Zebra mussels (Dreissena polymorpha) are small freshwater bivalves native to the Ponto-Caspian region of Europe. Since their arrival in the 1980s, they have become one of the most damaging invasive species in North American aquatic systems. Their rapid proliferation and ability to attach to submerged surfaces have caused severe ecological disruption, including the displacement of native species, and massive economic costs from clogging water intake pipes and other infrastructure. Despite this damage, the mussels possess powerful filtering and bioaccumulation capabilities that suggest a hypothetical, highly controlled utility for improving water quality in specific, isolated contexts.
Mechanism of Water Clarification
The primary impact of zebra mussels is their ability to clear the water column through extremely efficient filter feeding. As suspension feeders, the mussels draw water into their shells and filter out suspended particles using ciliary action on their gills. A single adult zebra mussel can filter a substantial volume of water, with estimates ranging from 5 to over 400 milliliters per hour. This intensive filtration removes fine silt, detritus, and phytoplankton, resulting in dramatically increased water clarity and greater light penetration in invaded lakes.
The particles they consume are either digested or rejected as dense clumps called pseudofeces, bound together with mucus. Because pseudofeces are heavier, they quickly sink, transferring organic matter and nutrients from the open water column to the lake bottom, or benthic zone. This nutrient shift is the primary mechanism behind the observed water clearing effect.
Bioaccumulation for Pollutant Removal
Zebra mussels possess a remarkable capacity for bioaccumulation, allowing them to sequester chemical pollutants within their body tissues. This process differs from general filtration, involving the concentration of substances dissolved in the water or bound to ingested particles. They accumulate heavy metals, such as cadmium, chromium, lead, and mercury, as well as persistent organic pollutants (POPs) like polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs).
The concentration factor can be significant; for organic pollutants, the level in the mussel tissue can be up to 300,000 times higher than in the surrounding water. This ability has made the mussels useful as biomonitors for tracking pollution levels. If deployed in a closed system and then harvested, they could function as a biological “sink” to remove these toxic substances from the active water cycle. However, this accumulation also poses a risk, as the toxins can be passed up the food chain to predators consuming the contaminated mussels.
Impact on Phytoplankton and Algal Blooms
Zebra mussel grazing exerts pressure on the lake’s primary producers, the phytoplankton, which is often viewed as a potential solution for harmful algal blooms (HABs). By consuming vast quantities of these microscopic organisms, the mussels reduce the overall phytoplankton biomass, decreasing the frequency and intensity of some nuisance blooms. This consumption disrupts native food webs, as zooplankton and small fish are starved of their usual food source.
The interaction becomes complex when dealing with cyanobacteria, commonly known as blue-green algae, which cause many toxic HABs. Research shows that zebra mussels selectively reject the toxic strains of cyanobacteria, such as Microcystis, often spitting them out as unpalatable pseudofeces. This selective feeding removes the non-toxic phytoplankton that would otherwise compete with the toxic strains for nutrients and light. Consequently, the mussels inadvertently create an environment where the toxic, rejected cyanobacteria gain a competitive advantage, leading to increased biomass and higher concentrations of associated microcystin toxins in the water.
Constraints on Controlled Usage
Despite their biofiltration and pollutant-sequestering mechanisms, the widespread use of zebra mussels for environmental remediation is limited by their invasive nature. A female zebra mussel is highly fertile, capable of releasing up to five million eggs per year, allowing populations to explode rapidly and colonize new areas. This reproductive capacity, coupled with a lack of natural predators, makes any open-system deployment unmanageable.
The structural damage they cause is a major constraint. Their attachment to hard surfaces can clog water intake pipes, screens, and cooling systems in power plants and municipal facilities, leading to costly maintenance and shutdowns. The ecological harm, including the starvation of native filter feeders and food web disruption, argues against their use in natural bodies of water. Therefore, any beneficial applications, such as a large-scale biofilter or a bioremediation system, must be strictly confined to highly isolated, controlled environments. These include specialized wastewater treatment facilities or localized experimental enclosures where the mussels can be introduced, monitored, and safely removed without risk of escape.