The zebra mussel, a foreign species that has ravaged the Great Lakes and spread to much of the rest of the United States, has turned up in the Chesapeake Bay watershed.
Scientists found the Caspian Sea native in a New York reservoir at the northern fringe of the Susquehanna basin, and some adults have been spotted moving downstream of the impoundment.
The rapidly reproducing mussels can quickly dominate systems where they become established, filtering huge amounts of algae from the water. In parts of Lake Erie, it’s possible to clearly see the bottom in 50 feet of water where little more than a decade ago, visibility was only a fraction of that.
But, cautioned Tom Horvath, of the State University of New York – Oneonta Biological Field Station, “they definitely shouldn’t be thought of as a good thing.”
They can quickly outcompete native species, and research from the Hudson River even suggests that they could complicate the Bay’s nutrient reduction efforts.
Horvath’s colleagues discovered the zebra mussels last fall in the Eaton Brook Reservoir. The reservoir, in Madison County, NY, is near the northern tip of the Chenango River, a tributary of the Susquehanna, and is about as far away as one can get from the Bay while still being in the watershed.
That toehold could provide the launching platform from which the mussels begin working their way downstream.
Although zebra mussel larvae, known as veligers, were spotted in the Susquehanna near Binghamton, NY, in the early 1990s, adults had never been found in the watershed, and veligers were never again seen. “But now with this population in Eaton Brook, there is going to be a full-time source pumping veligers into the watershed,” Horvath said.
Based on the size of the zebra mussels in the reservoir, Horvath said they likely arrived in 1999. Surveys this year found the mussels were still present, and multiplying. “The population seems to be doing pretty well this year,” Horvath said.
Zebra mussels are a freshwater species and scientists believe salinities in most of the Bay are generally too high for them to survive, although low salinity portions may support the invader.
While the Bay may not be conducive to zebra mussels, scientists and state officials have long been concerned that they could become widely established in the region’s rivers, where they would bring a host of problems, becoming a nuisance for boaters, an added expense for utilities and other water users, and a threat to aquatic life.
Zebra mussels were accidentally introduced to the Great Lakes through the release of ship ballast water in the mid-1980s. Since then, their numbers have rapidly multiplied.
Adult mussels build colonies that number tens of thousands per square meter. In the Great Lakes, buoys colonized by zebra mussels have sunk under their weight. They have also colonized shipping locks, preventing the gates from opening and closing.
While there may be some positive aspects — besides clearing the water, they are a food source for diving ducks, crayfish and a few other species — most of their impact has been negative. “You can think of some benefits. but the potential impact, in my opinion, would far outweigh the potential good that it could have,” Horvath said.Dense colonies of zebra mussels have clogged water intake pipes used by municipalities, industries and utilities, in some cases forcing them to shut down. When they attach to the hull of a boat, they increase the vessel’s drag. If sucked into the water intake, the engine may overheat. In some areas, beachgoers have to wear boots to protect their feet from the sharp edges of dead zebra mussels that have washed up on shore.
They outcompete native clams and mussels, and in some areas have eliminated entire populations. Many freshwater mussel species are already rare because of decades of pollution, and biologists worry that the zebra mussel could be the last straw for some, pushing them into extinction.
By filtering so much algae from the water, scientists believe zebra mussels may be altering the food chains in some areas, and reducing what is available for other species, including fish, to eat.
Since becoming established in the Great Lakes, they have rapidly expanded to other areas, including much of the Mississippi River basin, the Erie Canal, the Hudson River and other areas.
Their biology facilitates their spread. A typical female may produce 30,000 to 40,000 eggs per year, although some produce up to a million. Fertilized eggs develop into free-swimming larvae known as veligers, which swim for two to three weeks in search of food before settling to the bottom and attaching themselves to hard surfaces with tiny threads known as byssal fibers.
But zebra mussels seem to prefer lakes and impoundments to flowing rivers. Horvath said studies tend to show that moving streams usually can’t maintain a self-sustaining zebra mussel colony; they must constantly be resupplied with individuals from upstream populations.
Streams, however, can serve as distribution highways which allow zebra mussels to gradually spread from one suitable area to another backwater or reservoir downstream where a self-sustaining population can be established. In a sense, Horvath said, the zebra mussels can hopscotch their way through a system.
Already, Horvath has found adult zebra mussels below the Eaton Brook reservoir, but their numbers rapidly decline. A bigger concern is that veligers will be able to travel further downstream, possible settling at more suitable habitats.
“But,” Horvath said, “there has not been enough work in rivers and streams to say for sure what to expect here.”
Some clues come from the Hudson River, where research led by David Strayer at the Institute of Ecosystem Studies found that zebra mussels can dramatically alter river ecology. After they were discovered in the river in 1991, their numbers rapidly expanded, and they became the dominant plankton consumer in the Hudson.
In fact, the zebra mussels increased the filtering of river water from 3 percent a day to 100 percent a day. As a result, the biomass of phytoplankton and small zooplankton de creased 80–90 percent.
At the same time, dissolved oxygen levels declined significantly as respiration by the mussels consumed oxygen faster than it could be replaced.
Meanwhile, the river’s export of nutrients, especially phosphorus, increased sharply. By decreasing the amount of nutrient-consuming phytoplankton in the water, the nutrients went unused down the river. That’s something that could complicate nutrient reduction efforts in the Bay watershed.
Within the watershed, some streams would be off-limits to zebra mussels. They don’t inhabit acidic water, and they require relatively high calcium levels to build their shells. That means smaller streams impacted by acid rain or acid mine drainage would likely be unsuitable for zebra mussels. But scientists say most larger rivers would have water quality suitable for zebra mussels.
One problem with their presence within the watershed, Horvath said, is that they may be rapidly spread by human recreational activities. Zebra mussels easily move from one place to another by attaching themselves to boats and trailers. Their larvae can even be transferred in bait buckets.
“If they do establish in the Susquehanna, there is always that chance that the Susquehanna becomes a source for other nearby waterbodies, just through recreational activities,” Horvath said.
But he said the discovery does give time for states to step up efforts to hold the line on their spread through educational efforts aimed at boaters, fishermen and others. In addition, industries and other water users have time to explore different treatment systems that protect water intakes to help prevent zebra mussels from clogging their pipes.
“It’s not a thing where all hopes are lost,” Horvath said. “There there are new technologies that may show potential for limiting their population numbers. But we have to continue to watch and get a better understanding of their movement through flowing water systems.”
How to Help Halt Zebra Mussels’ Spread
There are several precautions that individuals around the Bay can take to help prevent the spread of the zebra mussel.
- Know how to identify a zebra mussel. They look like small clams with a yellowish or brownish D-shaped shell, usually with alternating dark– and light-colored stripes. (Hence the name “zebra”) They can be up to 2 inches long, although most are less than one inch. Zebra mussels grow in clusters containing many individuals and are usually found in shallow (6–30 feet), algae-rich water. They are the only freshwater mollusk with the ability to firmly attach themselves to solid objects — submerged rocks, dock pilings, boat hulls, water intake pipes, etc.
- Check your boat, motor, live wells and boat trailer for hitchhiking mussels.
- Do not transfer your boat from waters infested with zebra mussels into uninfested waters without inspecting it and removing all mussels by scraping and washing in hot water (140ºF or hotter).
- Flush your engine with uncontaminated water to remove mussel larvae.
- Always drain bilges, live wells and motor wells before leaving mussel-infested waters.
- Boats and trailers should dry in the sun for at least two to four days before being transported from infested to uninfested waters. Boats should be dry docked for two weeks, if at all possible.
- Inspect bait buckets and containers for tiny mussels and rinse these with very hot, uncontaminated water.
- Because of the possibility of contamination, leftover bait should not be transported from infested to uninfested waters. Give unused bait to another angler or place it in a garbage container at the launch site.