Post by Ryan Smazal, Maritime Preservation Intern at the Wisconsin Historical Society, and Sara Fox, Aquatic Invasive Species Outreach Assistant at UW-Extension. Also on Wisconsin Sea Grant’s Great Lakes Takes.
As many people might know, the Great Lakes house about a fifth of the freshwater supply for the entire world. A less commonly known fact is that the Great Lakes contain more than 700 shipwrecks. Among those shipwrecks live over 3,500 plant and animal species who generally coexist with the shipwrecks in peace. However, the introduction and spread of aquatic invasive species might threaten existing ecosystems and the preservation of shipwrecks in the Great Lakes.
The Great Lakes were once a massively important trading region because of their connection to inland rivers and lakes which helped transport goods. The shipping industry took advantage of this connection, as well as the Great Lakes’ many natural ports. For various reasons, including gales, scuttling, and fires, some of the same ships from this era remain at the bottom of the Great Lakes today. Fortunately, the cold water and low salinity of the Great Lakes create an ideal environment for preserving these shipwrecks. Historians have interest in uncovering new shipwrecks as a way to investigate old trade routes, discover cargo, and study differences in ship design. Additionally, fish tend to make their homes near shipwrecks, so divers enjoy the opportunity to explore freshwater ecosystems.
The La Salle has been preserved at the bottom of Lake Michigan since 1874. Photo credit: Tamara Thomsen
Although new wrecks are being discovered every year, there’s concern to preserve the ones we’ve already found. An example historians look to is that of The Alvin Clark, a ship that began to deteriorate as soon as it was removed from its cold and wet home in Lake Michigan. Apart from the natural preservation system the lakes provide, there are intervention-related options for conservation such as polyethylene glycol treatment or designating a wreck as a National Historic Place. Unfortunately, options like these tend to be expensive and time consuming.
Despite efforts being undertaken to preserve the shipwrecks, they face a new potential threat: aquatic invasive species. Today over 180 nonnative species, both plant and animal, inhabit the ecosystems of the Great Lakes. The vessel most likely responsible for the introduction of many of these invaders are cargo ships that enter through the St. Lawrence Seaway. Large cargo ships uptake water at the beginning of their journey to provide stability on the high seas. When the ships arrive to their destination and it’s time to make room for cargo, they release the necessary amount of ballast water, along with aquatic animals and plants that may have hitched a ride. Some of these foreign species die immediately, but those that survive can thrive due to their reproductive capabilities or lack of native predators.
Aquatic invasive species that might pose a threat to the preservation of shipwrecks include mussel species like the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena rostriformis bugensis). Here’s why:
- Zebra and quagga mussels begin their lives as microscopic veligers, slowly taking in nutrients and growing until they become heavy enough to sink. Once they begin to sink, they use feet-like assemblages called byssal threads to stick on to the first hard substance they encounter whether it be a boat propellor, a dock, or a shipwreck. Many mussels on the same structure could cause a heavy pile up, and sometimes can corrode certain metals.
- Both zebra and quagga mussels are filter feeders. Did you know a single zebra mussel can filter up to a liter of water a day? Mussel respiration also produces carbon dioxide. Both of these characteristics can affect water quality and alter waterbodies in a way that may not be favorable to shipwreck preservation.
- Zebra and Quagga mussels reproduce quickly and in large quantities. A female zebra mussel can produce as many as one million eggs in a season! Once introduced to an area, invasive mussels can take over and continue to affect water quality and damage underwater structures for decades.
Zebra mussels piled up on an old tennis ball. Photo credit: Paul Skawinski
Even though there are over 180 kinds of nonnative species on the Great Lakes, we shouldn’t be discouraged. It’s important to still be conscious of the cleanliness of our watercraft vehicles. Not every patch of every Great Lake has a zebra mussel presence and shipwrecks could exist in an acre of water not yet infested. Aquatic invasive species can be accidentally transported in boat motors, livewells, and on fishing and diving gear. Individual lake users can help stop the spread of invasive species by following the WDNR recommended prevention steps:
INSPECT boats, trailers, and equipment
REMOVE all visible plants, animals, and debris
DRAIN water from boats, motors, bilges, and live wells before transporting away from a water or entering Wisconsin
NEVER MOVE live fish away from a waterbody
If every boater were conscious of their ability to spread invasive species and took actions to prevent the spread, existing populations could be easily contained. If populations were contained forever, shipwrecks in uninfested areas could be preserved and studied for decades to come.
For more information about aquatic invasive species, consult the NAS database: https://nas.er.usgs.gov/queries/default.aspx
For more information about shipwrecks in the Great Lakes, consult: http://www.wisconsinshipwrecks.org/