Bottom Deposits

 

Bottom deposits are formed by the natural process of both plant and animal material decomposing after death, and also input from external sources. This can be in the form of simple leaf litter which falls to the pond in the Autumn, of sediment in suspension from stream inlets or surface runoff especially in times of flood.

These deposits contain all the nutrients that have been consumed by the living plants and animals, and the natural process of bacterial utilisation recycles these back once more to the pond.

This is not the whole story though, as there are various forms of
bacteria, each surviving in different environments. You may well have heard the terms "aerobic" and "anaerobic". These two conditions within the bottom deposits greatly change both how the nutrients are released back into the water column, and also the speed at which this process occurs.

In a normal pond a very narrow area of the bottom deposits are aerobic and those below are anaerobic. These two different forms of bacterial activity greatly effect how much nutrient is released back into the pond for reuse. This narrow band of aerobic sediment is called the oxidised microzone. Within this area oxygen diffused from the water allows aerobic bacterial activity. These bacteria are able to breakdown organic material to carbon dioxide and inorganic matter, such as ammonium and phosphate. The ammonium can then be converted to nitrate by bacteria before it can escape the sediments. The phosate in the presence of oxygen is then tied up by iron particles to form oxidised iron. If these circumstances exist, all that escapes is a little ammonium carbon dioxide and some methane from the anaerobic area.

If the aerobic area does not exist, then things are very different.
Normally this is due to bacterial activity being too great and the
oxygen is used up faster by the bacteria than it can diffuse. As oxygen is not present the bacteria have to use other oxidising agents such as nitrate and sulphate. This causes the nitrate to be converted to nitrogen gas and sulphate to sulphide. The sulphide normally combines with iron to create iron sulphide, which is a very insoluble substance which turns the sediments black. Phosate is also produced and under these conditions remains in solution, as does some ammonium.

Things are very different within a weed bed, as due to activity above the sediment matter falls like a constant rain upon the bottom. This tends to turn all sediments anaerobic in the area. Plants have adapted to be able to tap into this source of nutrient supply and absorb both the ammonium and phosate. Therefore a plant bed is better able to recycle nutrients, and also to cope with build up within the sediment much more than would be a years supply.

To sum up it would appear that in open water a layer of aerobic
sediments returns the least amount of nutrient back into solution,
whilst those within plant beds are better able to cope with excess
material over a longer time scale.