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Dieren en planten

Flagellates   Dinoflagellates   Dinophysis   Alexandrium   Phytoplankton   Harmful algae   

Water en land

Mens en Milieu

Red tide, Marijke de Boer

Harmful algae

One-celled algae, or phytoplankton, are considered harmful if they produce toxic substances during an algal bloom. When consumed by marine animals, such as shellfish, these materiials don't necessarily affect the animals negatively. However, if humans consume shellfish in which the toxic material has accumulated, it can turn lethal. An algal bloom occurs when large amounts of algae grow rapidly, use up the necessary nutrients to grow and die in massive numbers. This can cause a discoloration of the water, often brown or red, which explains why people also refer to an algal bloom as a red tide. Sometimes algal blooms are smelly and unattractive for swimming. However, those aren't often the harmful blooms. A total of nineteen species of harmful algae are found in the Dutch part of the North Sea.

  • Research

    Because the blooms can have tremendous negative impacts on human health as weel as the economy, governments carefully monitor them. The presence of harmful algal blooms in coastal waters is a fisherman's nightmare. However, little is known as to what causes them. Sometimes they occur entirely naturally, other times human activities seem to be responsible (pollution).

    The most prominent algae in the North Sea region that excrete harmful materials are the dinoflagellates Alexandrium and Dinophysis. Alexandrium is known to produce saxitoxin, which is resposible for paralytic shellfish poisoning (PSP). Dinophysis produces okadaic acid which causes Diarrhetic shellfish poisoning (DSP). When people consume shellfish which have accumulated too much of these toxic materials, it can lead to all sorts of illnesses, such as intense stomache aches, diarrhea, nausea, slurred speech or death.

    Scientists are studying the predictability of toxic algal blooms in both fresh and salt waters. They make models based upon satellite images of these events. Water samples are taken daily to determine which species are present and in what amounts. A good prediction can lead to closing sluices on time and announcing swimming bans. It can also warn controllers who monitor shellfish quality.

    There are also studies taking place of how climate change will influence harmful algae in the future. Momentarily, the average temperature of the North Sea water in the summer is 18 degrees Centigrade. Scientists expect that the summer water temperature will average 22 degrees Centigrade in the year 2100. In that case, experiments have shown that there will be less Phaeocystis blooms (foam algae) but greater amounts of algae species which are harmful for fish.