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Of course, river water will be more concentrated than rainwater simply because of evaporation. Using the world average runoff ratio of 0.35, which means that 35% of precipitation becomes runoff, the concentration of ions in river water should be nearly three times greater than the concentration in rain. Because the true differential is roughly 20-fold, rock weathering, other natural sources, and anthropogenic inputs must account for the majority of dissolved ions in river water (Berner and Berner 1987). Figure 4.1 illustrates this discrepancy for North America. Roughly 10-15% of the calcium, sodium, and chloride in US river water comes from rain, compared to one fourth of the potassium and almost half of the sulfate. In contrast, almost none of the SiO2 or HCO^ comes from rain. This emphasizes the need to examine the origins of each of these major cations and anions in order to understand what influences their concentrations.

River Ecology Structure

FIGURE 4.1 Dissolved ionic concentrations for river (clear bars) and rain water (shaded bars) from North America. Rainwater concentrations are multiplied by 2.6 to correct for evaporation. Cross-hatching shows anthropogenic contribution to river water values. (Reproduced from Berner and Berner 1987.)

FIGURE 4.1 Dissolved ionic concentrations for river (clear bars) and rain water (shaded bars) from North America. Rainwater concentrations are multiplied by 2.6 to correct for evaporation. Cross-hatching shows anthropogenic contribution to river water values. (Reproduced from Berner and Berner 1987.)

Calcium is the most abundant cation in the world's rivers. It originates almost entirely from the weathering of sedimentary carbonate rocks, although pollution and atmospheric inputs constitute small sources. Its concentration (along with magnesium) is used to characterize "soft" versus "hard" waters, which are discussed fully below. Magnesium likewise originates almost entirely from the weathering of rocks, particularly Mg-silicate minerals and dolomite. Atmospheric inputs are minimal, and pollution contributes only slightly.

Sodium is generally found in association with chloride, indicating their common origin. Weathering of NaCl-containing rocks accounts for most of the Na found in river water. However, rainwater inputs from sea salts can contribute significantly, especially near coasts. Pollution, due to domestic sewage, fertilizers, and road salt, is an especially important factor. Berner and Berner (1987) estimate that, worldwide, ~28% of the sodium in rivers is anthropogenic.

Potassium is the least abundant of the major cations in river water, and the least variable. Roughly, 90% originates from the weathering of silicate materials, especially potassium, feldspar, and mica. Concentrations thus vary with the underlying geology, and also increase substantially from polar latitudes toward the tropics, apparently due to more complete chemical weathering at higher temperatures. Silica is used by diatoms in the formation of their external cell wall and can on occasion limit algal productivity.

Bicarbonate (HCO^) ultimately derives almost entirely from the weathering of carbonate minerals. However, the immediate source of the majority of bicarbonate is CO2 dissolved in soil and groundwater, which is produced by bacterial decomposition of organic matter, and derives in turn from the photosynthetic fixation of atmospheric CO2. Bicarbonate is a biologically important anion. High concentrations are reflected in measures of alkalinity and are indicative of fertile waters. The carbonate buffer system, alkalinity, and hardness are interrelated, as will be discussed more fully below. Anthropogenic increases in acidity, caused by acid precipitation or mining, reduce bicarbonate levels through the formation of H2CO3.

The origins of chloride are essentially the same as sodium: mostly from weathering of rocks, but inputs of sea salts and pollution including the application of road salts to reduce ice can be important. Chloride is chemically and biologically unreactive, and so is useful as a tracer in nutrient release experiments.

Sulfate has many sources, including the weathering of sedimentary rocks and pollution from fertilizers, wastes, mining activities, and especially the burning of fossil fuels; biogenically derived sulfate in rain and volcanic activity are additional inputs. In areas of sulfuric acid rain, such as Hubbard Brook, New Hampshire, sulfate concentrations are high relative to overall ionic concentrations (Likens and Bormann 1995). Sulfate and bicarbonate concentrations tend to be inversely correlated in streamwater, especially in low alkalinity areas.

The key nutrients N and P are discussed more fully in Chapter 11, where their forms (nitrate, ammonium, phosphate, etc) and influence on ecosystem processes are discussed in detail. Dissolved inorganic P and N are primary nutrients that limit plant and microbial production, and cycle rapidly between their inorganic forms and their incorporation into the food web.

Lastly, the concentration of hydrogen ions is very important both chemically and biologically, because they determine the acidity of water. This is expressed as pH, and is a logarithmic scale in which a tenfold change in hydrogen ion activity corresponds to a change of 1 pH unit. A pH of 7 is neutral, higher values are alkaline, and lower values are acidic.

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