Jay Poutu, of South New Brighton School, asks :-

Why are there so many types of animals in the Estuary?

Islay Marsden, a zoologist at the University of Canterbury, responded.

Estuaries are amongst the biologically richest habitats in the world. They may not have the diversity, or range of species found on a rocky shore but the animals are numerous( tens of thousands per cubic metre) living both on and under the mud surface.

Water from the Avon and Heathcote Rivers and the Oxidation Ponds mix with oceanic water from Pegasus Bay in the Estuary. This provides a nutrient rich, sheltered habitat for small animals, over-wintering areas for migrating birds and nursery grounds for coastal fishes.

About 33 different types of invertebrate animals (those animals without a backbone) are found here. Although they include snails and crabs the most numerous types are small marine worms called polychaetes. Just below the mud surface are filter feeding cockles, at densities greater than 400 per square metre. Mud crabs scuttle about on the surface and mud snails feed on sediment, plant detritus, and bacteria.

In the more salty water near the mouth of the Estuary we find different types of animals. At low tide on walls, boulders or rocks near the Causeway, Cave Rock and Sumner there are chitons, sea anemones, crabs and starfish that occur on rocky shores around Banks Peninsula.

The Estuary, where the rivers meet the sea, forms a distinct marine habitat containing high densities of relatively few invertebrate species. These specialised animals thrive in the widely fluctuating salinities that define the estuarine environment.