BACKGROUND

The Hudson River estuary stretches about 150 miles from New York Harbor up to Troy, NY. This is about half the total length of the river. Although the water isn't salty at Troy, the Hudson here "feels the tide" and water levels rise and fall at high and low tides (see also http://www.ecostudies.org/research_hres.html and http://www.dec.state.ny.us/website/hudson/hre.html).

Zebra mussels were first seen in the Hudson River in 1991 (see Figure Set 1 for general information about zebra mussel introduction to the U. S.). Within little more than a year the biomass of the mussels was greater than that of all other heterotrophic animals in the Hudson. Their filtration rates are estimated at about 1-3 days for the entire freshwater estuary, and as a result, phytoplankton and small zooplankton densities have dropped to 10-20% of pre-invasion levels. The change in the Hudson as a result of zebra mussels has been large, widespread, and likely long lasting - few other human induced ecological changes to the river have been so dramatic.

What has happened in the Hudson has been seen in other places where zebra mussels have become established. Lakes and rivers colonized by the mussels often see 50-75% declines in phytoplankton and small zooplankton biomass, rise in water clarity of 50-100%, drop of more than 50% in filter-feeding zooplankton and native bivalves, and increase in macrophyte beds and animals associated with zebra mussels.

In this exercise students describe and analyze several figures that help them understand the changes taking place in the Hudson River as a result of zebra mussel introduction.