Jaden Norton and Kacey Sauer, of South New Brighton School, asks :-

Why does the estuary smell so much?

Mike Gilson, a chemist with the Christchurch Drainage Board, responded.

There are many smells around the estuary, some of which are very pleasant like the smell of fresh clean salty air. However, at certain times of the year this clean fresh smell is spoilt by the presence of an unpleasant odour. It is this odour which I suspect you are referring to. It is the odour of the rotting seaweed washed up onto the estuary foreshore.

There are two main types of seaweed which grow in large numbers in the estuary, they are sea lettuce, proper name - Ulva; and a spiny branched seaweed with a proper name of Enteromorpha. It is the sea lettuce which appears to give most problems although they both have similar growing habits. These seaweeds grow on the mud flats which are exposed at low tide so they spend part of their life uncovered by water. They grow from a short stalk which is attached to stones, rocks and shells on the mud flats. During the winter they are dormant and grow very little, but during the spring as the seawater temperature rises, their growth becomes rapid and as summer approaches some of the mudflats look completely green where they are covered in seaweed. Sea lettuce grows in the form of a flat leafy rosette which may reach up to 0.5 metre across during the summer. At this stage of its life, because of its size, the action of the tide lifts the plant off the mud flat and it becomes free floating, drifting with the tide up and down the channels in the estuary. Due to winds and high tides the drifting sea lettuce tends to be washed onto the shore above the high tide mark.

From February to April large quantities of sea lettuce can be seen washed up onto the foreshores and it is now that the problem of odour arises. Whilst it is in the sea there is no smell but once it is thrown above the high tide mark in the summer it quickly rots producing that characteristic odour. In the warm sun the decay process quickly starts due to the action of bacteria. The bacteria use the sea lettuce as a food source and as they consume the plant they give off sulphur gases and vapours such as hydrogen sulphide, (rotten egg smell) and other more complex vapours called mercaptans which are also foul smelling. It is these gases and vapours which make the estuary smell. The process of decay is the natural process of recycling dead plants and animals back into environment as the raw materials for the next generation of plants.