Paul Tait, of South New Brighton School, asks :-

Where does seaweed in the estuary come from?

Murray Parsons, a Marine Phycologist (one who studies marine algae) with Landcare at Lincoln, responded.

The seaweeds in the Avon-Heathcote Estuary are all very important in the ecology of the estuary. Seaweeds or algae are important as food for the animals of the estuary.

In the estuary the most important algae are the diatoms, tiny, single, glass-walled cells that you cannot easily see. Sometimes these tiny cells are found in sufficient numbers as to colour the sand and mud with a brownish coating. Diatoms are food for crabs, worms, cockles and other shellfish. The seaweed easily seen in the estuary is the green sea lettuce or Ulva. Sometimes the whole of the estuary seems green with it. These sheets of Ulva tissue are usually two layers of cells thick. At the beginning of their growth, they have a small holdfast that anchors them to the shells on the mud. As the sheet grows larger it breaks away and floats about. The contents of each cell of Ulva is able to divide up into 32 or 64 little swimming spores which when released into the water swim about to look for a shell to settle on. Each spore can grow into a new Ulva plant. Later in the growing season some of these spores act as gametes. They fuse together in pairs just before they settle down. In this stage they are able to survive during any adverse conditions.

Another seaweed in the estuary is Gracilaria, forming dark red-black or brown, branching strings. Again it has a small holdfast on a shell. It occurs as three kinds of similar plants, male, female and asexual ones. The only one that is easily distinguished is the female one which after fertilisation develops spherical lumps, cystocarps, on the upper parts. These cystocarps release spores which continue the life-history of the alga.

Recently Gracilaria was harvested in the estuary. It is a good source of agar jelly which is used as a thickener in foods and in hospital laboratories for making plates for growing disease bacteria.