Still water stratification
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.
with autumn, letís see how an average lake changes throughout the year.
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.
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.
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.
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
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
............................................ Footnotes .............................................
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.
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
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.
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.