Lake Magadi

Lake Magadi , c.20 mi (30 km) long and 2 mi (3.2 km) wide, S Kenya, in the Great Rift Valley. Formed and constantly resupplied by volcanic springs, the lake has a thick crust of carbonate of soda. The crust is removed by a floating dredge and then pumped to refineries, where it is processed into soda ash (used in glassmaking).

geochemistry of lakes The sources of dissolved solids in lake water are in many respects those of dissolved solids in rivers. The chemistry of the two parts of a freshwater lake, the epilimnion and the hypolimnion (see lakes), is affected by biological processes and by circulation.

The process of photosynthesis and respiration in lakes can be represented by the following reaction:106 CO₂ + 16 NO₃⁻ + HPO₄²⁻ + 122 H₂O + 18 H⁺ (+ trace elements and energy) → C₁₀₆ H₂₆₃ O₁₁₀ N₁₆ P₁ + 138 O₂.

Photosynthesis is the combination of carbon dioxide and nutrients (nitrogen, N, and phosphorus, P), aided by trace elements and solar energy to produce organic matter and oxygen (the reaction being driven to the right). The process of aerobic respiration involves the breakdown of organic matter and the consumption of oxygen but also the release of nutrients, carbon dioxide, protons, trace elements, and energy (the reaction being driven to the left). There is more photosynthesis than respiration in the epilimnion. The excess photosynthesis in surface waters manifests itself in the precipitation of dead organic matter to the bottom of the lake. However, in the hypolimnion (deep water) there is a net respiration, which causes the oxygen concentration to decline in the water but the nitrogen, phosphorus, proton, and carbon dioxide concentrations to increase according to the reaction for photosynthesis. Prolonged isolation of deep waters from the atmosphere (with no overturn) results in dramatic changes in water quality. The deep water becomes anoxic, and bacterially mediated chemical reactions, such as the reduction of nitrate, sulphate, iron, and manganese, and the formation of methane can take place.

Lakes are classified as oligotrophic or eutrophic according to whether their concentration of plant nutrients or their productivity of organic matter. Trophic means concerned with nutrition, and oligotrophic lakes are poorly fed; that is, they have a low concentration of nutrient elements such as nitrogen and phosphorus. On a geological timescale, lakes are short-lived. Streams bring sediment in to the lake and the accumulation of organic deposits may cause shallow lakes to change to bogs or swamps. Eventually they may become dry land. Relatively long-lived lakes are those located in deep basins or in arid regions where the drainage will integrate only very slowly. Eutrophication is the process in which a lake gradually fills in with organic-rich sediments, eventually becoming a swamp and then disappearing. Humans have greatly accelerated the process by artificially enriching lakes with too many nutrients or with an excess of organic matter, which can result in oxygen-depleted bottom waters. This process has been called cultural eutrophication.

Of the nutrient elements needed for photosynthesis (shown in the equation above), hydrogen and oxygen are readily available and carbon is generally supplied from the atmosphere, but nitrogen and phosphorus are not always available. They are the limiting nutrients. Relatively small amounts of nitrogen and phosphorus can produce relatively large amounts of organic matter. According to the equation for photosynthesis, only 7 g of nitrogen and 1 g of phosphorus are required to synthesize 100 g (dry weight) of algae. If the mass ratio of nitrogen to phosphorus is greater than 7 in a particular water, phosphorus is the limiting nutrient; conversely, if the ratio is smaller than 7, nitrogen is the limiting nutrient. However, nitrogen deficiencies (N/P ratios smaller than 7) can be made up by the development of blue-green algae, which are capable of fixing nitrogen from the atmosphere. Phosphorous is thus usually the limiting nutrient.

The primary sources of phosphorus and nitrogen in lakes are direct rainfall and snowfall on the lake itself and runoff from the surrounding drainage area. In oligotrophic lakes most of the phosphorus in the runoff comes from rock weathering and soil transport. However, in areas influenced by humans there are additional sources of phosphorus, including agricultural runoff containing phosphorus from fertilizer and animal waste, and sewage containing phosphorus from human waste, detergents, and industrial waste. The pollution of lakes has been shown to be proportional to population density per lake volume and energy consumption per capita.

In past decades dilute freshwater lakes and streams in southern Scandinavia, south-eastern Canada, and the north-eastern United States have been acidified by acid precipitation. These lakes and their drainage areas are underlain by weathering-resistant igneous and metamorphic rocks or noncalcareous sandstones and have thin, patchy, acid soils. In contrast, other areas that are also receiving acid precipitation, but have limestone and calcareous sandstone, contain lakes whose pH values are essentially unaffected by acid precipitation. The rapid dissolution of calcium carbonate in the rocks is able to neutralize the acid precipitation. In addition to permanently acidified lakes, there are also episodic decreases in pH in dilute, but usually non-acid lakes, caused by runoff of acid snow melt in the spring. Acid pollution accumulates in the snow in the winter and is preferentially released by the first snow melt in the spring.

Saline and alkaline lakes are common in arid to semi-arid climates. Arid conditions, however, do not always produce saline lakes. The necessary conditions for saline lake formation and persistence are: (1) the outflow of water must be restricted, as it is in hydrologically closed basin; (2) evaporation must exceed inflow during the initial stages; and (3) for persistence, the inflow must be sufficient to sustain a standing body of water. Lakes are saline and alkaline because salts derived from the surrounding rocks are brought into the lake in solution and evaporation causes the proportion of dissolved salts to increase steadily. Eventually, with continued evaporation, saturation is reached with respect to soluble minerals, and they are precipitated, resulting in chemical fractionation of the remaining water. Among the first minerals to be precipitated are calcium and magnesium carbonates. The pH of a saline lake can rise to values greater than 10 if the concentration of bicarbonate (HCO₃⁻) in inflow waters exceeds that necessary to precipitate all the Ca and Mg ions. Bicarbonate can build up with continued evaporation, causing the reactionH⁺ + HCO₃⁻ → H₂O + CO₂

to be driven to the right, resulting in the loss of hydrogen ions and a loss of CO₂ to the atmosphere. In other words, the pH of the remaining water increases. An example of a saline but non-alkaline lake is the Great Salt Lake in the United States; in contrast, Lake Magadi in Kenya is saline and alkaline.

Sigurdur Reynir Gislason

Bibliography

Lerman, A., Gat, J., and Imboden, D. (eds) (1995) Physics and chemistry of lakes (2nd ed). Springer-Verlag, New York.