Zebra mussels (Dreissena polymorpha) are small, striped, D-shaped freshwater mollusks native to the Caspian and Black Sea regions. They were unintentionally introduced to North America in the late 1980s, likely via ballast water from cargo ships discharged into the Great Lakes. Since their 1988 discovery in Lake St. Clair, they have rapidly spread, becoming a significant invasive species.
Direct Physical Dangers
Zebra mussels pose immediate physical hazards. Their sharp shells can cause cuts and lacerations to individuals walking barefoot near infested shorelines or swimming in affected waters. Boaters also risk injury from mussel-encrusted surfaces on docks or submerged structures. These cuts are painful and carry a risk of bacterial infection if not promptly treated.
Less commonly, prolonged contact with mussels or water containing their byssal threads may cause skin irritation or mild allergic reactions. Given the risk of cuts, wearing protective footwear in or near infested waters is a prudent measure.
Impact on Water Systems and Safety
Zebra mussels significantly affect water systems and infrastructure. As prolific biofoulers, they attach in dense colonies to underwater surfaces, including intake pipes for drinking water plants, power facilities, and industrial operations. This colonization obstructs water flow, reducing system efficiency and requiring costly maintenance to restore operation.
Their filter-feeding clarifies water but can paradoxically promote harmful cyanobacteria, or blue-green algae. By removing competing phytoplankton, mussels create conditions favoring these toxic algal blooms. These blooms produce dangerous toxins, like microcystins, posing health risks if ingested through drinking water and requiring extensive treatment.
The decay of large numbers of zebra mussels can also impart unpleasant tastes and odors to drinking water. As they decompose, they release organic compounds affecting water quality. Treatment facilities face increased complexities and costs to deliver potable water.
Economic and Recreational Consequences
Zebra mussels impose substantial economic burdens on industries and municipalities. Significant resources are allocated to monitoring, controlling, and removing infestations from infrastructure like water intake pipes and industrial cooling systems. Increased operational costs for utilities can translate into higher consumer bills or taxes.
Recreational activities are also heavily impacted. Sharp shells make beaches and swimming areas hazardous for bare feet. Boaters face equipment damage, as mussels encrust hulls, clog cooling systems, and foul engines, leading to costly repairs. Mussels attaching to boat lifts and docks further increase maintenance for property owners.
Beyond direct damage, zebra mussels disrupt aquatic food webs, negatively affecting native fish populations. This diminishes commercial and recreational fishing, impacting industries reliant on healthy fish stocks. Aquatic tourism may also decline as the appeal of infested waterways for fishing, swimming, and boating decreases.
Food Web Contamination
Zebra mussels can indirectly affect human health through their role in the aquatic food web, specifically by accumulating environmental contaminants. As filter feeders, they process large water volumes, accumulating toxins like persistent organic pollutants (PCBs) and heavy metals such as mercury in their tissues.
This process, known as bioaccumulation, means contaminant levels in mussels can be significantly higher than in surrounding water. When consumed by fish and other aquatic organisms, these toxins transfer up the food chain. Predators, including larger fish, then accumulate higher concentrations through biomagnification.
Humans consuming fish from infested waters face a potential, indirect health risk from these contaminants. Though mussels are not typically eaten by humans, the transfer of accumulated PCBs and heavy metals to harvested fish highlights a food safety concern. This emphasizes the broader ecological implications of invasive species.