Allelopathy is a biological phenomenon by which an organism produces chemicals and/or toxins that influence the germination, growth, survival, and reproduction of other organisms (generally in plants). Allelopathy in the aquarium is a little understood subject and evidence for allelopathy in aquarium plants is limited. This article will not explore the science behind allelopathy, rather it will seek to provide readers with a better understanding of the potential consequences of allelopathy in the aquarium.
Allelopathy in Aquatic Plants
Often strange things happen in planted aquariums for which there appears to be no rational explanation. Analysing nutrient levels, water chemistry and light does not provide concrete explanations for certain events within the planted aquarium and this has led some to believe that allelopathy must play a role in the planted aquarium.
Aquatic plants contain a variety of allelochemicals whose primary function is to protect the plant from being eaten by herbivorous organisms or being destroyed by disease. However, the specific effects of these chemicals are not understood very well.
A major problem in investigating allelopathy in aquatic environments, is that allelochemicals are water soluble and thus, the origin of these chemicals (i.e. which specific plant/organism produced them) and the desired target are hard to pinpoint.
Allelopathy, Plants and Algae
Tanks with heavy plant growth often seem to have very little algae, even though those same tanks may have many hours of light, high nitrate and phosphate levels, all of which are traditionally claimed to cause algae. The fact that algae does not do well despite these apparent advantages, suggests that allelochemicals produced by plants might help control algae growth. It would be advantageous for plants to limit algae growth, as algae compete with plants for resources.
In the same way, allelopathy is thought to be a competitive strategy employed by plants to limit the growth and reproduction of other plants. This phenomenon is apparent in many terrestrial plants, however, this type of allelopathy has yet to be proven in aquatic plants. High-tech planted aquariums may provide evidence against this type of allelopathy in aquatic plants. High-tech aquariums are designed to produce rapid plant growth and as plant growth is generally accompanied by the release of allelopathic chemicals, the argument goes that if aquatic plants did employ allelopathy, High-tech aquariums would not be as successful as they are.
Note from Author
Only once have I ever witnessed a possible case of allelopathy. One of my tanks contained the following plants; Anubias sp., Rotala rotundifolia, Hygrophila polysperma and Hygrophila difformis (single stem). The Hygrophila difformis had shown decent growth in previous years. However, once I added the Rotala rotundifolia and Hygrophila polysperma, the Hygrophila difformis stopped growing. Initially I attributed this to lack of light and/or nutrients, as the plant was placed directly next to a large bunch of Hygrophila polysperma. Once I pruned back the Hygrophila polysperma the Hygrophila difformis began to grow better. Once the plant reached a significant height, I trimmed the stem and placed the cutting in another area of the tank which contained recent cuttings of the Hygrophila polysperma of about the same height. The Hygrophila difformis once again stopped growing and eventually withered away. I have no explaination for this change in growth rate and attribute it to allelopathy.