Salinity


 * S alinity **

Salinity can be defined as the measure of the total amount of dissolved salts in a sample of water. This is commonly expressed as parts per thousand (or ppt), for example:

Whilst the salinity levels in ponds or dams can be affected by things like run-off from soils, evaporation and available water source – it is unlikely that most aquaponics systems will be affected unless the water source is high in salts. However depending on the water source you may have salinity issues – here is a table which provides details of salt tolerance of some plants.
 * ** S ea water has a salinity of 30 ppt (3.0%) to 40 ppt  **
 * ** Freshwater has a salinity level of 0-5 ppt. **
 * ** Brackish water is somewhere in between sea water and freshwater. **



The level of salinity in the water influences the salt and water balance in aquatic organisms. In freshwater species elevated salinity levels in the surrounding water tend to result in the loss of water from the animal. This may eventually lead to the death of the animal, unless it has a mechanism for preventing this loss of water. Some species have a mechanism to prevent this imbalance from occurring. This mechanism is termed Osmoregulation and the organisms **osmoregulators**.

The salinity levels of the water affect growth rates. If stock need to osmoregulate in extreme salinities then energy will be expended. This energy reduces the amount of energy available to the stock for growth.

However, salinity can also decrease the toxicity of certain substances in the water. For example, nitrite is a toxic substance that is produced by the breakdown of ammonia by bacteria in a process called nitrification. The ammonia is produced through the metabolism of food and is a waste product. Nitrite is toxic at relatively low levels (1mg/l +) and it interferes with the ability of the blood to carry oxygen around the fish's body.

Salt, in particular chlorine, is a major contributor to salinity, and affects the reaction between nitrite and blood, and will decrease the toxicity of nitrite as a result. Saltwater Silver Perch can therefore tolerate higher levels of nitrite.