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After being accidentally introduced into Lake St. Clair via ship ballast water from their native Black and Caspian Sea in the late 1980's, the invasive Zebra mussel has caused a severe array of problems both for the human communities surrounding the Great Lakes region and the aquatic communities in the Great Lakes. These tiny little bivalves, due to the fact that a single female can lay up to 1,000,000 eggs a year, have quickly established themselves in the area and have become quite the nuisance, to say the least. Their impacts are distributed far and wide when examining indirect consequences, but their most direct impacts on the Great Lakes include out-competing native species, over-filtering the water, and attaching themselves to almost any surface imaginable. Now, all these things might not seem like such a big deal, but indirectly, they pose serious and dire problems to life as we know it surrounding the Great Lakes.
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Besides affecting areas of human concern in the form of economic damages and management difficulties, the establishment of Zebra mussels in the Great Lakes ecosystem has had severe detrimental effects on the local ecology and has even lead to the disruption of the local food web.
One of the most apparent ecological effects that the zebra mussel has had on the Great Lakes ecosystem is the manner in which it filters the lake water. Since they are filter-feeders, a single Zebra mussel can filter up to a quart of water a day. Multiply that by a number on the order of millions and it becomes possible to filter all of the water in a lake or stream in just one day. This has yielded much clearer lakes, which may seem great and beautiful and everything, but in actuality the increased solar penetration, along with other factors, has actually generated massive bright green toxic algae blooms. Lake Erie, pictured below, has fallen victim to said algae blooms. The Zebra mussel filters out most plankton and algal species, but chooses to leave behind a certain cyanobacteria called microcystis. This macroalgal species, mixed with their newly acquired lack of competition and phosphorous run-off from local farms and industry, is able to thrive. Their eventual decay can deplete oxygen from the water and create "dead zones" where oxygen levels are so low that fish can't even survive. (That's not good at all)
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This survey, taken from Lake Ontario (Table 1), shows sharp Diporeia declines in most samples between 1990 and 1995. Of the 15 sites sampled, 7 had become completely devoid of the important shrimps, 5 were drastically reduced, and 3 actually showed an increase in Diporeia density. Interestingly enough all of the sites that experienced a total depletion of Diporeia numbers were at depths lower than 50 meters, while those who experienced population increases were all at relatively deeper sites, perhaps hinting at the possibility that water depth is correlated with the stability of a Diporeia population.
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Because of this, the alewife fish has also seen a 23% reduction in energy density between pre-invasion and post-invasion surveys, which has caused the Chinook salmon, who prey upon them, to need to consume a corresponding 22% more alewives than before the mussel invasion to attain ideal body weights.
Any way you look at it, these Zebra mussels are posing quite the problem for the Great Lakes region, as well as everywhere else they are spreading to. While the easiest and closest to home problems are easy to conceptualize in monetary terms, the bigger changes that they are causing in the ecosystem should be cause for larger alarm. The difficulty of removing them only adds to this gargantuan problem. Unless a way to successfully eradicate established populations and halt their spread can be found, we just might have to get used to these tiny rapidly-breeding pests and the non-refundable ecological collapses that accompany them.
Ronald Dermott - Sudden Disappearance of the Amphipod Diporeia from Eastern Lake Ontario, 1993–1995