“Based on previous studies, scientists estimated how much plankton the zebra mussels could filter out of the water. (Phytoplankton and zooplankton are microscopic organisms that are two critical components of the river's food web.) The numbers suggested the impact of zebra mussels on the river could be huge.”
Since large zebra mussels tend to feast on zooplankton, the increase of zooplankton can spread the concern of the large zebra mussels' population.
Text evidence (Paragraph 3): If there were fewer large zebra mussels, it made sense that there was more zooplankton. That's because large zebra mussels feed on bigger food particles like zooplankton. Smaller zebra mussels can only eat smaller particles like phytoplankton and bacteria.
