is pH Important?
pH is a measure of the acidity of alkalinity of water, expressed
in terms of its concentration of hydrogen ions. The pH scale ranges
from 0 to 14. A pH of 7 is considered to be neutral. Substances with
pH of less that 7 are acidic; substances with pH greater than
7 are basic. The chart below shows the pH of some common substances:
term pH was derived from the manner in which the hydrogen ion
is calculated - it is the negative logarithm of the hydrogen ion (H+)
concentration. What this means to those of us who are not mathematicians
is that at
higher pH, there are fewer free hydrogen ions,
and that a change of one pH unit reflects a tenfold change in the concentrations
of the hydrogen ion. For example, there
are 10 times as many
hydrogen ions available at a pH of 7 than at a pH of 8.
pH of water determines the solubility (amount that can be dissolved
in the water) and biological availability (amount that can be utilized
by aquatic life) of
chemical constituents such as nutrients
phosphorus, nitrogen, and carbon) and heavy metals (lead, copper, cadmium,
etc.). For example, in addition to affecting how
what form of phosphorus is most abundant in the water, pH may also determine
whether aquatic life can use it. In the case of heavy metals, the degree
which they are soluble determines their toxicity. Metals
tend to be more toxic at lower pH because they are more soluble.
for Natural Variation
Photosynthesis uses up dissolved carbon dioxide, which acts like
carbonic acid (H2CO3) in water. CO2 removal, in effect, reduces the
acidity of the water and so
pH increases. In contrast, respiration
of organic matter produces CO2, which dissolves in water as carbonic
acid, thereby lowering the pH. For this reason, pH may be higher during
daylight hours and during the growing season, when photosynthesis is
at a maximum. Respiration and decomposition processes lower pH. Like
oxygen concentrations, pH may change with depth
in a lake, due again to changes in photosynthesis and other chemical
reactions. There is typically a seasonal decrease
in pH in the lower layers of a stratified lake because CO2 accumulates.
There is no light for plants to fix CO2 and decomposition releases CO2.
Fortunately, lake water is complex; it is full of chemical "shock
absorbers" that prevent major changes in pH. Small or localized
changes in pH are quickly modified
by various chemical reactions,
so little or no change may be measured. This ability to resist change
in pH is called buffering capacity. Not only does
capacity control would-be localized
changes in pH, it controls the overall range of pH change under natural
conditions. The pH scale may go from 0 to 14, but the pH of
waters hovers between 6.5 and 8.5.
Expected Impact of Pollution
When pollution results in higher algal and plant growth (e.g.,
from increased temperature or excess nutrients), pH levels may increase,
as allowed by the buffering
capacity of the lake. Although
these small changes in pH are not likely to have a direct impact on
aquatic life, they greatly influence the availability and solubility
chemical forms in the lake and may aggravate nutrient
problems. For example, a change in pH may increase the solubility of
phosphorus, making it more available for
plant growth and
resulting in a greater long-term demand for dissolved oxygen.
for pH are reported in standard pH units, usually to one or two decimal
places depending upon the accuracy of the equipment used.
pH represents the negative logarithm of a number,
is not mathematically correct to calculate simple averages
other summary statistics. Instead, pH should be reported
as a median and range of values; alternatively the values could be converted
to hydrogen ion concentrations, averaged, and
to pH values. Generally, during
the summer months in the upper portion of a productive or eutrophic
lake, pH will range between 7.5 and 8.5. In the
the lake or in less productive lakes, pH will be lower, 6.5 to 7.5,
perhaps. This is a very general statement to provide an example of the
differences you might
Case of Acid Rain
An important exception to the buffering of pH changes in lakes
is the case of lakes affected by acid rain. Lakes that have received
too much rain with a low pH
(acid rain), lose their buffering
capacity. At a certain point, it takes only a small bit of rain or snowmelt
runoff for the pH to change. After that point, change occurs
quickly. According to the EPA, a pH of 5-6 or lower has been found to
be directly toxic to fish (for additional information, see our acid
Michaud, J.P. 1991. A citizen's guide to understanding and monitoring
lakes and streams. Publ. #94-149. Washington State Dept. of Ecology,
Olympia, WA, USA (360) 407-7472.
Moore, M.L. 1989. NALMS management guide for lakes and reservoirs.
North American Lake Management Society, P.O. Box 5443, Madison, WI,
53705-5443, USA (http://www.nalms.org).