Turbidity . This is a measure of the amount of suspended solids in the 

 water column. These solids are usually fine organic or inorganic materials. 

 They are essential to biological processes as sources of nutrients but in 

 excess can cause serious problems. Extreme suspended sediment loads may be of 

 natural origin or due to human activities (dredging or spoil disposal, 

 construction, agricultural, or timberland runoff). High levels of solids, as 

 they settle, are a particular hazard to demersal eggs and the integrity of a 

 spawning area in general. The main effects are direct and acute for eggs, 

 young, and adults. The two major effects are interference with oxygen 

 exchange (smothering) or clogging of fish gills (Clayton et al. 1976). The 

 extent of harm due to settling of suspended solids depends on the type of 

 material, time of year, and the species involved. Clay particles are apt to 

 form a hard, compact crust upon settling. Organic materials, such as wood 

 pulp fibers, can form an impenetrable mat over the bottom (Bell 1973). This 

 can render a spawning area unusable and suffocate invertebrates (fish food) . 

 Silt may also contain toxic residues (from agricultural or industrial wastes), 

 which may be lethal to local fishes or destroy fish food organisms. Excessive 

 turbidity from organic wastes may seriously reduce the availability of oxygen 

 through microbial action. Turbidity may also be caused by living material, 

 such as plankton, usually in concentrations greater than . 1% by volume (Bell 

 1973). 



Suspended sediments in excess reduce the penetration of light into the water 

 column which may reduce the populations of submerged vascular plants, 

 phytoplankton, and algae. This decreases primary productivity and affects 

 available food supply in the system. High turbidity is most common in 

 sluggish waters near shore and in partly enclosed areas. In general the less 

 mobile (demersal) fishes have a higher tolerance for turbid water but are also 

 more heavily exposed as the sediment settles (Clayton et al. 1976). 



Dissolved oxygen . Most fishes are adversely affected by reduced levels 

 of dissolved oxygen (DO). Massive fish kills have been recorded as a result 

 of severe oxygen depletion. Fish-kill data are not systematically maintained. 

 Active, migratory fishes like the blueback herring, alewife, and menhaden, 

 have high oxygen demands and are particularly sensitive to dissolved oxygen 

 sags. For most cold water fishes (e.g., salmon and trout) it is desirable 

 that DO concentrations be at or near saturation levels (Bell 1973). Certain 

 human activities increase the oxygen demand in aquatic systems. Additions of 

 organic wastes, nutrients, and sediments increase the levels of microbial 

 decomposition, which consumes oxygen. Dissolved oxygen reductions are more 

 often a problem of sluggish, impounded, or enclosed waters. Temperature also 

 affects DO levels: higher temperatures decrease DO levels. 



Pathogens . A wide variety of pathogens, in the form of bacteria, 

 protozoa, viruses, and fungi, may enter aquatic systems from municipal waste 

 disposal activities or accidental spills. Chronic disturbances, such as 

 municipal sewage, may permit the population to remain in place but cause 

 morbidity, such as fin rot or other diseases (Clayton et al. 1976). 



Toxicants . Heavy metals, hydrocarbons, biocides, and industrial 

 chemicals are particularly hazardous and lingering toxicants. Effluents from 

 industrial plants and mines may contain heavy metals (e.g., copper, mercury, 

 cadmium, selenium, silver, mercury, lead, zinc, and iron) in concentrations 

 that are lethal to fishes or their food organisms. These elements can 



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