Gas dissolved in water still exerts a pressure and at equilibrium will exert the same pressure
as the fraction in the gaseous phase. This means that if the water in a lake completely absorbs
atmospheric gases and attains equilibrium with the atmosphere, then the pressure of oxygen in the air
will be equal to the pressure of oxygen dissolved in the water. Even though the concentrations are quite
different. It's that simple.
In the presence of thermal gradients and biological processes which tend to remove oxygen
through respiration, oxygen may be distributed in a variety of ways. A clinograde distribution occurs
when oxygen concentration continuously declines with greater depth. With sufficient mixing, the
distribution of oxygen may be uniform, surface to bottom. Positive and negative heterograde
distributions occur when oxygen departs from the clinograde trend. These terms have less meaning
today than they did when lakes were classified according to the types of material and thermal
distributions but these terms are still occasionally used by limnologists and aquatic ecologists.
Carbon dioxide is an important product of respiration and enters a chemical reaction system
called the carbonate equilibrium system. At equilibrium for this system, inorganic carbon will exist in
several forms dependent on pH.
pH = -log [H+]
pH is related to how acid or alkaline water is and it is one of two master variables for determining
chemical equilibria. (Don't let this scare you either) At equilibrium, carbon dioxide will exist in water
at pH less than approximately 4.3 as the dissolved gaseous form and the hydrated form, carbonic acid
(H2CO3). At pH greater than 4.3 carbon dioxide may take ionic forms, bicarbonate (HCO3- )and at
even greater pH values, carbonate (CO3=). The carbonate ion is not a major form of inorganic carbon
for most lakes and it is unavailable to plants as a source of carbon. Please refer to the accompanying
illustration for the relationships of pH to the forms that carbon dioxide takes in water.
Metals may be dissolved in water in varying concentrations depending on the source, the pH, and
the presence or absence of dissolved oxygen. If oxygen is absent, then a chemical condition called a
reducing environment might form. This is in opposition to an oxidizing environment (see the connection
to oxygen?). Anaerobic environments may or may not be reducing ones but aerobic environments are
nearly always oxidizing ones. Chemically reduced conditions allow the dissolution of certain metals,
especially iron and manganese. These elements are important micronutrients for biota but can be toxic
in high concentrations.
Other important metals include the alkaline earth metals such as calcium, magnesium, sodium, and
potassium. Calcium is an important nutrient for plants and animals, especially those forming calcareous