<P> A major decrease in the concentration of dissolved oxygen was observed in the Seneca River in central New York in the summer of 1993 . This decrease was due to extremely high concentrations of zebra mussels in the watershed . Additionally, the Seneca River had significantly less chlorophyll in the water, which is used as a measure of phytoplankton biomass, due to the presence of zebra mussels . </P> <P> The cost of fighting the pests at power plants and other water - consuming facilities is substantial, but the exact magnitude of the damage is a matter of some controversy . According to the Center for Invasive Species Research at the University of California, Riverside, the cost of management of zebra mussel in the Great Lakes alone exceeds $500 million a year . A more conservative study estimated total economic costs of $267 million for electric - generation and water - treatment facilities in the entire United States from 1989 through 2004 . </P> <P> Zebra mussels are filter feeders . When in the water, they open their shells to admit detritus . As their shells are very sharp, they are known for cutting people's feet, resulting in the need to wear water shoes wherever they are prevalent . </P> <P> Since their colonization of the Great Lakes, they have covered the undersides of docks, boats, and anchors . They have also spread into streams and rivers throughout the US . In some areas they completely cover the substrate, sometimes covering other freshwater mussels . They can grow so densely that they block pipelines, clogging water intakes of municipal water supplies and hydroelectric companies . Zebra mussels do not attach to cupronickel alloys, which can be used to coat intake and discharge grates, navigational buoys, boats, and motors where the species tends to congregate . </P>

What happened to the number of zebra mussels in ontario rideau river between 1993 and 1996