The Zebra Mussel Invasion of the Great Lakes and its Ecological and Economic Impacts

image from: 
http://www.lakegeorge.net/images/zebra_mussel_toon.gif

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.

Image from: http://www.learner.org/courses/envsci/visual/img_med/zebra_mussels.jpg

Because of their strong byssal fibers, the zebra mussel can attach to most hard surfaces and have been known to cling to shopping carts, water pipes, and boats.  In the case of water pipes, they cannot serve their given purpose if they have a bunch of little mussels taking up space inside and clogging the flow, so they need to be removed which can be costly and never-ending.  Maintenance crews will have to come back time and time again to remove pesky mussels from clogged water intake pipes for electric power generation and municipal water filtering facilities.  This can typically cost a lot of money; $3.1 billion between 1993-1999 for the power industry alone, to be exact (source).  Other methods of getting rid of the Zebra mussel include chlorine-based chemical controls, which can be effective in inducing mortality but yield dangerous environmental consequences, such as killing other non-target organisms, bioaccumulation, and the release of known carcinogens into the ecosystem.

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)

Image from: http://eoimages.gsfc.nasa.gov/images
/imagerecords/76000/76127/lakeerie_tm5_2011278.jpg

The other major effect that the Zebra mussel has been shown to inflict upon colonization sites lies in its ability to out-compete a tiny little shrimp-like amphipod called Diporeia.  Since around the time that zebra mussels were discovered in the great lakes, the numbers of Diporeia have simply plummeted  These little shrimp-like substances are a very important player in the Great Lakes food chain, for they are high in lipids and are a good source of calories for many larger aquatic organisms in the ecosystem.   

Image from: http://www.iaglr.org/jglr/db/show_article.php?
file_name=2001/num4/27_4_423-433.pdf

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.  

Image from: http://www.iaglr.org/jglr/db/
show_article.php?file_name=2001/num4/27_4_423-433.pdf

As this graph (Fig. 2) shows, the density of Diporeia has quickly decreased to zero in almost all sample sites on Lake Ontario since the zebra mussel was established in 1989, with the exception of the mid-lake sample site.  The Diporeia populations near the shore and in the east side of the lake completely collapsed.  Even after two years of follow up surveying, none were to be found.   The increases in the mid-lake sample site make sense when combined with the numbers from the above table; the mid-lake sites are more likely to have a greater depth than nearshore sites, and thus this would also support the hypothesis that increases in water depth can be linked to increased probability of survival of Diporeia populations.

Since food webs typically function similarly to a stack of blocks, the removal of a single lower block can sometimes cause the whole stack to collapse.  Since the decline of Diporeia, a whole slew of other larger aquatic organisms have hopped on the band-wagon and started to decline in numbers as well.  The period between 1988 and 1996 saw a 95% decline in Lake Ontario lake trout and slimy sculpin numbers, while smelt stocks have also been heavily damaged by this lack of suitable food.  In Lake Erie, rainbow smelt and whitefish have seen dramatic declines.

In Lake Michigan, some fish populations are being forced to switch their diet from Diporeia to Zebra Mussels!  On first thought, this doesn't sound like such a bad thing because it shows that it is possible for native species to predate upon and hopefully contribute to the eradication of these hellishly invasive mollusks, but an examination into the caloric content of the Zebra Mussel will show that they offer little to no energetic value to their consumers.  One of the few things that they do seem to accomplish for those fish who do choose to consume them though, is that they take up space in their digestive tract and leave little room for food-bits that actually contain energy.  Thomas Nalepa, a research biologist at the Great Lakes Environmental Research Lab, said that "the [whitefish], whose guts became packed with shell material, were so skinny that fishermen could no longer get a fillet off them."
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.

More Reading:
Ronald Dermott - Sudden Disappearance of the Amphipod Diporeia from Eastern Lake Ontario, 1993–1995

Zebra Mussels - NationalAtlas.gov