LEWISTON — Higher-than-normal phosphorous levels in Lake Auburn were behind a fish-killing algae bloom this past summer, but water engineers Tuesday were still working to determine what caused the phosphorous levels to spike.
Water-quality engineers said phosphorus levels could be blamed on runoff into the lake's feeder streams and ponds or by a new species of alga.
They also are trying to determine whether the bloom was an isolated event or a troubling trend.
"What we know is that 2011 and 2012 were bad years," said Kenneth Wagner, a certified lake consultant for Water Resources Inc. "It's not something that had happened before and it seemed to happen very suddenly. We would expect to find a smoking gun, a cause that explains why it happened. But we have not found that yet."
Wagner was among a team of water-quality experts and engineers hired by the Lake Auburn Water Protection Commission last summer to investigate the algae bloom and come up with possible solutions.
The team gave an initial report to the commission and a group of residents Wednesday night at Lewiston City Hall.
Engineers found both elevated phosphorous levels and evidence of waterside erosion at several sites along Lake Auburn's feeder tributaries. These include Little Wilson Pond, the Basin and Townsend Brook.
They also found evidence of gloeotrichia, a kind of alga that's new to Lake Auburn. Unlike other algae, it sinks to the bottom of a lake and can bring phosphorous to higher levels. Other algae, more common in the lake, feed on that phosphorous and grow.
One or both could have caused the bloom, Wagner said. "But we don't know that it's not a one-time event. There could be some cause we are not aware of that's spiking the phosphorous elevations and it may go away."
Water-quality officials discovered more than 200 dead trout along the shore or floating close to the shore in mid-September, said Jim Pescatore of the water-quality engineering firm CDM-Smith. That's 150 more fish than reported earlier in the summer.
High phosphorous levels in the lake encourage common blue-green algae to grow. As the algae grows and dies, it sinks in the lake, decaying and using up oxygen in the process.
That layer of the lake, where the trout live, is already low in oxygen late in the summer. The decaying algae use up available oxygen, suffocating the trout.
Pescatore said the engineers hope to begin presenting short-term and long-term solutions in May when they release a second-phase draft. Short-term fixes could involve adding copper- or peroxide-based algaecides to the water, dredging the lake bottom to remove phosphorous-rich soil or adding aluminum to the lake. Aluminum binds with phosphorous in the water and sinks to the bottom.
Long-term solutions could involve stronger regulations around the lake and the ponds and streams that feed it.