Cladophora and botulism are both naturally existing organisms found in the Great Lakes, including Lake Michigan. A commonly held belief is that all native organisms are good and all non-native organisms are bad; however, native species can also be considered a nuisance. As invasive species continue to be introduced into the Great Lakes, the ecosystem has continued to change and the existing habitat as been altered, impacting the native species, including Cladophora and botulism.
Native to the Caspian Sea region, both zebra and quagga mussels were introduced into the Great Lakes through the ballast water in cargo vessels. In their larval form, they are about one millimeter long and were bypassed during any basic filtering methods utilized in the shipping industry at the time. Following the introduction of zebra mussels in the early 1990s, the clarity of the water in Lake Michigan began to steadily increase, and continued to increase following the introduction of quagga mussels in the early 2000s (see Graph 1, courtesy of Inland Seas Education Association). This increase in clarity was a direct result of the filtering capacity of these mussels. Both organisms draw water in through a siphon (similar to a straw), consume the plankton, and excrete the filtered water.
The clearer water stemming from these mussels has had many impacts on the Lake Michigan ecosystem, including an increase in near-shore habitat for Cladophora, a filamentous algae found on hard substrates along the lake bottom in Northern Michigan waters. The basic necessities for all plant growth, including algae, are the availability of water, nutrients and sunlight. Excessive nutrient inputs from human activity have been linked to pronounced growths of Cladophora, as has the increased photic zone, stemming from the increasingly clear water. The photic zone is defined as the area where there is enough sunlight for photosynthesis to take place. With the increased water clarity, more of the lake bottom is included in this zone, creating ideal habitat for Cladophora.
Botulism, is a bacterium found in the Great Lakes that under the appropriate conditions can create a neuromuscular toxin that affects waterfowl, fish, amphibians and some mammals. Increased water temperatures, combined with anoxic (oxygen deprived) conditions near the bottom lead to the production of the toxin by the bacterium. Current research has shown that Cladophora mats can act as a reservoir for the bacteria, and possibly even act as a promoter of its growth.
The increased presence of the toxin in Northern Michigan has resulted in large bird die offs along the shores of Lake Michigan. Loons, mergansers, cormorants, and gulls are all susceptible to botulism intoxication, because fish constitute a large percentage of their diet. Infected fish flounder and swim erratically near the surface, making them ideal prey. Once consumed, the infected fish pass the toxin on. In birds, type E botulism results in paralysis. Waterfowl may drown because they are unable to hold up their head or may drag their wing(s) while standing. Sleeping Bear Dunes National Lakeshore as well as other agencies in NWMI have taken part in beach surveys looking for waterfowl killed by botulism. In 2006, about 3,000 fish eating birds died in northeastern Lake Michigan from Botulism. It is difficult to definitively diagnose Type E botulism as the cause of death, because a blood sample from a live specimen must be tested.
As this issue continues, more research will go towards a better understanding of the relationship between all the parts of this issue. Once these pieces are understood, we will be better equipped to develop an effective management plan that will consider all factors.
–Emily Shaw, Contributing Writer