Catastrophic Oxygen Depletion and How to
During the warm spring and summer months,
we receive an increased number of phone calls regarding sudden
fish kills in ponds. The typical history includes observing
a very large number of fish dead in an otherwise normal pond
following a rainstorm or summer thunderstorm. Usually the
owner is very concerned that the fish may have died due to
"run-off" of farm chemicals into the pond. Most of the time,
these fish kills are a result of a phenomenon known as "pond
stratification." Pond stratification is somewhat of a misnomer,
since the stratification can also occur in lakes, creeks and
some rivers. The stratification leads to a catastrophic depletion
of oxygen which almost always results in a very high mortality
of aquatic animal life within 24-48 hours following the "de-stratification."
The scientific reasoning behind this
phenomenon of pond stratification relates to the temperature
of the pond. In the early spring, while the temperature of
the pond is still relatively low, the dissolved oxygen is
uniformly distributed throughout the pond. As the atmospheric
temperature increases, the pond begins to stratify, that is,
become layered, with the surface water becoming warmer and
lighter while the cooler and denser water forms a layer underneath.
Circulation of the colder bottom water is prevented because
of the difference in densities between the two layers of water.
Dissolved oxygen levels decrease in the bottom layer since
photosynthesis and contact with the air is reduced. The already
low oxygen levels are further reduced through the decomposition
of waste products, which settle to the pond bottom. After
a rain, or any other event which disrupts the two layers,
a "de-stratification" or "turn-over" of the pond occurs. This
has the effect of releasing all of the dissolved oxygen from
the upper layer of the pond into the atmosphere, hence, a
catastrophic oxygen depletion.
Once stratification of a pond occurs,
there is nothing that can be done to alleviate the situation.
However, pond stratification can be very easily prevented
by the use of supplemental aeration. Aerators come in all
sizes and shapes as well as different power sources, i.e.,
tractor p-t-o, electrical, mechanical, etc. It is important
to aerate the pond properly, i.e. match the size of the aerator
to the pond, since over-aeration is wasted and may even lead
to oxygen supersaturation, known as "gas-bubble" disease and
under-aeration will not prevent stratification.
In those cases where we suspect catastrophic
oxygen depletion, all other possible pathogens including bacterial,
viral, parasitic agents are eleminated from the differential
diagnosis list. However, the history of several days to weeks
of warm weather followed by a sudden rainstorm are highly
suggestive of this condition. If you suspect that you are
dealing with a pond turn-over situation following pond stratification,
it is imperative to have the pond owner take a water sample
and have the dissolved oxygen (DO) concentration evaluated
immediately. This water sample should be collected in a clean
glass jar or bottle with a screw-top lid and should be completely
filled by completely submersing the sample and container and
placing the lid on the container while it is still under water.
- by: Tim Muench, DVM, MS
- edited by: Randy White, DVM, PhD