AUBURN, Maine (NEWS CENTER) -- After months of testing, scientists say it's phosphorus that's killing fish in Lake Auburn, but the root of the problem is still unclear.
Last summer, about 200 lake trout died on the lake, 150 more than the year before.
According to the most recent report, elevated phosphorus levels have been allowing algae to grow, decreasing oxygen levels in the water.
While the phosphorus levels have killed fish, it has not contaminated public drinking water.
"We're talking such low levels of concentration, we have not approached a level that would concern public health," said John Storer, Superintendent of the Auburn Water & Sewerage Districts.
Lake Auburn is the public drinking water source for more than 45,000 people in the region.
The study contains several theories to explain the increase in phosphorus, but draws no definitive conclusion.
It could be because of intense storms in 2011 and 2012, whcih disrupts the water and adds phosphorus.
It could also be because of a new form of algae to the lake, called gloetrichia. This type of algae interacts differently with the phosphorus already present in the lake, and could elevate levels.
Ultimately, scientists have not found a "smoking gun," Storer said.
He said the next step is to pursue the permitting process for algecides, which would kill algae if the problem comes back.
On site testing has also increased. Scientists in the lab on Lake Auburn used to test the algae once a month. Now, they're testing three times a week.
Storer said the district wants to address the problem before it grows.
If phosphorus levels continue to rise, they might have to invest in more water treatment, "which could double or triple the water rates that we currently have," Storer said.