| After
several late season cold fronts, it looks like spring has
finally arrived and will likely quickly transition into
summer. This was confirmed over the last two weeks by the
rash of calls to the local AgCenter office concerning fish
pond management. The large majority of these calls are requesting
recommendations for controlling filamentous algae, also
commonly known as pond scum.
Filamentous
alga usually becomes a problem in clear water with shallow
areas, which allows sunlight to reach the bottom or near
the bottom. This promotes the growth of filamentous algae
and eventually other aquatic weeds. In most instances the
pond owner is not aware of the growth until the mats “overgrow”
and begin to die back. When this happens, the dead alga
begins to decompose, releasing gases. These gases form bubbles
in the algae mat and float it, forming a slimy green mass
on the surface.
In
extreme cases the decomposing mats can consume enough oxygen
to cause fish kills. These unsightly mats also make fishing
difficult.
The
best method to controlling filamentous algae is prevention.
If possible, during construction, deepen pond edges so that
the slopes and areas less than two feet deep are minimized.
Once soil particles settle out of the water following construction
and bank edges have established vegetation, ponds usually
clear up. This is the critical time in preventing aquatic
weed problems.
When
water clarity reaches 18 inches of visibility, it’s
time to either fertilize the pond to promote planktonic
algae bloom or dye the water. Since winter time fertilization
is ineffective, early spring is the best time to fertilize,
once waters have reached 60-65F. Liquid fertilizers specially
formulated for ponds work best when sprayed onto the surface
during bright sun. These pond formulations, usually high
in phosphorus, will have ratios such as 0-34-0 and are sometimes
called superphosphates.
Planktonic
algae not only prevents sunlight from penetrating shallow,
clear water areas and unwanted weeds, it also jump starts
the food chain in ponds. These microscopic plants undergo
photosynthesis, which releases oxygen into the water and
provide food for microscopic organisms, insect larvae and
larval fish. Larger fish feed on those, thus driving the
food chain up to the desired fish species we often harvest
for food. Research has shown that fertilized ponds produce
two to three times the fish as unfertilized ponds.
Another
alternative to controlling filamentous algae is dye. Several
manufacturers sell pond dyes under a variety of trade names.
They come in several colors including blue, green and black.
Dyes are recommended in excessive weed growths where fertilizers
may exacerbate the problem. These dyes are fairly inexpensive
and last for several months so long as the pond does not
receive heavy runoff and flow through.
Copper
sulfate is the recommended herbicide for controlling algae.
However, caution must be used not to treat the entire pond
at once since killing large masses may consume dissolved
oxygen. Also, copper can be toxic to fish so be sure to
follow label recommendations and rates.
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