Ausable River Association Water Quality Associate Leanna Thalmann demonstrates how a she uses a Kimmererer — the tool that helps her to sample water quality at the bottom of Mirror Lake — at the lake on Wednesday, May 18. (News photo — Lauren Yates)

LAKE PLACID — Mirror Lake completely turned over this spring for the first time since 2020, and for only the second time in the last five years.

Turnover is a process that most lakes go through each fall and spring, according to the Ausable River Association. It’s the natural process of a lake’s cooler, dense water sinking to the bottom of the lake and mixing with deeper water. Water is at its densest at around 39 degrees Fahrenheit, so when the ice melts and the top layer of cold lake water starts to warm up a bit, it drops to the bottom of the lake and mixes with the deeper water. Turnover is important for large lakes because the process replenishes oxygen and distributes nutrients throughout the lake.

In recent years, Mirror Lake’s turnover has been disrupted by high concentrations of salt in the lake. When a lake doesn’t turn over, there’s less oxygen on its floor, which can lead to a few adverse effects on the lake’s biome — like algal blooms. In Mirror Lake, turnover is especially important for rainbow and lake trout, which like to hang out in cooler water at the lake floor.

Mirror Lake failed to turn over in 2017, 2018, 2019 and 2021 because of a high concentration of salt in the lake — a direct result of road salt use, according to Ausable River Association Water Quality Associate Leanna Thalmann. Saltwater is dense and settles at the bottom of the lake, which can prevent mixing. This year, salt levels were lower and storm events after Mirror Lake’s ice went out on April 14 helped the lake turn over.

Adirondack Watershed Institute Senior Research Scientist Brendan Wiltse said in March that the institute, which studies water quality in Mirror Lake alongside the Ausable River Association, has recorded an increasing reduction in salt concentration in the lake over the last few years. It’s been hard to pin down exactly why salt levels are dropping in the lake, according to Wiltse and Thalmann.

Kayakers enjoy Mirror Lake on Friday, May 20. (Staff photo — Andy Flynn)

Warmer weather conditions as the climate changes could mean that less road salt is being put down, though Thalmann said this past winter was a relatively cold one. The Main Street construction project included an overhaul of the village’s stormwater system, which used to direct stormwater runoff directly into Mirror Lake. The new system directs runoff into water catchments, which hold the runoff in a basin until the basin eventually absorbs the water. The town of North Elba and the village of Lake Placid have also committed to reducing their road salt use, which could be a contributing factor in the lake’s improvement, Thalmann said.