For Fun / Holiday / Science / Travel

Its beginning to look a lot like meromixis

‘Tis the season for relaxing and enjoying a hot drink with friends and family indoors. The hundred degree days that graced chapel hill this summer are now a distant memory. Finals have ended and visiting my family over the holidays means trekking up north. As much as I wish I could stay cozy indoors from December to February, that isn’t always possible. When I do need to face the snow and ice, the streets are often blanketed with a fresh coat of rock salt. This is great for safely navigating the roads and sidewalks, but what happens to all that salt in the spring? As it turns out, much of it flows into rivers and lakes with spring runoff. Before looking at some of the implications of this influx of salt, here is a little background on *lakes.

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Lake Superior in February

 Many lakes, especially lakes in in cold northern latitudes, naturally stratify in summer and winter. Water is densest at 4°C. In the winter, water cools to this temperature and sinks to the bottom. Water being cooled further becomes less dense and snaps into solid form. In summer, water fallows the pattern most of us more accustomed to; top water is warmed by the sun and sits on top of colder bottom water. The dividing line, easily found if you try to dive beneath the surface and suddenly hit a wall of freezing water, is called the thermocline. Just before ice forms and just after it melts, called ice-out, there is a period where the entire water column is the same temperature. Lakes that have this twice a year mixing regime are called dimictic. When the temperature driven stratification breaks down bottom water, called the hypolimnion, is no longer trapped beneath the thermocline and mixes with top water, called the epilimnion. This process is very important for the ecology of lakes. During turnover, nutrients that have been locked in the hypolimnion are upwelled, and oxygen produced by algae in the epilimnion mixes with oxygen depleted bottom water. Turnover balances out the the excesses and deficiencies in the lake strata.

Adding salt to this equation changes the game. Spring melt brings with it density increasing NaCl along with cold water that becomes concentrated in the hypolimnion. Even when the thermocline briefly disappears as ice melts, it is replaced by a chemocline, a difference in density due to dissolved ions. Nutrients the phytoplankton need in the top water remains trapped in the hypolimnion, oxygen that bacteria and other benthic organisms need stays in the epilimnion.

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Anoxic conditions in bottom water not only harm bottom dwellers, but can have huge consequences on lake chemistry. When oxygen is low chemical reactions between sediment and overlying water result in a release of nutrients that cause eutrophication. Depending on lake characters such as depth and width, some NaCl loaded lakes still experience one turnover period in fall, if  there is sufficient wind to cause mixing. Lakes that have one period of turnover are called monomictic. Other lakes, especially small deep lakes that can not be easily mixed by the wind, become meromictic. This is when the hypolimnion and epilimnion remain permanently unmixed.

Meromixis is bad news if you want healthy lakes. Unfortunately, regions that get ice and snow are the same regions where small deep lakes are common. Glacially formed lakes, such as small, deep kettle hole lakes, were formed as ice retreated during the last ice age. The areas that were covered in glaciers thousands of year ago are now the the cities and towns where roads and sidewalks get a daily coating of rock salt during the winter. The ever growing impact salt is having on lakes has been termed cultural meromixis.

With lakes getting saltier and saltier can we expect the Midwest to be the next frontier for coral reef restoration projects? Unfortunately, no. As much as we love the ocean, we should take every effort to keep our lakes salt free. If you are like me, stuck in an ice box for the winter, take a look into the regulations on salt application in your area and try to limit your rock salt use. In the name of saving our lakes, I am pouring myself a big cup of cocoa and staying in. Cheers!

 

*Some lakes. This is the “classic model” lake stratification/mixing regimes but is dependent on depth, climate, etc. and does not happen in all lakes.

 

References

Novotny, E. V., & Stefan, H. G. (2012). Road salt impact on lake stratification and water quality. Journal of Hydraulic Engineering, 138(12), 1069-1080. doi:10.1061/(ASCE)HY.1943-7900.0000590

Sibert, R. J., Koretsky, C. M., & Wyman, D. A. (2015). Cultural meromixis: Effects of road salt on the chemical stratification of an urban kettle lake. Chemical Geology, 395, 126-137. doi:10.1016/j.chemgeo.2014.12.010

 

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