How do zebra mussels cause ecological harm in the Great Lakes?

Zebra mussels cause ecological harm primarily by filtering plankton from the water, which effectively removes this vital food source for native aquatic organisms, such as fish and other species that rely on plankton at various life stages. As zebra mussels proliferate, they can filter vast amounts of water, significantly reducing the concentration of phytoplankton and zooplankton. This decline in plankton populations can lead to adverse effects on the entire food web of the Great Lakes ecosystem, disrupting the balance and leading to declines in fish populations and other aquatic organisms that depend on these primary producers and consumers.

The other options do not accurately represent the primary ecological impact of zebra mussels. For instance, while they do affect the habitat, competition for physical space is generally not as significant a threat as their impact on food availability. They do not release toxins; instead, their presence can promote changes in water quality and ecological dynamics. Lastly, their impact on sediment levels is not their main harm; rather, it is their filtering capacity that alters the nutrient dynamics and ecological balance in the lakes.