Water pollution often brings to mind oil spills or industrial
waste discharges. However, in many south Louisiana waters,
there are types of pollution that don’t necessarily
kill fish directly but quietly harm fishing. These are sediment
and nutrient pollution.
Sedimentation
is the deposit of sands, silts and clays that cover the
bottom and fill in waterbodies. Nutrient enrichment (eutrophication)
involves the addition of unhealthy levels of nitrogen and
phosphorus. While much of the man-made portion of these
problems can be controlled, solutions aren’t always
easy.
Sediments
and nutrients are mostly nonpoint sources of pollution.
Nonpoint pollution is “sources of pollution which
enter surface or groundwaters through widely diffused small
increments,” as described in the Clean Water Act.
Sources include urban and industrial stormwater runoff,
agricultural runoff (sediment, fertilizers, chemicals and
manure), forestry activities, construction projects and
water control projects such as dams, levees, channels and
weirs. Point sources of nutrient pollution include sewage
discharges.
Sediment
tends to slowly fill basin-type lakes that collect lots
of drainage, such as Lake Fausse Point and Lake Verret.
(Major rivers like the Atchafalaya carry huge sediment loads,
but that’s another story.) Everyone knows that muddy
water makes fishing tough, but not everyone stops to think
about what happens as the water clears.
The
sediment drops out of the water and slowly fills the lake.
As large areas of a lake become shallower than four to five
feet, their capacity to hold fish drops dramatically. Not
only do muddy bottoms make for poor fish nesting, but the
shallow water heats up tremendously in the summer, and oxygen
levels can be depleted. Large areas of shallow water also
are ideal for growth of choking mats of aquatic vegetation.
Local
and area drainage is the main source of sediment for these
types of lakes, and we have considerable control over this
source of sedimentation. Everyone demands good drainage
for their homes, roads and crop fields, but we don’t
always take the extra steps to get the sediment out of the
runoff.
Sediment
fences, sediment traps and vegetated buffer strips all help
solve the problem. Every time you see a developer use good
sediment barriers around a construction site, you know that
a bit of the problem way down the bayou has been addressed.
Fencing livestock away from ditch banks also has positive
impacts because erosion from trampled banks can be severe.
Eutrophication
can be a problem in any lake, but it is almost always a
concern in lakes receiving lots of drainage.
Along
with sediment comes nutrients from agriculture, pastures,
yards, and municipal and camp sewage. The resulting deep
green algae blooms in the water are an indication of an
ecosystem that is “on the edge.”
The
normal oxygen cycle in a lake has highest dissolved oxygen
(D.O.) late in the day and lowest levels just before dawn.
In eutrophic systems this cycle tends to swing wildly from
extremely high D.O. at dusk to very low D.O. at dawn. These
waters tend to gradually become dominated by fish species
that can handle those conditions – the so-called rough
fish: carp, gar, bullhead catfish.
Fish
kills occur in areas where the dawn low D.O. drops below
about 1.5 milligrams per liter (mg/L). When a lake gets
to these conditions, public use drops and the Sportsman’s
Paradise becomes a little less like paradise.
As with
the local sources of sedimentation, we can make good decisions
about controlling nutrient pollution. State-of-the-art fertilizer
application and advanced sewage treatment with wetland or
dryland nutrient uptake components are solutions that should
be popular with people who are serious about quality fishing.
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