General Lake Chemistry
In the absence of any living organisms, a lake contains a wide array of molecules and ions from the weathering of soils
in the watershed, the atmosphere, and
the lake bottom. Therefore, the chemical composition of a lake is fundamentally
a function of its climate (which affects its hydrology)
and its basin
geology. Each lake has an ion balance of the three major anions
and four major cations (see Table 4).
|Table 4. ION BALANCE FOR TYPICAL FRESH WATER
means the sum of the negative ions equals the sum of the positive cations
when expressed as equivalents. These ions are usually present at concentrations
expressed as mg/L (parts per million,
whereas other ions such as the nutrients phosphate, nitrate, and ammonium are present at µg/L (parts per billion,
can have profound influences on lake chemistry. Excessive landscape
disturbance causes higher rates of leaching and erosion by removing
vegetative cover, exposing soil, and increasing water runoff velocity.
Lawn fertilizers, wastewater and urban stormwater inputs all add micronutrients
such as nitrogen and phosphorus, major ions such as chloride and potassium,
and, in the case of highway and parking lot runoff, oils and heavy metals.
Emissions from motorized vehicles, fossil fuel-burning electric utilities
and industry, and other sources produce a variety of compounds that
affect lake chemistry.
the best understood ions are H+ (hydrogen ion, which indicates
acidity), SO4-2 (sulfate) and NO3-
(nitrate) which are associated with acid rains. Mercury (Hg) is another
significant air pollutant affecting aquatic ecosystems and
in aquatic food webs, contaminating fish
and causing a threat to human and wildlife health (see also the Minnesota Pollution Control Agencys section on Hg).
high concentrations of the ions calcium (Ca+2) and magnesium
(Mg+2) are called hardwater lakes,
while those with low concentrations of these ions are
lakes. Concentrations of other ions,
are highly correlated with the concentrations of the hardness ions, especially Ca+2. The ionic concentrations influence the lake´s
ability to assimilate pollutants and maintain nutrients in solution.
For example, calcium carbonate (CaCO3) in the form known
as marl can precipitate phosphate from
the water and thereby remove this important nutrient from the water.
The total amount of ions in the water is called
the TDS (total
dissolved salt, or total dissolved solids concentration).
Both the concentration of TDS and the relative amounts or ratios of
different ions influence the species of organisms that can best survive
in the lake, in addition to affecting many important chemical reactions
that occur in the water. One example of particular interest in the Great
Lakes region involves the calcium requirement of the exotic zebra mussel
that is causing profound changes in Lake Erie (see National
Aquatic Nuisance Species Clearinghouse or Sea
Grant Nonindigenous Species Site). Lake Superior appears to be relatively
immune to infestation by this invader because of low calcium concentration.
Its bays, however, such as the lower St. Louis River and Duluth-Superior
Harbor, may not be immune to zebra mussel infestation.