Friday, March 16, 2012

Hitchhiking West: Journey of the Zebra Mussel

Most people in the American West are unaware that an experienced hitchhiker is making its way across the country. Once it reaches Oregon, the waterways will be unrecognizable. I am talking, of course, about the zebra mussel.




These coin sized bivalves were first sighted on Lake St Clair in 1988. Since then they have expanded their territory to the entire Great Lake system. They were able to do this by attaching themselves to boats with strong, sting-like strands of protein called byssal fibers. Zebra mussels are microscopic as larvae and are able to float in the currents for a couple weeks before settling. Once settled, the juveniles are less than a millimeter in length, making them easy to miss. A single boat carrying one male and one female can infest an entire body of water. Females produce around 1 million eggs a year, causing their populations to exponentially increase (information about the zebra mussel life cycle can be found at http://el.erdc.usace.army.mil/zebra/zmis/zmishelp4/life_cycle.htm). Today, there are between 450 trillion and 1 quadrillion invasive mussels in Lake Michigan alone! Considering there are only 10 trillion cells in the human boy, this is a lot of mussels.

Modeling their population in a simple exponential model,
N(t)=N(0)e^rt
assuming 2 zebra mussels sparked the colony in 1988, and using the conservative estimate of 450 trillion, the growth rate for Lake Michigan is 137% a year! At this rate they will reach 21 quintillion (that's 18 zeros!) by 2020.





Where there are zebra mussels there are also quagga mussels (Dreissena rostriformis bugensis). Quagga mussels are a close cousin to the zebra mussel. They also came here in 1988, but they have taken longer to establish themselves in the Great Lakes. Now that they have made it to the lakes, however, they are starting to displace zebra mussels. Quagga mussels require water with less dissolved oxygen, so they are able to survive at greater depths than the zebra mussel. They are also able to attach to soft surfaces, such as sand or silt, while zebra mussels need a hard surface. Zebra mussels do have one advantage, however; the byssal threads they produce grow twice as fast and are much stronger than the quagga’s threads. So while quaggas are taking over in the lakes, zebra mussels still dominate quick moving waters such as rivers and streams. Less zebra mussels sounds great, but this isn’t a victory for our lakes. Quagga mussels have the same devastating effects zebra mussels do on ecosystems. In fact, they are so similar it took 3 years for scientists to realize there were two different species of mussels colonizing Lake Erie. As shown in the image below, the mussels do look very similar. Christine Moskell, from Finger Lakes Institute, explains quagga mussels and their relationship with zebra mussels very well in a paper that can be accessed at fli.hws.edu/sos/quaggaspdf.pdf.




Zebra and quagga mussels are so small it isn’t readily apparent why the public should care about their presence, or why they cost the tax payers between $1 - $5 billion annually. Despite their size, once established these mussels are able to collapse food chains, completely cover boats and other equipment, clog pipes, kill wildlife, and poison humans. Everyone should be aware of the implications of a zebra mussel colony, especially now that they are headed West.



The fishing industry is collapsing in the Great Lakes. Over the past decade the walleye industry alone lost over $350 million. Sport fish are affected so drastically because zebra mussels over filter the water, causing phytoplankton and zooplankton populations to drop more than 50% in most cases (see population map of Lake Michigan below). Without plankton, the small shrimp-like crustacean, Diporeia, is unable to find food and their populations consequently decline. Diporeia are important because they are the food source for many small fish who then feed the larger game fish. The decline in sport fish combined with the large costs of property damage (see above) is putting an immense strain on the Great Lake’s fishing industry; in the last decade many small fisheries have been forced to sell or go bankrupt.