AUBURN — Work repairing erosion problems around Lake Auburn and the tributaries that feed it might be the best long-term fix for avoiding future algae problems, say Twin Cities water officials.

Members of the Lake Auburn Watershed Protection Commission on Thursday received copies of a study of the lake and the surrounding watershed that sought causes of a fish-killing algae bloom that occurred in the summer of 2012.

The study said that increasing phosphorus levels were responsible for the 2012 algae blooms. Water officials from Lewiston and Auburn knew that soon after the 2012 bloom and began looking for a reason.

John Storer, superintendent of the Auburn Water District, said he was disappointed the study did not spell out a single cause for the rise in phosphorus levels.

“As an engineer, it’s a little frustrating that they have not been able to find a smoking gun,” Storer said.

Water quality officials discovered more than 200 dead trout along the shore or floating close to the shore in mid-September 2012. They blamed the kill on high phosphorus levels in the lake.

Phosphorus in the water encourages algae to grow, using up available oxygen in the water and suffocating the fish. The effect is especially notable in the cooler water at the bottom of the lake, where trout like to be in the hot summers.

According to the study, heavy rains in 2012 and warm temperatures later in the year made the algae bloom worse. The study noted that rainfall last summer was about 8 inches below the seasonal average and that the fish-killing algae bloom did not return.

Storer said now the concern is where the phosphorus came from and what to do with the phosphorus that’s already in the lake.

“We have slightly higher phosphorus in the lake than we would like,” Storer said. “We’ve got it there; we’ve got to watch and monitor it and we need to be prepared should that phosphorus cause any problems.”

Sid Hazelton, district engineer for the Auburn Water District, said the study outlined about 35 places around Lake Auburn and its feeder tributaries that can be improved to keep soil from eroding and polluting the water.

“There are culverts to be replaced, there are ditches to be fixed, there are filtration basins to be installed,” he said. “The priority ones are actually closer to the lake.”

They include eroding areas along Spring Road, Route 4 and Holbrook Road or other places where soil could be eroding into the watershed.

“There are actually quite a few that are outside Auburn, up into Minot and Turner,” Hazelton said.

The study calls out other fixes for the phosphorus problems beyond erosion control.

Storer said the district already has state and local approval to use copper sulfate to interrupt any algae blooms if they occur this summer, but that’s a short-term solution, at best. The study outlined two longer-term solutions to get rid of phosphorus for the next 20 years.

“The copper sulfate is like taking an aspirin to treat a headache,” Storer said. “These could actually go to the root causes of what’s causing the headache and help knock it down.”

One suggestion is to create a system to pump oxygen — either air or liquid — directly into the bottom of the lake, improving the deep-water habitat for fish in the long term.

The other solution would use a single treatment of alum to bind up existing phosphorus, making it sink to the bottom and out of the water column.

The oxygen solution could cost up to $4.2 million to start and $155,000 per year — about $4.95 million over the next 10 years in the most realistic scenario.

The alum solution could cost up to $6.2 million the first year in the most expensive estimate, with no other annual costs going forward. Realistically, it would probably cost about $4.1 million over 10 years, according to the study.

“We need to explore these a little more ourselves,” Storer said. “We need to see what we would need to get permits and work on the costs and how the various options would work going forward.”

staylor@sunjournal.com