PORTLAND – An environmental organization is looking for more citizen scientists to expand water-quality testing in Casco Bay.

Friends of Casco Bay, a nonprofit group, needs around 20 volunteers to monitor nutrients as part of an effort to preserve the bay’s environment.

Volunteers in the past have monitored the bay’s temperature, salinity, clarity and oxygen levels. The new testing will measure nutrients like nitrogen and phosphorus, which are contained in fertilizers, wastewater and other runoff.

“This is the beginning of a long-term database,” said Dr. David Townsend, an oceanography professor at the University of Maine who is helping with the testing. “In 10 to 20 years, we can spot trends. If the water gets bad, we will know how we got there.”

The expanded testing is important because excessive nutrients can cause invasive species to grow or create algae blooms, which reduce oxygen levels for other sea life, said Peter Milholland of the Friends of Casco Bay.

“Once you upset the balance, then you have problems,” Milholland said. “You start seeing the loss of lobsters, fish and everything. Eventually, they all die off.”