In situations where eutrophication occurs, the natural cycles
are overwhelmed by an excess of one or more of the following:
nutrients such as nitrate or phosphate, or organic waste.
In the first case under aerobic conditions (presence
of oxygen), the natural cycles may be more or less in balance
until an excess of nitrate and/or phosphate enters the water.
At this time the water plants and algae begin to grow more rapidly
than normal. As this happens there is also an excess die off
of the plants and algae as sunlight is blocked at lower levels.
Bacteria try to decompose the organic waste, consuming the oxygen,
and releasing more phosphate and nitrate to begin the cycle anew.
Some of the phosphate may be precipitated as iron phosphate to
remove the soluble form from the water solution.
In the second case under anaerobic conditions (absence
of oxygen), as conditions worsen as more phosphates and nitrates
may be added to the water, all of the oxygen may be used up by
bacteria in trying to decompose all of the waste. Different bacteria
continue to carry on decomposition reactions, however the products
are drastically different. The carbon is converted to methane
gas instead of carbon dioxide, sulfur is converted to hydrogen
sulfide gas. Some of the sulfide may be precipitated as iron
sulfide. Under anaerobic conditions the iron phosphate in the
sediments may be solubilized into solution to make it available
as a nutrient for the algae which would start the growth and
decay cycle over again. The pond may gradually fill with undecayed
plant materials to make a swamp.