Eva Crossan, of Stirling, South Otago, asks :-

What is an algal bloom and how do they affect people?

Katie Ayers, a biologist at the Cawthron Institute in Nelson, responded.

Microalgae are very small organisms found in ocean waters in all parts of the world. When favorable environmental conditions arise, the algae may reproduce quickly resulting in a large amount of them in one place at one time. This is called an 'algal bloom'.

The conditions that may cause a bloom are extra nutrients in the water, unusual weather events or when algae are moved from one part of the world to another in the ballast water of ships. Blooms can occur with many different types of algae and can be any color, but the most common ones are red or brown. These blooms are called red tides or brown tides. Blooms can also have interesting properties, such as one seen in the Hauraki Gulf recently which had bioluminescence (it glowed in the dark).

Most microalgal blooms are not harmful and provide good food for sea creatures. However, some can affect fish, shellfish and humans, as well as other animals like birds and sea mammals. These are known as Harmful Algal Blooms (HABs). HABs can be dangerous to fish or shellfish by reducing the oxygen that they breath in the water, or producing toxins which harm or kill. When shellfish eat toxin-producing algae, the toxins can make the people who eat the shellfish sick as well, causing different effects from upset stomachs to memory loss. This means that it is important to monitor the algae around shellfish and fish farms, to prevent harm to the sea creatures as well as those who eat them.