Both positively and negatively.
Zebra mussels are molluscs that are native to Eurasia. These muscles were brought to the great lakes of the US through ships' discharges. The zebra muscles are currently considered a invasive species in the US mainland. They negatively impact the native species by feeding and filtering out the algal bloom which is food source for the native fishes in the lakes. However they also positively impact the lake ecosystem by filtering out the waste or dirt particles that cloud the lakes. These are then excreted as a dense mucus droppings that sink to the bottom of the lake. This becomes a food source for the native species that live in the lake floor.