Biological processes

In streams and small rivers, biological activity in the water has little influence on water chemistry because any effects are diluted by the rapid flow. Conversely, in large slow-flowing rivers and in lakes, biological activity can cause major changes in water chemistry.

All photosynthetic plants absorb light and convert this to chemical energy within a chlorophyll molecule. The liberated energy is then used to convert CO2 (or HCO-) and water into organic matter. This complex biochemical process is crudely represented by the familiar equation:

light

CH2O represents organic matter in a simplified way as carbohydrate. The reaction depicted by equation 5.19 requires the input of energy (AG° = +475kJmol-1) (see Box 4.8) to proceed and this is provided by light. In shallow freshwater, large plants and drifting microscopic algae (phytoplankton) are responsible for photosynthesis, while in deep lakes (and in the oceans) phyto-plankton account for almost all photosynthesis. The reverse of equation 5.19 is organic matter decomposition, i.e. oxidation or respiration, which liberates the energy that sustains most life:

CH2O(S)+ O2(S)^ CO2(g) + H2O(l) AG °=-475kJmol-1 eqn. 5.20

Since photosynthesis requires light, it is confined to the surface layers of waters — the euphotic zone (the region receiving >1% of the irradiance arriving at the water surface). The depth of the euphotic zone varies with the angle of the sun, the amount of light absorbed by suspended matter (including phytoplankton) and the presence of dissolved coloured compounds in the water.

The decomposition of organic matter, which is almost always bacterially mediated, can occur at any depth in the water column. Decomposition consumes oxygen (eqn. 5.20), which is supplied to the water largely by gas exchange at the water/air interface and partly as a byproduct of photosynthesis. Temperature influences the amount of oxygen that can dissolve in water. Oxygen-saturated freshwater contains about 450 |mmoll-1 oxygen at 1°C and 280 |mmoll-1 at 20°C.

In summer, the surface layers of many lakes are warmed by insolation. The warmer surface water is less dense than the cold deep water, causing a stable density stratification. Stratification limits exchange of oxygenated surface water with the deeper waters. Organic matter produced in surface waters sinks into the deeper waters where it is oxidized, consuming and depleting dissolved oxygen. In some cases, oxygen levels fall below those needed to support animal life. This

Fig. 5.9 Variation in (a) temperature and (b) dissolved oxygen for lakewater (Esthwaite in northwest England) between March and October 1971. Modified from Heany et al. (1986). © 1986. This material is used by permission of John Wiley & Sons, Inc.

process is illustrated in Fig. 5.9 using data from Esthwaite, a lake in northwest England. In March the water column is well mixed and oxygen concentrations are uniformly high, around 350-400 |mmoll-1 at all depths. By May, however, the water column is stratifying (warmer above 7 metres water depth, cooler below) and oxygen concentrations begin to fall in water depths below 8 metres. By the end of summer a large area of very low oxygen concentrations (<10 |mmoll-1) has formed below 8 metres depth. In the autumn the stratification is destroyed by cooling and by strong winds that mix oxygenated surface waters with the deep water.

The rate of oxygen consumption usually increases as the supply of organic matter increases, either due to enhanced photosynthesis in the surface waters, or due to direct discharge of organic waste, for example sewage. The Thames estuary (UK), Chesapeake Bay (USA) (see Section 6.2.4) and the Baltic Sea (see Section 6.8.1) are all examples of water bodies affected by low oxygen concentrations, and similar processes occur in groundwater when oxygen consumption exceeds supply.

Once oxygen has been used up, bacteria use alternative oxidizing agents (electron acceptors, see Box 4.3) to consume organic matter. These alternative oxidants are used in an order that depends on energy yields (see Table 4.7). Nitrate reduction (denitrification) is energetically favourable to bacteria, but is often limited in natural freshwaters by low nitrate concentrations. Anthropogenic inputs, however, have resulted in increased nitrate concentrations in rivers and groundwater (Section 5.5.1), increasing the availability of nitrate for bacterial reduction. One of the byproducts of denitrification is nitrous oxide (N2O; Fig. 5.12), a powerful greenhouse gas which is increasing in concentration, probably because of human perturbation of the nitrogen cycle.

Iron (Fe) and manganese (Mn), both potential electron acceptors, are common as insoluble Fe(III) and Mn(IV) oxides. In reducing environments (at about the same redox potential as nitrate reduction), these oxides may be reduced to soluble Fe(II) and Mn(II) (see Table 4.7). Indeed, iron is soluble only under low redox or acidic conditions (Box 5.4).

Sulphate reduction (see Table 4.7) is not an important mechanism of organic matter respiration in freshwaters because dissolved sulphate levels are usually low. In seawater, however, sulphate is abundant and sulphate reduction is very important (see Section 6.4.6). Methanogenesis (see Table 4.7) can be an important respiration process in some organic-rich freshwater lake and swamp sediment. The reduced reaction product, methane (CH4), a greenhouse gas (see Section 7.2.4), is known to bubble out of some wetlands, including rice paddy fields, contributing significantly to atmospheric CH4 budgets (see Section 3.4.2).

5.5.1 Nutrients and eutrophication

In addition to CO2, water and light, ions (or nutrients) are needed for plant growth. Some of these ions, for example Mg2+, are abundant in freshwater, but other essential nutrients, for example nitrogen (N) and phosphorus (P), are usually present at low concentrations in natural systems. If light availability does not limit algal growth, chemical limitation is likely to occur when demand for nitrogen and phosphorus exceeds their availability. Consequently, a great deal of attention has been focused on the behaviour of nitrogen and phosphorus in natural waters and their role as potential, or actual, limiting nutrients. In seawater, the ratio of nitrogen to phosphorus required for optimal growth is quite well known, being 16:1 on an atomic basis. In freshwater, the required nitrogen: phosphorus ratio (N:P) is more variable. If, however, either nitrogen or phosphorus are in excess of the ratio required for optimal growth, it follows that the less abundant nutrient may be totally consumed and become limiting. Of course, nutrient elements can be available in excess when artificially introduced into the environment, for example as nitrate- and phosphate-based fertilizers. Nitrogen and phosphorus that leach from fertilizer applications often stimulate exessive algal growth (biomass) in water courses, a problem referred to as eutrophication. The exess algal biomass can cause toxicity, clogging of water filters, unsightly water bodies, reduced biodiversity and low oxygen concentrations in stratified waters.

Phosphorus

In natural waters, dissolved inorganic phosphorus (DIP) exists predominantly as various dissociation products of phosphoric acid (H3PO4) (eqns 5.3-5.5). Phosphorus is usually retained in soils by the precipitation of insoluble calcium and iron phosphates, by adsorption on iron hydroxides or by adsorption on to soil particles. As a result, DIP in rivers is derived mainly from direct discharges, for example sewage. DIP concentrations vary inversely with river flow (Fig. 5.10), the input being diluted under higher flow conditions. Since phosphate is usually in sediments as insoluble iron(III) phosphate (FePO4), under reducing conditions (such as occur in sediments when oxygen consumption exceeds supply (Section

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