FIGURE 4.9 Relationship between aluminum concentrations (measured as inorganic monomeric Aluminum, AL) and pH for streams of northeastern United States. Note the wide range in AL at low pH values. (Reproduced from Wigington et al. 1996.)

In addition to the mobilization of toxic metals, there are numerous other pathways by which acidity is detrimental to the stream biota. Direct physiological effects of acidity are implicated by field and laboratory demonstration of increased mortality or failure of eggs to develop as pH is reduced (Willoughby and Mappin 1988). Inability to regulate ions apparently is responsible, including loss of body sodium and failure to obtain sufficient calcium from surrounding waters (0kland and 0kland 1986). Field collections generally show stoneflies and caddisflies to tolerate waters of lower pH than do mayflies and some dipterans. Hall et al. (1987) speculate that differences in life cycle and respiratory style of these groups account for their differential susceptibility to acid stress.

Episodic pulses of acid water during snowmelt or rainstorms appear to be more influential than stream chemistry during low flow periods. Lepori et al. (2003) were able to differentiate between streams that became acidic during snowmelt (pH reduced to 5) versus well buffered streams (pH remained above 6.6) in an acid-sensitive Alpine area of Switzerland. The streams that were sensitive to acid episodes had different invertebrate assemblages from well-buffered sites and soft-water, stable streams. Episodes of acidification and elevated aluminum concentrations restricted stream fishes from sites in the northeastern United States that had suitable chemical conditions (pH >6 and inorganic Al < 60 |g L_1) at low flow (Baker et al. 1996). Abundances of the brook trout Salvelinus fontinalis were reduced and the blacknose dace (Rhinichthys atratulus) and sculpin (Cottus bairdi and C. cognatus) were eliminated from streams that episodically experienced pH <5 and inorganic Al > 100-200 |g L_1. Behavioral avoidance is one cause of declines, and offers the possibility of subsequent recolonization if alkaline refuge areas are available. Baker et al. suggest that lower mobility in sculpins relative to brook trout may explain why the former were eliminated from acid-pulsed streams. The availability and spatial location of refuge streams of moderate to high alkalinity influenced brook trout population structure in streams of the central Appalachian Mountains of West Virginia. Spawning and recruitment occurred primarily in small tributaries with alkalinity above 10 mg L_1, whereas large adults apparently dispersed throughout the catchment (Petty et al. 2005).

Ecosystem processes also are affected by stream acidification. Inputs of autumn-shed leaves are an important energy supply to woodland streams, and breakdown rates respond to a number of environmental variables (Chapter 7). Breakdown rates of beech leaves Fagus sylva-tica varied more than 20-fold between the most acidified and circumneutral sites in 25 woodland headwater streams along an acidification gradient in the Vosges Mountains, France (Dangles et al. 2004a). Microbial factors associated with decaying leaves, particularly microbial respiration, declined with increasing stream acidity, as did the number of taxa. In some instances, however, ecosystem processes may be relatively unaffected due to species substitutions. Here again, evidence indicates that naturally acidic streams differ from anthropogenically acidified systems. Neither the number of taxa nor the rate of leaf breakdown were markedly depressed even at a pH of 4.0 in naturally acidic streams of northern Sweden (Dangles et al. 2004b). Because these naturally acidic Swedish streams contain a unique fauna, they could be adapted to those conditions. In addition, metal toxicity likely is less important in naturally brown- and blackwater streams owing to the chelating abilities of humic acids, which bind metal ions mobilized at low pH (Winterbourn and Collier 1987, Collier et al. 1990).

Addition of lime to neutralize acid conditions is widely practiced. The River Auda in Norway had lost its anadromous salmon and sensitive mayflies due to anthropogenic acidification when liming commenced in 1985. Within 2 years sensitive mayflies had returned, and additional macroinvertebrates appeared over the following 5-plus years. However, in some cases liming has not been sufficient to offset the effects of episodic acidification. In three acidified Welsh streams that were evaluated for 10 years following the liming of their catchments, pH increased to above six and the number of macroinvertebrate species increased, but relatively few acid-sensitive species recovered (Ormerod and Edwards 2002). The occasional appearance but limited persistence of acid-sensitive taxa in limed streams led the authors to suggest that episodes of low pH continued to affect acid-sensitive taxa even after liming. Whether it makes sense to add lime to naturally acidic streams adds a further complication. In Sweden, approximately US $25 million has been spent since 1991 to lime some 8,000 lakes and 12,000 km of streams to restore their condition, and as Dangles et al. (2004b) point out, expending funds to lime naturally acidic systems may not be wise management.

4.5 Summary

The constituents of river water include suspended inorganic matter, dissolved major ions, dissolved nutrients, suspended and dissolved organic matter, gases, and trace metals. The dissolved gases of importance are oxygen and CO2. Exchange with the atmosphere maintains the concentrations of both at close to the equilibrium determined by temperature and atmospheric partial pressure, especially in streams that are small and turbulent. Photosynthetic activity in highly productive settings can elevate oxygen to supersaturated levels and result in strong fluctuations between day and night. Respiration has the opposite effect, reducing oxygen and elevating CO2. High levels of organic waste can reduce oxygen concentrations below life-sustaining levels, and CO2 can be elevated from ground-water inputs or biological activity.

Many factors influence the composition of river water, and as a consequence it is highly variable in its chemical composition. The concentration of the dissolved major ions (Ca2+, Na+, Mg2+, K+, HCO-, SO2-, Cl ) is roughly 120 mg L-1 on a world average. However, river water is highly variable, ranging from a few milligrams per liter where rainwater collects in catchments of very hard rocks to some thousands of milligrams per liter in arid areas.

Variation from place to place is determined mainly by the type of rocks available for weathering, by the amount of precipitation, and by the composition of rain, which in turn is influenced by proximity to the sea. The concentration of TDS is roughly twice as great in rivers draining sedimentary terrain compared to igneous and metamorphic terrains, due to differential resistance of rocks to weathering. Areas of high rainfall and surface water runoff usually have less concentrated streamwater chemistry compared to arid areas where evaporation is greater and dilution is less. Precipitation inputs typically are of lesser importance to streamwater chemistry, except in areas of very hard rocks and high surface runoff. Human-generated pollutants enter river water via precipitation and dry deposition, by stormwater transport of fertilizers, road salt, etc., as well as by direct disposal.

River chemistry changes temporally under the multiple influences of seasonal changes in discharge regime, precipitation inputs, and biological activity. Groundwater typically is both more concentrated and less variable than surface waters, because of its longer association with rocks. In undisturbed catchments some ions are remarkably constant across discharge fluctuations spanning several orders of magnitude. However, because rainfall increases the surface water contribution, ion concentrations often are diluted by increases in flow.

Natural waters contain a solution of CO2, carbonic acid and bicarbonate and carbonate ions in an equilibrium that serves as the major determinant of the acidity-alkalinity balance of fresh waters. Fresh waters can vary widely in acidity and alkalinity, and extreme pH values (much below 5 or above 9) are harmful to most organisms. The bicarbonate buffer system, consisting of the CO2-HCO--CO^- equilibrium, provides the buffering capacity that is critical to the health of the freshwater biota.

Although fresh water is highly variable in its chemical composition, and rivers more so than lakes, the biological importance of such variation is mainly evident at the extremes, and where human-generated pollutants are substantial. Water of very low ionic concentration appears to support a reduced fauna, particularly of crustaceans and mollusks. The number of species commonly increases with hardness, and many taxa are distinctly "soft-water" or "hard-water" forms. Anthropogenic additions of strong inorganic acids set off a number of changes in water chemistry, and at a pH much below 5.0, the biological consequences are serious.

Chapter five

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