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volume, and in more smoothly flowing rivers because of less turbulence. In these circumstances, high naturally occurring biological activity can alter the concentrations of oxygen and CO2, organic pollution can greatly increase respiratory demand for oxygen, and acid precipitation can alter the carbonate buffer system, which influences the concentration of free CO2 in solution.

Respiration and photosynthesis are the two important biological processes that alter the concentration of oxygen and CO2. Oxygen consumption increases with increased loadings of organic matter due to direct chemical reactions and aerobic respiration. Oxygen demand can be high in certain areas and seasons, for example, within accumulations of fallen leaves in woodland streams in autumn, in backwaters with abundant decaying plant matter, and due to anthropogenic sources. In highly productive waters, whole-system photosynthesis results in elevated concentrations of oxygen during the day, while whole-system respiration causes oxygen to decline at night. These diel (24 h) changes in oxygen concentration provide a means of estimating photosynthesis and respiration of the total ecosystem and are discussed in Chapter 6. First applied to productive, slow-moving rivers and lentic waters where diffusion is relatively low and more easily estimated (Odum 1956, Edwards and Owens 1962), recent improvements in measuring diffusion rates and detecting small changes in oxygen concentrations are extending this approach even to small woodland streams (Young and Huryn 1998).

CO2 concentrations in streamwater are influenced not only by atmospheric diffusion and instream metabolism but also by groundwater inflows, which commonly are substantially enriched with CO2 due to soil respiration throughout the catchment. In Walker Branch, Tennessee, streamwater was always supersaturated with respect to the atmosphere and so outgassing occurred at all times (Jones and Mulholland 1998). Free CO2 exceeded the concentration expected for atmospheric equilibration by a factor >10 at the source of a moorland stream in northeastern Scotland, and decreased to near equilibrium at 2 km downstream, indicating the extent of outgassing of soil-derived CO2 (Dawson et al. 1995). In highly productive lowland streams that support luxuriant growths of macrophytes and microbenthic algae, diel shifts in dissolved CO2 can be large (Rebsdorf et al. 1991). Because of the interdependence of CO2 concentration and pH (a measure of acidity, discussed below), midday pH can increase by as much as 0.5 units. In larger rivers receiving a substantial organic load, outgassing of CO2 is unable to compensate for excess CO2 generated by microbial respiration. As a consequence the partial pressure of CO2 (jOCO2) in the water column can exceed that of the atmosphere by as much as 2-5 times, and occasionally by even more (Small and Sutton 1986, Rebsdorf et al. 1991). The CO2 concentration of the Rhine is a good example. Water leaving the source, Lake Constance, in summer is lower than the atmospheric partial pressure due to the productivity of lake phytoplankton. In winter, however, water leaves the lacustrine source at about twice the atmospheric partial pressure. Because organic pollution increases as one proceeds downriver, the pCO2 increases also. High summer temperatures permit high microbial respiration, with the result that the downstream average pCO2 is about 20 times the atmospheric value (Kempe et al. 1991).

The River Thames is a good example of the effect of organic pollution on dissolved oxygen concentrations (Gameson and Wheeler 1977). Human and animal wastes have been a documented source of foulness since at least 1620, and 1858 was known as the "Year of the Great Stink,'' but the volume of untreated human sewage reduced water quality to an all-time low in the mid-1950s. Parts of the Thames around London became anaerobic from microbial respiration driven by organic waste, and sparked pollution-control efforts that led to substantial recovery. The impact of high oxygen demand due to pollution can be exacerbated by high summer temperatures, which reduce the solubility of oxygen in water, and by ice cover, which minimizes diffusion.

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