Herbicides, insecticides, and fungicides are frequently asso ciated with agricultural runoff, but can also come from suburban areas and golf courses due to an increased applica tion rate per area. The enormous variety of pesticides released into the environment (over 600 different com pounds 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, con sumer products, and created by fossil fuel combustion also have significant detrimental effects on ecological stability. Polychlorinated biphenyls (PCBs) and polycyclic aro matic 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 gaso line 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 chemi cals. 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 muni cipal 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.
The reduction of species diversity is a common result of stream pollution. Sensitive species that cannot tolerate changing conditions are replaced by more tolerant spe cies. This usually leads to a community with lower complexity and reduces energy flow through the system. An example of this occurred worldwide before the use of secondary treatment in municipal wastewater. Below sewage outfalls, streams would commonly be void of dissolved oxygen due to the high biological and chemical oxygen demand from organic matter decomposition. With increasing distance downstream, water conditions steadily improved due to instream biotic and abiotic processes. In the areas that were anoxic or had very low dissolved oxygen immediately below the outfall, sewage fungus, cyanobacteria, and tolerant invertebrates (such as Tubifex) dominate. As conditions improve, these organ isms are replaced by less and less tolerant species until the algal, invertebrate, and fish community is similar to that immediately above the outfall point.
Another common change in the biotic community that accompanies human development is the alteration of native species composition. Species are often introduced purposely, such as fish stockings for sport, or accidentally through ship ballast water exchange. Changes in environmental conditions such as temperature or nutrient availability allow non natives that are better suited to new conditions to displace native species. However, even when basic environmental conditions remain, non natives may be better competitors for available resources and thus dominate communities once introduced. Reduction or alteration in higher trophic levels such as fish is the most noticeable, but changes in the microbial community occur more rapidly. Although most introduced species do not become established, when they do, non native species can alter nutrient cycling and retention, change food web lin kages, and possibly eliminate vulnerable native species. Ecosystem integrity is compromised when species with important functional roles are displaced without the avail ability ofother similar species to take over the lost role (i.e., there is low functional redundancy within the system).
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