North American water quality has generally increased in recent decades, thanks to restoration programs and the creation of anti-pollution laws like the Clean Water Act. However, these actions have not come close to eliminating water pollution, which remains a serious environmental issue in both marine and fresh waters. Pollution disrupts the balance of aquatic ecosystems, causing changes in species composition and biomass that can be used to monitor water quality.
Sewage and Sediment
Sewage overflow, farm effluent, and eroded sediment are common water pollutants. Fecal matter contains high levels of pathogenic micro-organisms, such as Giardia, Escherichia coli and Campylobacter, as well as organic matter. Pathogenic microorganisms can infect both humans and aquatic organisms, like fish and shellfish. Organic matter and sediment become suspended in the water column where they can clog the gills of fish and invertebrates. Suspended solids diffuse light, making water clarity poor. This can make it difficult for fish and mammals to see their prey, and also impacts aquatic plants, which require light for photosynthesis. Organic matter and sediment eventually falls to the bottom, where it depletes oxygen by blocking spaces between gravel and sand, burying sedentary invertebrates, and destroying habitat for other bottom-dwelling animals.
Nutrients are present in leachate — waste water that drains into surface and underground water through soil — and runoff from urban and agricultural land. This includes treated waste water and industrial emissions. Water quality problems occur when nutrients are present in concentrations that promote the growth of plants and cyanobacteria, also known as blue-green algae. Excess growth of these organisms chokes waterways and estuaries, altering flow regimes and reducing water clarity. Some waters are prone to large algal blooms during warm months. A few types of algal blooms produce toxins that can be harmful to terrestrial and aquatic organisms. Toxins produced by blooms have been known to kill livestock, dogs, fish and marine mammals. Toxic or not, blooms of algae or plants can kill aquatic organisms when they die because the decay process depletes dissolved oxygen in the water.
Metals and Organic Chemicals
Urban, industrial and agricultural runoff often contains metals and organic chemicals that are toxic to aquatic life. Metals include selenium, copper, nickel, mercury and lead. Organic chemicals include the by-products of manufacturing, pesticides, dry-cleaning solvents and chemicals found in many household products. Aquatic plants readily take up metals and organic compounds, which then accumulate in plant tissue and inhibit growth. Metals and organics disrupt a wide range of biological systems within the bodies of aquatic animals, the effects of which can be sub-lethal or lethal. The threshold at which toxins become lethal varies between organisms. Sub-lethal effects of aquatic pollutants include reduced growth, abnormal development, reduced reproductive success, immune deficiency and skin necrosis. Metals and organics can move up food chains, accumulating in organisms at each position in the chain and disrupting the balance of whole ecosystems.
Certain species of plants and animals are useful as bioindicators of water pollution. Bioindicators provide information about water quality and the biological integrity of a given aquatic community. Good bioindicators possess one or more of the following qualities: predictable response to pollution, easy to collect and measure, moderate sensitivity to pollution, readily accumulate contaminants, or respond rapidly to pollution. Algae, diatoms, bacteria, fish and aquatic invertebrates all make good water quality bioindicators. A combination of bioindicators is usually used for bioassessment, as this provides better information about pollutant exposure and the extent of ecological damage.