The Cayuga Fisher

The Water:  Seasonal Variations

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Seasonal Variations:  It's All About the Temperature

We all know bodies of water warm in summer, cool in winter, and freeze over if small enough.  But what exactly is happening down there?  Sunshine or lack thereof drives the annual temperature cycles, just like our land-based seasons.  The lakes even have four seasons to match:  spring "turnover", summer stratification, fall "turnover", and the winter mixed period.  The length of the two turnover periods are relatively short and usually lasts for a few weeks to a month or so.  Winter mixing occurs for approximately five months, and stratification approximately six months in Cayuga Lake. 

During winter, the lake is considered isothermal (the same temperature) from top to bottom, and the wind and currents mix the lake water throughout the water column.  While this is essentially true, there are often little variations in this sameness.  The sun may heat the surface water during calm periods and cause weak stratification, but the wind, length of night and cold air temperatures prevent it from lasting more than a few days. 

The bottom of the lake may contain pockets of slightly warmer water, as water is most dense at 39 degrees F.  The surface may be 35° with the warmer water below, as well as most of the fish community.  The earth itself is relatively warm and adds a slight amount of heat to the lake water, and biological activity on the bottom does too.  The 39 degree water will tend to accumulate in certain areas of the lake bottom under light winds.  However, the average winter wind is far stronger than any slight stratification or pocket heating and will easily mix the water again.  As spring approaches, the entire lake begins to gain heat and the water warms slightly from top to bottom.  With continued warming, the lake begins to seperate into distinct temperature layers.

The lake warms from late March until September and loses heat the rest of the year.  There are year to year variations mostly due to weather patterns, but in general, the lake begins to gain heat energy around the spring equinox, and lose heat at the fall equinox.  Just as with our seasons, the cycle depends on available sunlight: when the days are longer than the nights, the lake gains heat and when the nights are longer, it loses heat. 

There are also yearly variations in the annual heat budget, that is, one year it may gain more than it loses, or even for several years in a row. This can cause slight variations in the average yearly lake temperature, but it all averages out in the long run. (More or less... and global warming, sorry, but it's here, will undoubtedly change our waters, and the average lake temperature will start increasing.eventually.)

Weather conditions during this period are critical for determining the annual heat budget.  Any excess heat, above the annual average, is stored in the hypolimnion, the cold deep layer of water in the lake.  Varying conditions lead to slight differences in water column structure and temperature.  A sunny, windy spring will warm the hypolimnion the most, as increasing amounts of solar energy enter the lake and high winds keep it well mixed.  This will also lead to a deeper than average metalimnion (which includes the thermocline).  A sunny, calm spring leads to a colder lake bottom and shallower metalimnion.  Eventually the density gradient (due to water temperature difference) becomes too great for the wind to overcome, and the lake becomes fully stratified and remains so until fall.

The summer months feature warm surface water and good recreational oppotunities.  Unfortunately, my favored trout species are found far below!  Early on, in late spring, the thermocline is shallow and sharp, allowing lake trout and other salmonids near the surface.  As time passes, the thermocline deepens and all the salmonids in the lake follow.  Excessive wind events can bring cold water back to the surface as upwellings, but in general, the trout and baitfish are in the depths.

As the lake waters cool into fall, several things occur.  The depth of the thermocline increases, and it becomes wider.  Heat energy is no longer concentrated on the surface and begins to dissipate into the depths as the epilimnion disappears and metalimnion deepens and widens.  Eventually the temperature and density gradient breaks down and the wind is able to mix the entire water column.  The lake continues to cool into winter, reaching its coldest temperatures in February and early March.

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