Alkalinity refers to amount of carbonates and bicarbonates in the water and water hardness refers to the concentration of calcium and magnesium.
As calcium and magnesium bond with carbonates and bicarbonates, alkalinity and water hardness are closely interrelated and produce similar measured levels.
Waters are often categorised according to degrees of hardness as follows:
A lack of calcium in the water will also result in soft shelled marron as they rely on the intake of calcium from the water column to harden their shells after moulting.
Water alkalinity and hardness can be increased by liming ponds which involves adding a measured amount of lime to the pond. However there is no practical way of decreasing alkalinity and hardness when they are above desirable levels.
Ammonia:
Ammonia in ponds is produced from the decomposition of organic wastes resulting in the breakdown of decaying organic matter such as algae, plants, animals and uneaten food.Ammonia is also produced by fish and crayfish as an excretory product.
Ammonia is present in two forms in water – as a gas NH3 or as the ammonium ion (NH4+). Ammonia is toxic to culture animals in the gaseous form and can cause gill irritation and respiratory problems.
Ammonia levels will depend on the temperature of the pond’s water and its pH. For example at a higher temperature and pH, a greater number of ammonium ions are converted into ammonia gas thus causes an increase in toxic ammonia levels within the freshwater pond.
If high levels are ammonia are present within the pond’s water, a number of measures can be taken. These include:
- reduce or stop feeding,
- flush the pond with fresh water,
- reduce the stocking density,
- aerate the pond,
- in emergencies – reduce the pH level.
The amount of ammonia present in a pond can be calculated by recording the total ammonianitrogen (TAN), pH and temperature (Table 1).
For example to obtain the concentration of NH3: Water at pH 8.4, 280 C and 2mg/l of TAN (sampled measurement) contains 15% NH3 (from table). Therefore 2mg/l x 15% / 100 = 0.3 mg/l of NH3.
Nutrient levels:
Nutrient levels refer to the amount of phosphorus and nitrogen that is present in the water column.
Nutrients are important as they promote healthy plankton blooms which are necessary to maintain turbidity levels and provide feed for fish.
Nutrient levels can be increased in the ponds by adding inorganic or organic fertilisers in measured doses.
Increased levels of nutrients however may be harmful. It can cause excessive plankton growth, potential blue-green algae blooms and oxygen depletion.
High levels of nutrients can be caused by high stocking densities, over feeding (very common especially during winter), high productivity, and dead plant and animal matter.
To decrease high nutrient levels, feeding rates should be decreased (or stopped) and the pond may need to be flushed with clean water.
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