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Phytoplankton is the natural marine flora of the sea and several species naturally produce toxic substances or, in other ways, may have harmful effects on other species. The nature, effects and consequences of these toxins for human aquaculture activities, and particularly fish and shellfish culture and harvesting, are being actively researched by Marine Scotland Science and around the world. The need for management and mitigation of their occurrence and effects has increased with the increased use and utilisation of the coastal waters. Besides fisheries, fish farming and the harvesting/cultivation of shellfish have experienced problems due to harmful algae, including extensive economic losses and risks to public health.


The harmful effects of phytoplankton blooms are not always due to toxin production. Large blooms of phytoplankton, although often occurring naturally, may be exacerbated by external inputs of nutrient from human activities (excessive sewage outfalls or industrial or farm run-off from the land), a process called eutrophication. Blooms will often collapse when the chemical nutrients in the water run out and then the dying phytoplankton can rapidly settle in a smothering mass to the seabed. When a bloom is in this senescent state, the cells' production of oxygen through photosynthesis is outstripped by the oxygen demands of respiration and the demands of the bacteria consuming the decaying cells. This can lead to de-oxygenation of the water and suffocation of other marine life in the area. Some species of phytoplankton form chains that can have sharp spikes on their cell walls, which may abrade and damage the fragile gill lamellae of farmed fish when these phytoplankton types bloom in the sea.

Strategies for detecting and managing problems and minimising losses can be;

  • careful site selection of aquaculture installations,
  • regular monitoring for algae species abundances and toxins,
  • effective communication of monitoring data and incidents,
  • effective health and economic risk assessments and advice provision, and
  • research to increase understanding of these natural toxins and their effects.

There are a number of known toxic species of phytoplankton in UK waters. These are being monitored in the Scottish coastal seas, and active research is under way by Marine Scotland Science and in laboratories across many other countries. This is a global problem.