Some Background

Electrical Conductivity (EC) of water is a readily obtained measure of its salt content. EC values in the area of 30-60 uS/cm are typical of pristine lakes in the Halifax region. EC values for Sandy Lake were in that range in 1955 and 1971, and 1980 (not in 1977 however) but samples taken from 1985 onward were well above 100 with an overall upward trend.

EC is one of the variables, along with temperature, dissolved oxygen and pH measured by probes used characterize the vertical or limnological profiles of lakes, such as those obtained for Sandy Lake in Oct of 2017.

One feature of the Sandy lake profiles that raises some concern is an elevated EC or salt content in the deeper layers of the lake (the “hypolimnion”). Sandy Lake is a “dimictic” lake meaning that it normally turns over (mixes from the top to the bottom), twice a year.  In late spring into summer,  the deeper parts of the lake become “thermally stratified” – as water warms it becomes lighter and stays on top of the lake, leaving the heavier cold water at the bottom, with a “thermocline” – a narrow band in which the temperature changes sharply from warm to cold – in between. The deep, cool layer is called the hypolimnion.

This is a desirable feature because the deeper, cooler waters provide a summer refuge for cool water fish, especially salmon and trout. Oxygenation of the hypolimnion is reduced over time, but then as the lake cools in the fall it  becomes “isothermal” (the same temperature and same density from top to bottom), and a good wind will cause it to mix from top to bottom, re-oxygenating the deep layers.

In winter, the lake again stratifies, but this time it is because water has a maximum density at 4 degrees C, and when surface water goes below 4 degrees, the cooler water sits on top and then freezes and forms ice when it gets down to zero. In the spring as the ice melts and the water warms, the lake reaches a point at which it is again isothermal and and can be turned over by the wind.

Dimictic lakes mix from the surface to bottom twice each year.  From Wikipedia

We say such lakes are “thermally stratified”.  If salt accumulates in the deeper waters, those waters become heavier because of the salt and as the salt content increases, there is more resistance to the normal, temperature induced turnover, and the lake may not turn over at all. Then the deeper waters can go anaerobic (devoid of oxygen), making them unsuitable for cold water life. As well, that can trigger release of phosphorous  a plant nutrient-  from the sediments, causing the lake to produce an excess of algae which in turn further degrades the lake.

EC values in the area of 30-60 uS/cm are typical of pristine lakes in the Halifax region. 6 EC values for Sandy Lake were in that range in 1955 and 1971, and 1980 (not in 1977 however) but samples taken from 1985 onward were well above 100 with an overall upward trend. The low values in 1955 and 1971 suggest the lake was likely well below the mesotrophic range (re: figure above), i.e. it was oligotrophic in those earlier years – see this post

Water on the Web
A comprehensive set of learning materials developed under auspices of U.S. National Science Foundation 1997-2005
See for example:
Lake Ecology Primer
Water Science Curriculum