They came in 1988. An invasion that is unparalleled in
proportion by any other. On arrival they numbered in the millions,
and now number in the billions, colonizing and engaging in an ecological
take-over of our liquid environment.
No I am not writing of alien invaders
from the watery subsurface of Jupiter's icy moon Europa, but of the exotic
species Dreissena polymorpha (ZEBRA MUSSELS) brought to us courtesy of
the Caspian sea and international trade. It is believed that a
transoceanic ship discharged its mussel laden ballast into Lake St. Clair
where It is likely that the pesky bivalve awaited its attack in it's larval
stage of life (veligers). Once here they adapted quickly to the plankton
rich waters, attaching themselves to hard substrates with their strong
byssal threads. Today the Zebra mussel has spread throughout the Great
lakes and surrounding land-locked waterways as far as rivers and lakes in
Arkansas, Tennessee and just recently California. Since their arrival the
Zebra mussel along with the quaga mussel (close relative of the zebra
mussel) have reeked havoc on both the ecology and infrastructure of our
Great Lakes system. They have done some good you say. Well, maybe a little for the
Divers and Mussels
What does the Zebra Mussel mean for us privileged few who
enjoy exploring our underwater environment?
You might believe that It means catastrophe, and in many ways you would
be right, but for the Great Lakes diver it has most commonly meant
AMAZING VISIBILITY. The changes are spectacular, where just a
few years ago divers were lucky to get 10-20 feet of vis in the Great Lakes,
now you can expect as much as 70 feet or more. The benefit of this is
that you can now see what your diving on or in. In some situations
you can see from bow to stern on a wreck, and it's now possible to see
large curious fish such as carp checking you out from a distance.
As well you will now see colorful sponges which branch out like beautiful
corals in a marine environment (every year they seem to get larger and
larger). The mussels themselves are interesting (kind of) to watch with
there constant siphoning of the nutrient rich water. These are only a few
interesting somewhat beneficial (to the diver) elements of the zebra mussel
invasion I have noticed over the last couple of years, and to the diver
these benefits are quite evident on every dive.
Since the mussels are filter feeders they take in the nutrients
and food (phytoplankton) they need to grow and reproduce along with
suspended solids, toxic chemicals and heavy metals such as PCB's, Mercury,
chromium, cadmium, Selenium etc. The removal of phytoplankton
(partly responsible for the great vis) is reducing the food supply
(zooplankton) small and juvenile fish need to survive, causing their numbers
to dwindle :(where are all the fish):. Although this is a well known
problem, their filtering of pollutants is just as detrimental. These
pollutants accumulate into the mussels fatty tissues making them
potentially toxic to divers who might cut themselves on the sharp calcium
shell. An important thing to keep in mind when diving in a current to
strong to fin in is to wear some form of hand protection
(standard neoprene gloves in colder water or a thin canvas or leather glove
(such as gardening gloves) when the water is warm). This practice will
protect your hands from numerous potentially infectious cuts while your
pulling yourself through a current. This accumulation and filtering of
toxins not only affects divers directly, but indirectly as
well (changing our lakes ecology). Pollution in the Planktonic life zone
(free floating zone e.g. plankton) and Nektonic life zone
(free swimming zone e.g. Fish) is being transferred (NOT removed as many
people think) to the benthic life zone (lake/river bottom). This transfer
occurs in a couple of ways. The first way is through the mussels filtering
the water and rejecting most of the pollutants and other materials
they don't want. This material combines with mucus and produces
pseudofeces (waste product) which is then expelled from the mussel to the
lake/river bottom, where it exposes the benthic organisms to its toxic
nature. The second way occurs when the mussels die and the pollution
in there degrading tissues moves on to this new life zone. Both examples
are dangerously changing the ecology where important benthic organisms such as
crawfish and worms live by increasing there exposure to pollution.
In turn, organisms that feed on these bottom dwelling creatures are also
subjected to higher concentrations of pollutants. In short, all organisms
in our lakes/rivers are affected from this transfer of pollutants, making
it easier for hazardous materials to make there way through the food chain
(bioaccumulation), potentially affecting the health and number of fish
we see during our dives.
It's always an amazing experience doing a dive on a 19th century schooner.
That is, if you can see the wreck. The zebra mussel seems to have made
this both a pleasurable experience, providing great vis, as
well as an eye-opening experience to the infestation problem. For example
some wrecks such as the Vickory
in the St. Lawrence are completely blanketed with a foot or more of the
mussel, diminishing the original
appearance of the age-old wreck.
Many organisms are being displaced by the mussel. For example the native
clams of the Lakes are being killed off due to zebra mussels jamming their
shell open allowing predators easy access to there tasty
innards. This is an effect that diminishes the natural beauty and
ecology of the lake or river bottom. An interesting observation myself
along with many other divers have witnessed over the last couple of seasons
is that those sponges mentioned earlier seem to be overtaking the mussels.
Their growing overtop of the
mussels possibly smothering the bivalve from being able to filter.
Who's to say what the result of this will be
in a couple of years, maybe they'll control the zebra Mussel problem
(doubt it). (For more info on these
sponges stay tune to this web site for a future article)
Unfortunately scientists believe that zebra mussels will be impossible to
eradicate so control strategies and
the prevention of spreading is our only means of defense (for now). As the
new millennia draws near the
current trend of ecological change and degradation continues to mount.
Remember, Zebra Mussels are only
a small part of our environmental problems.
MISCELLANEOUS INFORMATION ON THE ZEBRA MUSSEL
Preferred Parameters of the Zebra Mussel: Water temp between 4 and 27 degrees C, salinity levels
below 8 ppt (parts per thousand), and Calcium levels no less than 25 ppm (parts per million). These
parameters can be found in just about every lake in North America.
Zebra mussels generate a tuft of fibers known as byssal threads from glands in their feet. These threads
attach to hard surfaces with an adhesive secretion that anchors the mussel in place. When necessary,
young mussels can break away from their attachments and generate new, buoyant threads that allow
them again to drift with the currents and find a new surface. The mussels attach to any firm surface
such as rock, metal, wood, glass, rubber, fiberglass, plants and even other mussels. Mussels prefer
depths of less than 12 meter's but can be found at depths of more than 100 meter's at densities of over
Through my research I have found conflicting data on just how much a ZM filters. This number varies
from between 1 and 10 L a day for an adult mussel with 1 L being the most common quantity.
Egg production in the mollusk occurs when the water temperature warms to 12 degrees C and are
fertilized outside of the shell (they hatch within a couple of days). One adult female Zebra mussel
produces up to 40,000 eggs per year. This figure coupled with the 3-5 year life span of a mussel equals
a possible 200,000 total eggs produced per mussel.
Phytoplankton concentrations have been reduced by as much as 80% from the great appetites of the
zebra mussel. This is causing huge decreases in some forms of zooplankton which feed on the
diminished supply of phytoplankton. In turn the smaller fish species and juvenile fry which feed on the
zooplankton our finding it harder to feed themselves.
The increased visibility makes it possible for photo energy to penetrate deeper into the water causing
heavy blooms of aquatic vegetation to increase greatly (changing the ecology).
Zebra mussels are rejecting blue-green algae which has allowed the plant to flourish over species of
algae the mussels do eat. The problem with this is that some forms of this blue-green algae
(Microcystis) are toxic to fish and cause gastro-intestinal problems in humans.
The colonization of the mussels on indigenous clams has eradicated some species in some areas.
Fresh water Drum and Carp as well as some species of waterfowl are eating the mussels. This will not
help stop the invasion but will only transfer the pollutants Zebra's absorb through filtering to the food
It is estimated that the entire volume of Lake Erie's western basin is filtered between one and three
times per week due to it's high concentrations of zebra mussels.
Out of the water, Zebra Mussel's can survive a few days but in moist conditions they can survive
almost two weeks. They can spread in many ways (downstream drift, boat traffic, float planes, fishing
gear, scuba divers and swimmers, construction and research) making it impossible to completely stop
their spread. By attaching themselves to pleasure boats they've spread through the Trent Severn
waterways and are now found in the Muskoka's. Ontario's Ministry of Natural Resources spends
heavily to educate boaters about how to prevent the spread of Zebra mussels via seminars and
MUNICIPAL AND INDUSTRIAL PROBLEMS
The Zebra mussel presents a very significant threat to industry. Because Zebra mussels are attracted to
moving water, they have created major biofouling problems for municipal and industrial raw water
users in areas of the Great lakes and surrounding water ways. Where there are established colonies,
Zebra mussels have fouled raw water intakes of powerplants and other industry that utilize raw water in
there processes. These fouled intakes block pipes and intake valves causing taste and odor problems
with freshwater supplies and have caused increased friction and reduced velocity in the pipes of various
industrial water intakes. Critical systems effected by the mussel are raw water cooling systems, fire
protection systems, raw water contact instruments and sump/dewatering pumps. Besides the industrial
problem, the mussels also cause havoc by weighing down navigational buoys and floating docks causing
them to become partially and even completely submerged. The shells from shallow water colonies are
washing ashore, causing bad odors from there decay and are imposing a danger to swimmers who could
cut there feet on the sharp shells. Industry is trying many techniques to combat this infestation
problem. UV radiation, ozone, chemical treatment, anti fouling paints etc. are being experimented with,
but Chlorine seems to be the method of control for now. With all this in mind it's easy to see how facilities
on the Great Lakes expect to spend upwards of 5 billion dollars by the year 2000 to control the
The information presented in this article is only as accurate as the latest research done. New studies
are both supporting and disproving current theories of the effect zebra mussels are having in our waterways.
For more information on zebra mussels and other exotic species contact the invading species hotline at
1-800 563-7711 or type in Zebra Mussel at your preferred search engine for a complete list of informative