As boating season winds down, what’s happening below the surface of our lakes?

Have you ever leapt off a dock and felt the water suddenly turn colder the deeper you went, only to warm up again when you resurfaced? That cold rush is evidence of thermal stratification, the natural layering that develops in Lake Winnipesaukee, and other large lakes, each summer. Warm, lighter water stays near the surface, while colder, denser water settles below. In between is a zone where temperature drops rapidly, known as the thermocline.

All summer long, Lake Winnipesaukee holds onto these distinct layers. The sun-warmed surface layer, called the epilimnion, mixes with the air and generally remains well oxygenated. It is the part of the lake most familiar to swimmers and boaters and provides habitat for warm-water fish such as bass and sunfish. Cold-water species, including lake trout, seek deeper, cooler water where temperatures are suitable and enough dissolved oxygen remains available.

Below the thermocline, the hypolimnion remains relatively cold and receives little to no sunlight, largely isolated from the atmosphere while the lake is stratified. The middle layer, or metalimnion, limits mixing between the warmer surface waters and the colder depths. As summer progresses, organisms throughout the lake continue to use oxygen, while little new oxygen reaches the deepest waters from the surface. By late summer, dissolved oxygen can become depleted, particularly near the lake bottom.

Since April, the Lake Winnipesaukee Alliance has been monitoring temperature and dissolved oxygen at 12 of Lake Winnipesaukee's deep-water sites each month. These measurements allow us to see changes occurring throughout the water column that are invisible from the surface. By tracking temperature and oxygen from spring through fall, we can follow the development of stratification, conditions in deep-water habitat, and eventually the breakdown of those layers as the seasons change.

That breakdown happens during fall turnover. As air temperatures cool, the lake's surface water also cools and becomes denser. Eventually, the temperature difference between the upper and lower layers becomes small enough that wind can mix the water column. Once the lake reaches a nearly uniform temperature from top to bottom, oxygen-rich surface water can circulate into deeper areas.

This seasonal mixing is an important part of Lake Winnipesaukee's natural cycle. It redistributes oxygen throughout the water column and helps replenish deep waters before winter. The process also moves nutrients and other dissolved materials that accumulated at depth while the lake was stratified.

Lake Winnipesaukee typically mixes twice each year, once in the spring after ice-out and again in the fall.

Lakes that follow this pattern are known as dimictic lakes. After fall turnover, the lake continues cooling until winter stratification develops beneath the ice. In spring, the process reverses as the ice disappears and the water column mixes again before summer stratification becomes established.

So, the next time you dive in and feel that sudden chill, remember that you are passing through one of the lake's seasonal layers. What feels like a quick change in temperature is part of a much larger annual cycle occurring beneath the surface.

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Bree Rossiter is the associate director of the Lake Winnipesaukee Alliance, the only nonprofit dedicated to protecting the water quality and natural resources of Lake Winnipesaukee and its watershed through science, education, advocacy, and collaborative solutions. Her work focuses on advancing long-term watershed protection initiatives and connecting science with practical solutions to help preserve the lake for future generations. To learn more, visit winnipesaukee.org.