Water temperature and light affect both biological and chemical processes in aquatic systems. Although natural seasonal variation in these parameters can be great, resi dent aquatic organisms have evolved to deal with these regular fluctuations, by regulating metabolism, annual reproductive cycles, and changes in pigment concentra tion. Thermal deviations from this natural variation can occur locally due to an input of industrial or municipal wastewater, power plant effluent, increased solar input from the removal of riparian canopy shade, or reduced from input of groundwater. On both local and regional scales, hypolemnetic release dams can lower stream water temperature drastically by releasing colder water from the bottom of a reservoir, or increase it by releasing warmer water from the epilimnion.
Increased water temperature can increase the rate of metabolic activity within a system, leading to faster microbial nutrient cycling, and altered reproductive suc cess and juvenile development of aquatic plants, macroinvertebrates, mussels, and fish. Light availability in streams can be lowered by excessive sediment loads caused by dredging, watershed erosion from agri cultural practices, deforestation, or urban development. Depending on stream size, the loss of riparian cover can also increase light availability to the channel. Light avail ability and primary productivity are directly linked within aquatic system.
Inorganic nutrients, mainly forms of nitrogen and phos phorus, 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 indus trial 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 (NH^), but nitrite (NO2) 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 NO2 in high concentrations can be toxic to aquatic life. High levels of dissolved NO2 and NH3 are rarer because NO2 is quickly transformed into NO3 through microbial nitrification, and NH3 is quickly trans formed into NH| in neutral to acidic waters. The proportion of NH3 to NH^ is regulated by water pH and temperature with a shift toward NH3 at higher tem perature and pH. Once in the stream, further nitrogen transformations occur through processes such as biotic assimilation of NH4 and NO3, nitrification (NHÍ to NO3 ), and denitrification (NO3 to N2 gas). Phosphate (PO4) pollution tends to enter adsorbed to sediments;
however, high levels of soluble phosphorus readily avail able 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 con sidered 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 composi tion 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.
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