Lake Erie’s “dead zone”
 |
| Image courtesy: Earth Sciences and Image Analysis, NASA - Johnson Space Centre. Mission: STS096; Roll: 701; Frame: 18 |
|
 |
|
‘Musseling’ in on an ecosystem
Tracing the natural history of
Lake Erie’s zebra mussels
Michael Bardwaj
As ice sheets a kilometer high crept out of the arctic almost 1.5 million years ago to
blanket vast parts of the northern hemisphere, much of life on Earth had a choice: either
pick up and move south to survive or freeze in the ice like insects caught in amber.
One seemingly innocuous species, the now-infamous zebra mussel (Dreissena polymorpha),
outlasted the ice age by hiding out in fresh water habitats scattered across southern and
eastern Europe. Before the glaciers retreated, many of these isolated pocket populations
became distinct from each other.
When the walls of ice eased their grip and began their slow northward retreat 10,000
years ago, they scored the landscape along the way, altering the routes of rivers and gouging
new channels. With new waterways open, zebra mussels surged into uncharted territory, swapping
their time-evolved genes back and forth, which increased the genetic variation throughout
the population.
This high genetic variation, combined with a reproduction cycle that sees these mussels
produce legions of offspring, forged the perfect invasive species. All they needed was
a chance.
Enter the industrial revolution. Through the early 19th century in Europe, rivers were
diverted, artificial channels were constructed and canals were built to link otherwise
isolated areas. While trade ships were steaming through these newly constructed waterways,
the mussels were burgeoning underneath. By the 1830s, zebra mussels had colonized all of
mainland Europe and had begun to invade Britain.
 With
the progress of technology during the 20th century and the globalization of industrial
trade, transatlantic tankers began carrying the wealth of nations across the open sea.
Ballast water sucked up from European waterways was carried in the bellies of these tankers
only to be dumped into the Great Lakes, along with the silent stowaway floating in the
ballast water.
In the mid-1980s, the first zebra mussel colonies were recorded in the shallow waters
of Lake St. Clair. Since then, the tiny mollusc has invaded the five Great Lakes and worked
its way up many of the inland waters of the United States and Canada.
The zebra mussel has altered aquatic ecosystems beyond recognition. Filtering up to one
litre of water a day, a single zebra mussel extracts enormous amounts of microscopic phytoplankton — small
plant life that forms the base of the aquatic food chain. Combined with an explosive breeding
capacity where one adult female can release over 40,000 eggs a year and a tenacious ability
to swarm any surface, sometimes counting as high as 30,000 to 100,000 per square metre,
this alien species has changed the very face of the lake.
So far, chemical controls such as chlorination and with mechanical controls, such as scraping,
have had a minor impact on zebra mussel populations, and often come with other environmental
consequences such as chemical pollution and habitat destruction. Public awareness campaigns
as well as tougher laws regarding lake and ocean travel have also stemmed the impact. Unexpected
biological controls have cropped up among invasive fish species, like the round gobi, also
an invasive species, which relies on the zebra mussel for food.
Despite all efforts, the zebra mussel continues to spread through North American waters.
Although extensive research has been done to map the causes and consequences of the mussel
invasion, a definitive solution remains beyond reach. Now, it is a matter of compromise
rather than conquer.
top
|
