Inorganic nutrients, mainly forms of nitrogen and phosphorus, are some of the most widespread and biologically important substances released into and transported by streams. The large number of sources, as well as multiple reactions and transformations within both the terrestrial and aquatic environments, make these additions very difficult to control and predict. Major sources of nitrogen and phosphorus into stream ecosystems can enter through both point and nonpoint sources. Point source loadings
come from a discrete source such as municipal and industrial wastewater effluent outfalls, and are more easily incorporated in a management strategy since the general location of the source is known. Nonpoint sources are much more difficult to identify and address, and include fertilizer in runoff from cropland, urban lawns, golf courses, waste from animal operations, atmospheric deposition, precipitation, soil erosion, and contaminated groundwater inflow.
Most nitrogen pollution enters as dissolved nitrogen in the forms of nitrate (NO3), and ammonium (NH4), but nitrite (NOT) and dissolved ammonia gas (NH3) can be present in areas with high nutrient pollution. Nutrients can also enter streams in particulate or dissolved organic forms. NH3 and NOT in high concentrations can be toxic to aquatic life. High levels of dissolved NOT and NH3 are rarer because NOT is quickly transformed into NOT through microbial nitrification, and NH3 is quickly transformed into NH^ in neutral to acidic waters. The proportion of NH3 to NH4 is regulated by water pH and temperature with a shift toward NH3 at higher temperature and pH. Once in the stream, further nitrogen transformations occur through processes such as biotic assimilation of NH^ and NOT, nitrification (NH^ to NO 3), and denitrification (NO3 to N2 gas). Phosphate (POT) pollution tends to enter adsorbed to sediments; however, high levels of soluble phosphorus readily available for biotic uptake are common with secondary treated municipal wastewater and runoff from large animal operations.
The most common effect of nutrient addition is an increase in primary (photoautotrophic) production. N, P, or both can limit algae and macrophyte production in streams. Thus, the limiting nutrient for each system should be evaluated, and both N and P should be considered when developing nutrient goals. Increased primary production can have positive and negative effects on the ecosystem. Expansion of the aquatic foodweb base provides a larger energy supply for consumers, which can support a greater biomass at higher trophic levels. Negative effects include a shift in algal species composition and edibility, for example, a dominance of long filamentous Cladophora, or toxin-producing cyanobacteria such as Microcystis. In streams with low discharge or in areas with minimal physical aeration, reduced dissolved oxygen levels can occur due to increased nighttime respiration and algal decomposition.
Metals such as mercury, lead, copper, cadmium, zinc, selenium, and arsenic can be introduced into streams through industrial wastewater discharges, runoff from urban and industrial areas, mining wastes, and landfills. In addition, metals can be transported long distances into remote sections of streams by atmospheric deposition in rain, snow, or dust. Metals can undergo chemical alterations to form more harmful substances once they enter aquatic systems. For example, inorganic mercury, which has a strong affinity for sediments, can be changed to organic methyl-mercury (CH3Hg) by sulfate-reducing bacteria in anoxic sediments. This toxic form of mercury is lipophilic and is the major form of mercury that bioaccumulates in the tissues of aquatic organisms. Biomagnification, the increase in concentration with increasing trophic status, results in top predators becoming highly enriched in mercury.
Metals on aquatic systems tend to accumulate on benthic organic sediments, where they can persist for long periods of time even though water column concentrations are relatively low due to continuous flushing. Fluctuations in the release of metals from the sediment are quite variable and depend on the physical characteristics of sediments (e.g., texture and composition), environmental conditions (e.g., redox state and microbial composition), and individual metal properties.
Metal accumulation in aquatic organisms can have both acute and chronic effects, and negatively affect all components to the ecosystem. For example, copper levels near 2mgl can greatly reduce algal productivity, and the bioaccumulation of mercury, cadmium, and zinc causes reproductive and juvenile developmental problems in macroinvertebrates, mussels, and fish. Metal toxicity can also change with different environmental conditions such as temperature and pH. Much is still unknown about the effect of metals on aquatic systems, including the effect of chronic low doses and the interactive effects of multiple metals.
A reduction in pH can occur in streams with limited buffering capacity (alkalinity). One of the main causes of anthropogenic acidity is acid precipitation. Nitric and sulfuric acids from coal and other fossil fuel combustion such as automobiles' exhaust form within clouds and are deposited onto the watershed with rain and snow. Although streams located in regions with significant industrial, urban, or mining influences are at a higher risk, acidic deposition can be transported long distances in the atmosphere and deposited in pristine watersheds that are otherwise unaffected by humans. pH-lowering acids also often enter streams through industrial waste-water discharges.
pH regulates many biogeochemical processes within a stream. For example, it regulates the proportion of NH3 to NH4, the solubility of potentially toxic metals such as aluminum, and microbial decomposition rates. Increased acidity can reduce the diversity of every biological component from microbes to fish, and especially harm pH-sensitive species such as invertebrates with shells composed of calcium carbonate as well as salmonid fishes.
Salinity pollution in streams can be due to the leaching of salts from soils, or caused by runoff of road salt used in cold, snowy areas. Dryland salinity occurs when a reduction of natural vegetation allows more rainfall to penetrate deeper into the soil and bring up excess salts to surface waters. Irrigation salinity occurs through the same process but the role of rainfall is replaced by irrigation water. Excess salinity in streams causes increased channel erosion due to a breakdown of the soil structure, as well as an increase in salt-tolerant species.
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