ALEXANDRIA, Minn. (AP) — Scientists still don’t have a clear answer on whether invasive species are destroying Minnesota’s lakes more than 20 years after they started spreading in the state’s waters.

Minnesota Public Radio News reports that researchers are struggling to understand the impact that species like zebra mussels have on the underwater ecosystem. Zebra mussels are now found in more than 330 lakes and rivers statewide.

Nick Phelps runs the Minnesota Aquatic Invasive Species Research Center. He says some lakes with invasive species will have low-level populations, while others will see massive impacts to their ecosystem.

Scientists and anglers worry when invasive species enter a lake because they start a chain reaction by eating microscopic algae called phytoplankton. Less phytoplankton means less food for tiny animals like zooplankton. Some fish depend on zooplankton.