaver and Fraley, 1993).

There are also detrimental effects of SABS on aquatic invertebrates. SABSs impact the density, diversity, and structure of invertebrate communities. High and sustained levels of sediment may cause permanent alterations in community structure including diversity, density, biomass, growth, rates of reproduction, and mortality. Direct effects on invertebrates include abrasion, clogging of filtration mechanisms – thereby interfering with ingestion and respiration – and in extreme cases, smothering and burial, resulting in mortality. Indirect effects are primarily from light attenuation leading to changes in feeding efficiency, behavior (i.e., drift and avoidance), and alteration of habitat from changes in substrate composition, affecting the distribution of infaunal and epibenthic species. Three major relationships between benthic invertebrate communities and sediment deposition in streams have been reported, including correlation between abundance of micro-invertebrates and substrate particle size, embeddedness of substrate and loss of interstitial space, and change in species composition with change in substrate composition.

Sedimentation alters the structure of the invertebrate community by causing a shift in proportions from one functional group to another. Sedimentation can lead to embeddedness, which blocks critical macroinvertebrate habitat by filling in the interstices of the cobble and other hard substrate on the stream bottom. As deposited sediment increases, changes in invertebrate community structure and diversity occur.

Invertebrate drift is directly affected by increased suspended sediment load in freshwater streams. These changes generally involve a shift in dominance from ephemeroptera, plecoptera and trichoptera (EPT) taxa to other less pollution-sensitive species that can cope with sedimentation. Increases in sediment deposition that affect the growth, abundance, or species composition of the periphytic (attached) algal community will also have an effect on the macroinvertebrate grazers that feed predominantly on periphyton. For example in the Chattooga River watershed, accelerated sedimentation was identified as the leading cause of habitat loss and reduction in bed form diversity (Pruitt et. al., 2001). A significant correlation was observed between aquatic ecology and normalized total suspended solids (TSS) data. Effects on aquatic individuals, populations, and communities are expressed through alterations in local food webs and habitat. When sedimentation exceeds certain thresholds, ensuing effects will likely involve decline of the existing aquatic invertebrate community and subsequent colonization by pioneer species.

SABS also have a negative affect on the survival of freshwater mussels. Increased levels of SABS impair ingestion rates of freshwater mussels in laboratory studies. However, it has been suggested that survival may be species-specific. Mussels compensate for increased levels of suspended sediment by increasing filtration rates, increasing the proportion of filtered material that is rejected, and increasing the selection efficiency for organic matter. Species-specific responses to SABS are adaptations to sediment levels in the local environment, such that species inhabiting turbid environments are better able to select between organic and inorganic particles. Many of the endangered freshwater mussel species have evolved in fast flowing streams with historically low levels of suspended sediment. Such species may not be able to actively select between organic and inorganic particles in the water column. Therefore, even low levels of sediment may reduce feeding and, in turn, reduce growth and reproduction.

Corals differ greatly in their ability to resist SABS, with most species being highly intolerant of even small amounts while a minority are able to tolerate extremely embedded sediment conditions, and a few are even able to live directly in sedimented bottoms. Excessive sedimentation can adversely affect the structure and function of the coral reef ecosystem by altering physical and biological processes through a variety of mechanisms. These all require expenditure of metabolic energy and when sedimentation is excessive they eventually reach the point where they can no longer spare the energy to keep themselves clean, and the affected tissue dies back. Excess SABS cause reduced growth rates, temporary bleaching, and complex food web-associated effects, with SABS killing not only corals but other reef dwelling organisms. Coral larvae will not settle and establish themselves in shifting sediments. Increases in sedimentation rates alter the distribution of corals and their associated reef constituents by influencing the ability of coral larvae to settle and survive.

Changes in the supply rate of sediment causes drastic changes in aquatic, wetland, and riparian vegetation. Undesirable changes in vegetation can be induced by both decreases and increases in SABS from natural levels. For example, in the Platte and Missouri Rivers, decreases in both sediment supply and scouring flows have resulted in the growth of stable riparian forests (including many exotic eastern tree species), and the loss of sandbar habitat for several wildlife species (e.g., cranes, piping plovers) (Johnson 1994). In the Colorado River, decreased sediment supply (but continuing scouring flow) has resulted in the loss of riparian wetland habitat dependent on sandbars (Stevens 1995). The magnitude and timing of sedimentation may influence structure and recolonization of aquatic plant communities. The effects of reduced primary production on aquatic invertebrates and fishes at higher trophic levels are compounded when SABS settles on remaining macrophytes. The macrophyte quality also is reduced as a food source. The periphyton communities are likely to be most susceptible to the scouring action of suspended particles or burial by sediments. For example, large-scale declines of submerged aquatic vegetation (SAV) in the Chesapeake Bay is directly related to increasing amounts of nutrients and, secondarily, to sediments entering the Bay (Staver et. al., 1996).

Indirect impacts of excess sediment on water quality can occur through its influence on aquatic plant communities, organic exchange substrates, and microbial populations. In environments with high concentrations of SABS, reductions in plant species density, biomass, and diversity throughout a trophic level are translated into reductions in energy input to the next trophic level. Decreases in plant populations may result in decreases in populations of zooplankton, insect abundance and overall biomass, which may initiate reductions in herbivore, omnivore, and predatory fish. SABS deposition may cover microbes or organic matter needed for microbial processes, or alter redox profiles important in the performance of water quality processes.

For other uses of water bodies, excessive SABS can, among other things, affect water clarity and the aesthetic quality of swimming waters, increase pre-filtration efforts and expenses at drinking water purification facilities, and lead to accelerated in-fill of dredged shipping channels, harbors and marinas.

In summary, the current literature suggests SABS are significant contributors to declines in populations of North American aquatic life and can impact other beneficial uses of waters. Improved SABS water quality criteria are needed to properly manage the level of SABS in aquatic ecosystems to minimize or avoid these effects.

References Cited

Griffiths, W., and B. Walton. 1978,
The effects of sedimentation on the aquatic biota. Alberta Oil Sands Environmental Research Program, Report #35.

Idaho DEQ, 2003.
Guide to Selection of Targets for Use in Idaho TMDLs. M. Rowe, D. Essig and B. Jessup. June 2003.

Jha, M. 2003.
Ecological and Toxicological Effects of Suspended and Bedded Sediments on Aquatic Habitats - A Concise Review for Developing Water Quality Criteria for Suspended and Bedded Sediments (SABS). US EPA, Office of Water draft report, August 2003.

Johnson, W.C. 1994.
Woodland Expansion in the Platte River, Nebraska: Patterns and Causes. Ecological Monographs 64: 45-84.

Pruitt B. A., D. L. Melgaard, H. H. Morris, C. Flexner, A. S. Able. 2001.
Chattooga river watershed ecological/sedimentation project; FISC Proceedings, Federal Interagency Sedimentation Conference, Reno, Nevada, March 26-30, 2001.

Staver, L. W., K. W. Staver, and J. C. Stevenson. 1996.
Nutrient Inputs to the Choptank River Estuary: Implications for Watershed Management. Estuaries 19: 342-358.

Stevens, L.E. 1995.
Flow regulation, geomorphology, and Colorado River marsh development in the Grand Canyon, Arizona. Ecological Applications 5: 1025-1039.

Weaver T. M. & Fraley J. F., 1993.
A method to measure emergence success of Westslope cutthroat trout fry from varying substrate compositions in a natural stream channel. N-Am J Fish Man 13: 817-822.

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