Monitoring plankton, blue-green algae and macrophytes

Plankton comprises all the microscopic organisms that are suspended in water and includes small plants (phytoplankton), small animals (zooplankton), and bacteria. When there is enough plankton in the water to discolour it, the water is said to contain a plankton "bloom". Because plankton forms the base of the food web, there is a strong relationship between plankton abundance and fish production. Plankton blooms are a common feature of fish culture ponds.

Type of phytoplankton include green algae, yellow-green algae, blue-green algae and diatoms. In summer, phytoplankton blooms contain blue-green algae, which can form scums at the surface. These scums absorb heat during the day and may cause shallow thermal stratification. During the night, heavy plankton blooms consume large amounts of dissolved oxygen and may cause oxygen depletion before the next morning. Plankton may suddenly die, decompose, and cause oxygen depletion. Factors causing this include exhaustion of available nutrients, increased clay turbidity and changes in the water quality (e.g. cold change, cloudy and rainy days). Excessive phytoplankton blooms can also cause large diurnal fluctuations in water quality variables (e.g. very high pH and NH3 levels in mid afternoon, such conditions are stressful to fish.

Blue-green algae can cause other problems in aquaculture besides fluctuating water quality parameters. They can produce toxic substances that are lethal to some fish. They can also produce compounds that impart a strong "off-flavour" to fish. Fish from these ponds may have sufficient off-flavour to make them unmarketable. This is why it is important to clean or "purge" your fish by placing them in clean water before sending to the market.

The larger aquatic plants or macrophytes include pondweed and milfoil. They are undesirable in fish ponds because they:

  • interfere with fish management such as seining, feeding and harvesting
  • complete with plankton for nutrients
  • provide havens for undesirable fish
  • contribute to oxygen depletion and high ammonia levels when they decompose
  • contribute to water loss through evapotranspiration.

Management options to reduce macrophyte growth include:

  • drying and desilting of ponds every 1-2 years
  • mechanical harvesting
  • increasing phytoplankton turbidity (fertilising)
  • application of herbicides.