Once at my sampling location, I anchored my canoe and got to work. I wasn’t expecting to find anything intriguing during this visit. But as I collected data on the profile of the water column, a level of sampling only completed on Mirror Lake
Once at my sampling location, I anchored my canoe and got to work. I wasn’t expecting to find anything intriguing during this visit. But as I collected data on the profile of the water column, a level of sampling only completed on Mirror Lake a handful of times in its history, I noticed something curious. The conductivity of the water, an indirect measure of the amount of dissolved salt, was much higher at the bottom of the lake than the top. There was also a surprisingly low amount of dissolved oxygen at the bottom considering spring turnover should have occurred about six weeks earlier.
These two phenomena, high salt and low oxygen at the bottom of the lake, presented an intriguing situation. If the lake had mixed just weeks earlier, why was salt apparently built up at the bottom of the lake and why wasn’t there higher oxygen concentrations? The obvious answer was that the lake hadn’t mixed that spring as we would have expected it to. It would take two years, over a thousand measurements of the water column, hundreds of hours, and dozens of water samples to confirm the answer to this very important question. We now know that dense salt-laden water running out of the stormwater system circling the lake accumulates at the bottom of Mirror Lake throughout the winter. We also know that the density differences in the water column due to salt substantially increase the amount of energy required to mix the lake in the spring. In 2017, we observed enough salt build up over the winter to prevent the lake from mixing.
Mirror Lake’s name alludes to one of the reasons why it is particularly susceptible to road salt inhibiting its natural mixing regime. With its long axis oriented perpendicular to the primary direction of the wind and being sheltered by a large hill on the windward side of the lake, the surface of Mirror Lake is often still and calm even on a moderately breezy day. This lack of wind exposure allows the lake surface to frequently offer perfect reflections of the surrounding mountains, but it also means the lake receives very little wind energy to help it mix. Instead, lake mixing depends on water temperature driven density differences. Without the addition of salt, the change in density due to water temperature would be enough to mix the lake. But with the salt, it isn’t enough. And so, without additional wind energy, the lake doesn’t mix.
As a result of the lack of mixing, the salt-laden water gets trapped at the lake bottom until the fall, the oxygen in that water isn’t replenished, and it gets used up by respiration from bacteria, fish, and other aerobic organisms. This poses significant problems for the lake and, if the lake were to stop mixing in the fall, there would be a cascade of negative effects over the following winter that would threaten much of the life in the lake. Our addition of salt to sidewalks, parking lots, and roads has d