Still water stratification

Lakes and ponds are subject to great changes throughout the course of a year. Deep lakes can vary as much as 40 f between the surface and the bottom and the surface temperature is often higher at one side of the lake than the other. Unlike rivers where the current is constantly mixing the water, dissolved oxygen is fairly evenly distributed through all depths with the exception of rapids and waterfalls and amounts are determined by temperature. Conversely, lakes rely mostly on wind for mixing and without it the oxygen content will be lowered considerably by temperature rise and decaying vegetable matter. Complete de-oxygenation may occur in some lakes below a certain depth during summer and fish will not enter it. In small shallow ponds during hot dry conditions the oxygen content of the entire water can fall so low as to cause fish kill. Long before this happens fish will likely have stopped feeding.

Starting with autumn, letís see how an average lake changes throughout the year.

All the water is above 42 f and most of it considerably warmer. The first frosts and cold wind lowers the surface layer and sinks, warm water rises and replaces it and is chilled in turn. This circulation process continues until all the water reaches 39.2 f, at this temperature water is at its heaviest. Any cooling beyond 39.2 f does not sink, instead forms a thin layer at the surface and only in very shallow lakes reaches the bottom which otherwise remains at 39.2 f. When the weather is cold enough the surface freezes at (32 f). Providing the temp at the top is between 32 f and 39.2 f this cold layer stays on top and the bottom is the warmest place in the lake. All circulation has stopped. 

When spring arrives the surface is warmed by the sun and when the surface reaches 39.2 f it sinks, this thin layer circulates until all the water in the lake is 39.2 f and circulation stops. A layer of warm water now forms at the top and both temperature and depth increases as time passes. This water will be warmed as far as the sunís rays penetrate and the warmest place will be at the top. Between the warm upper layer(epilimnium) and the cold layer(hypolimnium) is a thin layer of rapidly reducing temperature called the thermocline and usually somewhere between 15 and 20 feet deep, depending on the size of the water.

One effect of wind is the lowering of temperature; proof of this is to wet a hand and hold it in the wind. During wind, not only will the top water be cooled it will be blown along with the wind. As it is blown along it sinks and a current is caused which travels along the surface in the direction of the wind, and then, falling to the top of thermocline, goes back to where it came from. Wind has several effects. It mixes the warm upper layer, so that its temperature and oxygen dissolved in it are fairly constant and pushes the whole of the warm upper layer to wards one side of the lake to a greater or lesser extent, depending on its force and duration. When these are great, the thermocline will be tilted toward the windward side of the lake and the cold bottom layer may rise to the surface on the leeward side. If the wind ceases suddenly, the thermocline swings back to level and beyond, sometimes it overshoots and continues to swing back and forth for some time before settling down. The bigger the lake, the longer time cycle of the swing.

In shallow lakes, the upper warm layer reaches the bottom in the spring, and the cold layer and thermocline are both eliminated. The deeper the lake and the more coloured its water, the longer this will take, and in deeper lakes and reservoirs the thermocline and cold bottom layer remain all summer, only increasing in temperature by a degree or two, through conduction. The actual depth of the thermocline varies widely in lakes and can only be found by experiment.

Many kinds of plants continually die, sink to the bottom and their decay and that of other organic substances will use up oxygen. This cold water never comes to the top except in very strong sustained winds, and gets very little oxygen. If a water is fairly small and much decay takes place, its oxygen may be partly or wholly used up. In very deep lakes where its volume is many times the layer above it, or where plant growth is poor and little decay takes place, it will retain most of its oxygen throughout the summer but will still remain cold. Fish will avoid it and plants will not grow in it

In autumn, the effects of wind, rain and radiation loss continue to cool the warm upper layer until its temperature falls below the cold water beneath it. This then rises to the surface, and circulation with re-oxygenation takes place until the conditions are reached where this originally started.


............................................ Footnotes .............................................

A particular case must be mentioned in considering the tilting of the thermocline by wind. In reservoirs made by damming a watercourse in valleys and other waters with similar bottom contours. A wind with sufficient strength and duration blowing towards the deep end will cause the lower layer of cold and perhaps deoxygenated water to spread over a very wide area at the shallow end, The more gradual the slope of the bottom, the greater this effect will be. An opposite wind will have much less effect, causing the cold water to rise at the deep end over a small area; but if the wind drops rapidly, the swing back will produce temporarily the effect of an opposite wind.

You will realize that the effect of these changes on fish will be enormous. Dividing lakes and ponds into two categories; the shallow ones in which the thermocline and the bottom layer are eliminated early in the year, and the deep ones in which these layers remain all summer. In the shallow lakes, water temperature fluctuates greatly, since the volume of water is smaller, and such lakes are much more temperamental than those with deep water reserves. In these, fish can always find some part of the lake where the temperature lets them feed, whereas in a shallow lake all the water may be too cold or too warm. When fish have stopped feeding for reasons as this, you have to calculate when and where they will begin, but in shallow lakes it is easier to see the fish or signs of their whereabouts.

The effect of wind on fish will be great in all lakes, but greater in deeper ones. If it tilts the thermocline, fish will be driven out of parts of the lake and may be concentrated in others. Wind and rain increase the amount of dissolved oxygen in water as well as lowering its temperature except sometimes when it is already very cold, then rain or a warm breeze may have the opposite effect. Where there are wide areas of very shallow water with adjoining deeps, radiation losses at night or the cooling effect of wind may reduce the temperature of the shallows faster than the circulation caused can carry the chilled water to the depths and replace it by warmer water from them. There will be a current along the bottom travelling from shallow to deep and a return from deep to shallow along the surface. The effect upon fish will be great and must be considered if their movements are to be understood and predicted.

In calm, still weather, further temporary stratification by temperature occurs, in which the upper two or three feet of a lake, irrespective of depth becomes very warm. Something you will notice when swimming in lakes. Under these circumstances, this temporary layer above will circulate when wind or radiation losses cause cooling, until it is eliminated. Circulation of the water below will only then commence, and meanwhile there may be considerable loss of oxygen due to decay. Under these conditions in shallow smaller lakes, fish will be found very near the surface or in shallow water and will tend to move towards and feed in that part of the lake which gets re-oxygenated by any breeze that may spring up. As temperature rises, fish need more oxygen, but at higher temperatures less is available. Very hot weather conditions have different effects on fish in different lakes. In shallow lakes they are driven to the surface; in deep ones they go down to the thermocline where it is cooler and their oxygen requirements are less.

Most anglers know that many species of fish feed more freely at dawn and dusk. This was thought to be due to the cooling of the water in the evening, but it now known that some species of fish commence feeding in the evening, even in the depths of winter with ice forming at the waterís edge. Some train of thoughts came to the conclusion the angle of the sun causes these effects. Because of refraction, the light from any source that makes an angle of less than 10 degrees to the surface of the water will fail to penetrate. On a clear day as the sun falls below this 10 degrees angle there will be a sudden and sharp reduction of light that penetrates the surface and a sharp reduction in underwater lighting. Once this happens it will be more difficult for predatory birds to see below the surface, whereas fish will be better able to spot them above the surface or on it. Fish will feel safer once the sun sinks below that critical angle of 10 degrees.