Herbicides, insecticides, and fungicides are frequently associated with agricultural runoff, but can also come from suburban areas and golf courses due to an increased application rate per area. The enormous variety of pesticides released into the environment (over 600 different compounds in the United States alone are in agricultural use) make this pollutant difficult to manage. Ecological effects of pesticides can be substantial and occur at all trophic levels. For example, atrazine, which is water soluble, harms photosynthetic organisms at very low concentrations (~2 mgl" ), while dichlorodiphenyltrichloroethane (DDT), an endocrine disrupter, and chlordane, a carcinogen, can bioaccumulate in fish tissues, disrupting biochemical signals, increasing organ damage, and reducing reproductive success.
Many other organic chemicals used in industry, consumer products, and created by fossil fuel combustion also have significant detrimental effects on ecological stability. Polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) are introduced through industrial and municipal wastewater, urban runoff, groundwater intrusion, and atmospheric deposition. In addition, petroleum-based pollutants such as oil and gasoline enter through urban and road runoff, and leaking underground storage tanks. Many of these chemicals are closely associated with the sediments and can persist for many years within the ecosystem.
Endocrine disrupters have been recognized as a threat to aquatic ecosystems within the last few decades. These chemicals can mimic natural biologically active chemicals. Antibiotics and hormones, for example, estrogen and its mimics (ecoestrogens), are becoming more prevalent in stream ecosystems. Presently, the two main sources of lotic pharmaceuticals are animal feedlot runoff and municipal wastewater treatment effluent. The complete effects of pharmaceuticals on aquatic biota are not known, but have been linked to reduced fertility in reptiles, mollusks, fish, and mammals, and cause male organisms to exhibit feminine traits.
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