While more visually appealing than murky or dark waters, clear blue waters may not necessarily reflect a lake's true cleanliness and health. Lake Erie, for example, was once considered to be a "dead lake" in the late 1960s because it was so badly polluted. It now has increasingly clear and pristine waters due in great part to the contributions of the zebra mussel, an incredibly destructive invasive species that entered the Great Lakes system almost 2 decades ago.
It has been able to accomplish this through one simple mechanism: kill every other living organism underwater. As a filter feeder, the zebra mussel draws in a large quantity of water each day through its incurrent siphon (close to 1 liter) and consumes all the organic matter and plankton it catches with its gills. Though this helps leave the water clear, it also robs other organisms of a potential food source."The impact that the public is seeing is with that increased water clarity, but they're not equating that with the impact zebra mussels have had on the lakes' ecosystems," said Francine MacDonald of the Ontario Federation of Anglers and Hunters. In addition to forcing fish and other aquatic wildlife from their natural habitats, the mussels create a razor sharp carpet on the lake's floor that makes it practically unwalkable.
More worrying is the fact that zebra mussels have a knack for quickly spreading from one body of water to another, often jumping into streams, rivers and inland lakes across the region. In their larvae stage, they are virtually invisible and can peacefully drift through large tracts of water for a considerable period of time.
They were first able to spread to Lake Erie and the rest of the Great Lakes from Lake St. Clair by attaching themselves to the hulls of boats. The only way to stop this invasive species from taking over new bodies of water and annihilating all present life is through prevention, as MacDonald notes: "With zebra mussels there is no control, so prevention is the key way to stop them or prevent their spread to new water bodies."
See also: ::Chinese Water: A Picture is Worth... [Updated], ::Burn it Where You Buy it to Stop Invasive Species
UPDATE: We replaced the original picture as JiltedCitizen pointed out it wasn't exactly appropriate.
Image courtesy of dwstucke