Salinity

Salinity is usually defined as the sum of ionic compounds dissolved in water and can be measured in several ways. Specific conductivity quantifies the relative ease with which an electrical current passes through a water sample and is usually expressed as milli or micro Siemens per centimeter (mS or p.S cm-1). Because the conductivity of NaCl increases 2% for every degree increase in temperature, specific conductivity is always measured at 25 °C.

Salinity can also be expressed as milligrams per liter of total dissolved solids (mg l-1 TDS). For TDS, water is filtered through a 2 mm pore-size filter, evaporated to dryness at <100 °C, and then weighed. Lakes with TDS >3000 mgl-1 (3% or 5500 mScm-1) are commonly referred to as saline and this is the salinity at which most people start to taste salt.

Figure 1 Percentage of water in various compartments worldwide. Data from Wetzel RG (1983) Limnology 2nd Edn. Philadelphia: Saunders (black bars) and from Shiklomanov IA (1990) Global water resources. Natural Resources 26:34-43 (white bars) as cited in Williams WD (1996) The largest, highest and lowest lakes of the world: saline lakes. Verhandlungen Internationale Vereinigung Limnologie 26: 61-79.

Figure 1 Percentage of water in various compartments worldwide. Data from Wetzel RG (1983) Limnology 2nd Edn. Philadelphia: Saunders (black bars) and from Shiklomanov IA (1990) Global water resources. Natural Resources 26:34-43 (white bars) as cited in Williams WD (1996) The largest, highest and lowest lakes of the world: saline lakes. Verhandlungen Internationale Vereinigung Limnologie 26: 61-79.

Figure 2 Endorheic drainage basins of the world. With permission from Shiklomanov IA (1998) World water resources. A new appraisal and assessment for the 21st century. Paris: UNESCO.

Freshwater species start to disappear at salinity above 3000 mgl-1 and below this higher salinity biota are not found. Saline lakes can be further differentiated into hyposaline, mesohaline, and hypersaline (Table 1). Freshwater lakes have TDS of 500 mgl-1 while subsaline lakes are those whose TDS ranges from 500 to 3000 mgl-1 (Table 1). In Africa and Australia, lakes lie at the extreme end of the salinity gradient with salinities up to 270 000 mScm-1. Some Antarctic saline lakes have recorded salinities as high as 790 000 mS cm-1. Such high salinities result not only from evaporation but the fact that salts are frozen out of the ice cover. In the North American north Temperate Zone, however, specific conductivities are more typically in the range of 20-600 mS cm-1. Across the semiarid prairie pothole region of Saskatchewan,

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Figure 3 Declining water levels (predicted: solid line; actual: solid circles); and increasing salinity (predicted: broken line and actual: triangles) in Redberry Lake, SK, Canada, as a result of drought and land use changes in the basin during the period 1904-2003. Data from G. Van der Kamp and M.J. Waiser (unpublished data).

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Figure 4 Increases in specific conductivity from May to October, 2000, in a shallow saline prairie wetland - Pond 50, St. Denis National Wildlife Refuge, SK, Canada. Data from Waiser MJ (2001) The Effect of Solar Radiation on the Microbial Ecology and Biogeochemistry of Prairie Wetlands. PhD thesis, Edinburgh: Napier University.

May -October

Canada specific conductivities as high as 72 000 m S cm-1 have been recorded in some saline wetlands.

Although ions that comprise salinity can originate from marine aerosols deposited on lake surfaces (those lakes found near to marine environments and closely related hydrologically and biologically to the sea are known as thalassic lakes), they usually originate in soils or rocks within a lake catchment (such lakes are known as athalassic or true inland saline lakes). Surface catchment geology, therefore, is one factor that will determine whether receiving water bodies are saline or not. For example, igneous rocks that characterize the Precambrian Shield of North America and areas of Scandinavia, Russia as well as the Amazon and Congo River Basins, are of low solubility. Their receiving waters will consequently be low in salinity and precipitation will determine lake salinity. On the other hand, soils formed after retreat of the last glacier (i.e., Pleistocene in the Prairie Pothole region) tend to have higher solubility and therefore receiving waters will be higher in salinity. Aside from air borne salts, and weathering of rocks, the other major source of salts is from springs rich in minerals leached from underground rocks and sediments.

Within saline lakes, however, ionic composition tends to differ greatly from incoming waters and geochemistry of catchment soils. This is due to the fact that salinity arises from differential ion precipitation as water evaporates from the lake. In general terms, CaCO3 is precipitated first during evaporation and results in relative enrichment of Na, Mg, Cl, and SO4 in remaining water. Next, MgCO3 precipitates out as dolomite [CaMg(CO3)2] in waters where concentration of Ca and Mg is high. If concentration of Ca is high enough then gypsum will precipitate (CaSO42H2O). After all of this, precipitation has occurred, the remaining water tends to be enriched in Cl relative to other anions, and Na relative to other cations. For this reason there are more saline lakes with Na and Cl as their major ions owing to the high solubility of Na and Cl.

Table 1 Classification of saline lakes according to total dissolved solids (TDS), specific conductivity and salinity

Lake type

TDS

Specific

Salinity/

(mg r1)

conductivity

(ppt)

(mS cm1)

Fresh

<500

<850

0.5

Sub-saline

500-3000

850-5500

0.5-3

Hyposaline

3000-20 000

5500-30 000

3-20

Mesohaline

20 000-50 000

30 000-70 000

20-50

Hypersaline

>50 000

>70 000

>50

Seawater

35 000

53 000

35

Freshwater and seawater values are included for comparison.

Freshwater and seawater values are included for comparison.

Saline lakes exhibit a diverse ionic composition (Table 2). Globally, NaCl lakes are particularly common in Australia, South America, and Antarctica. In East Africa, however, waters are enriched in HCO3, but poor in Ca and Mg and these are the so-called soda (sodium carbonate) lakes of the African Great Rift Valley (Lakes Nakuru and Magadi). Across the prairie pothole region of North America, many of the lakes are dominated by Mg and SO4 (e.g., Redberry Lake).

Major determinants of salinity, therefore, are local geology, climate, soil age, and distance from the sea (ion deposition).

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