One way zebra mussels impact ecosystems is by reducing the amount of phytoplankton. Phytoplankton are consumed by Zooplankton. Zooplankton are small bodies of organisms that float in the water. Zebra mussels consume phytoplankton, diminishing Zooplankton's food supply. Zooplankton block the sun's light, so when absent, the sun increases the temperature of the water. Many organism are very sensitive to change of temperatures​ in their environments.
Also, clearer water allows more sunlight to reach shallow areas where producers, like plants, start to thrive, dramatically changing the ecosystem.
Phytoplankton also produce Oxygen during photosynthesis, but with zebra mussels dminishing the amount of phytoplankton, Oxygen levels can drop to dangerous levels.