Variability in ionic concentrations

The chemistry of fresh waters is quite variable, rivers usually more so than lakes. Natural spatial variation is determined mainly by the type of rocks available for weathering, how wet or dry the climate is, and by the composition of rain, which in turn is influenced by proximity to the sea. The ionic concentration of rivers draining igneous and metamorphic terrains is roughly half that of rivers draining sedimentary terrain, because of the differential resistance of rocks to weathering. All of these factors provide the opportunity for substantial local variation in river chemistry. As a consequence the concentration of total dissolved ions can vary considerably amongst the headwater branches of a large drainage. However, these heterogeneities tend to average out, and concentrations tend to increase, as one proceeds downstream (Livingstone 1963).

Although small streams in the same region often are chemically similar, they also can differ markedly. Within a series of small streams in southwest England, Walling and Webb (1975) reported a concentration range of total ions from 25 to 650 mg L_1, resulting from small-scale shifts between igneous and sedimentary rocks. Small streams draining a volcanic landscape in central Costa Rica exhibited pronounced differences in solute concentrations depending on geology, soil types, and elevation (Pringle 1991). Phosphorus, several major cations and anions, and trace elements were high in headwater streams draining younger volcanic landscapes and much lower in streams draining older lava flows. The blackwater Rio Negro and whitewater Solimóes (the Amazon mainstem) dramatically illustrate the chemical differences between tributaries draining distinctive landscapes. The Rio Negro drains well-weathered, crystalline rock and is much lower in ions and much higher in organic acids, whereas the Amazon mainstem drains the comparatively young Andes and has a much higher dissolved load. The unique chemical signatures of these two mighty rivers can be detected as much as 100 km below their confluence.

Climate exerts considerable influence over regional variation in the chemical composition of rivers. Across a gradient from arid to humid conditions, a general inverse relation is seen between annual precipitation and total solute concentration. High concentrations of total dissolved ions are found in rivers draining arid areas due to the small volumes of precipitation and runoff, salt accumulation in the soil, and evaporation (Walling 1984).

River chemistry also varies over time, due to the influence of seasonal changes in discharge regime, precipitation inputs, and biological activity. Flow variation has especially strong effects on ionic concentrations. Rivers are fed by a combination of groundwater and surface water, depending upon local geology and rainfall. Because of its longer association with rocks, the chemistry of groundwater typically is both more concentrated and less variable than surface waters. As a consequence, increases in flow due to rain events typically dilute streamwater, although it is not a simple relationship (Livingstone 1963). Golterman (1975) states that there are two common patterns. TDS may decline with increasing discharge, which is expected when the input of materials is constant. Alternatively, ion concentrations might not change greatly with fluctuations in discharge. This is expected when water chemistry reaches an equilibrium with the soil through which it percolates, or when concentrations approach saturation values. In addition to these two common patterns, however, some ions have been found to increase in concentration with rising discharge. The Orinoco River has very low concentrations of geologically derived nutrients because a large fraction of its catchment is underlain by resistant shield rock and covered with undisturbed forest, and because it has a very high runoff rate (Lewis and Saunders 1990). Seasonal increases in discharge result in a dilution response of major ionic solids, soluble silica and P; however, soluble organic fractions and all particulate fractions showed a purging response in which concentrations increased with increasing discharge.

Long-term studies of streamwater draining a hardwood forest in New Hampshire illustrate how ionic concentrations can change in response to seasonal variation in precipitation inputs, discharge, and the cycle of growth of the terrestrial vegetation (Likens and Bormann 1995). The most significant point is the relative constancy in stream chemistry, which probably is typical of intact, undisturbed ecosystems (Figure 4.2). Most dissolved substances vary within a narrow range (less than twofold), whereas streamflow can vary as much as four orders of magnitude over an annual cycle. In the Hubbard Brook Experimental Forest virtually all drainage water must pass through its mature and highly permeable podzolic soils. This affords considerable buffering capacity, and accounts for the relatively constant chemical composition of streamwater (Likens et al. 1970).

Figure 4.2 illustrates how concentration trends with discharge may differ among cations. Both magnesium and calcium showed no significant correlation with discharge, although the latter was the more variable of the two. Sodium concentrations exhibited a significant inverse relationship with discharge, presumably because of its low availability, and so rising discharge caused dilution. With the exception of some very high values during summer drought, potassium concentrations generally increased with increasing discharge. The explanation for this is complicated, and apparently includes biological activity as well as soil buffering. Stream discharge is low during the summer, higher throughout the winter, and highest at snowmelt. Plant growth during the summer corresponds with low potassium concentrations, and so it appears that seasonal changes in biological demand correlate with seasonal changes in flow conditions.

Although solute concentrations may exhibit only modest variation in response to discharge fluctuations in an intact forest, timber harvest and road building are significant disturbances that are reflected in solute export. Following deforestation and suppression of regrowth by herbicides in a catchment of Hubbard Brook,

FIGURE 4.2 The concentration of major ions in relation to stream discharge in a small forested catchment in the Hubbard Brook Experimental Forest, New Hampshire, from 1963 to 1965. (Reproduced from Likens et al. 1967.)

most major ions exhibited large increases in streamwater concentration and total output increased sixfold. Only ammonium and carbonate remained low and constant, and sulfate declined because of reductions in sulfate generation by sources internal to the ecosystem. The average values for calcium and magnesium increased by over 400%, sodium by 177%, and potassium over 18-fold. Altered ion concentrations were attributed to increased discharge, changes in the N cycle within the ecosystem, and higher temperatures (Likens and Bormann 1995).

Use of best management practices can moderate the response of streamwater chemistry to the disturbances associated with timber harvest. A long-term study of a well-managed clearcut of a hardwood forested catchment found that almost all solutes and nutrients showed elevated concentrations and exports, but because the forest was harvested using cable logging, which minimizes the need for roads and dragging of felled trees, and other best management practices were employed such as reseeding roads, overall losses were small and judged not detrimental to forest productivity (Swank et al. 2001).

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