NATURAL HISTORY

Pāuatahanui Inlet is an estuary.

An estuary is a place where a river or stream meets the sea and where the fresh water mixes with and dilutes the sea water so that a gradient exists between sea water at the mouth and freshwater at the head of the estuary.
This gradient is reflected in the changing nature of the plants and animals that live in the estuary.
At the mouth live true sea creatures intolerant of much reduction in the salt content (salinity) of the water while those at the head are fresh water forms intolerant of any increase in salinity. In between these extremes live marine species that have become adapted to a greater or lesser degree to deal with the physiological demands of reduced salinity.
Species with low tolerance (e.g. pipi) inhabit the lower reaches and those with high tolerance (e.g. mud snail) inhabit the upper reaches. Other species are distributed within these limits according to their particular degree of tolerance. Very few freshwater animals and plants have adapted to anything other than a very small rise in salinity.
A second common feature of estuaries that affects the distribution of animals and plants is that the intertidal area is composed of mud banks at the head grading into sand banks at the mouth.

but ...

Pāuatahanui Inlet is an unusual estuary.The range of salinity from head to mouth is much smaller than normal, so that the sand flats at the head are subjected to almost fully saline sea water for much of the year. This is because the amount of fresh water delivered to the Inlet from its catchment by its relatively small streams is not sufficient to make much difference to the salinity of the incoming tide.
The low stream outflow versus the strong inflow of sea water with each tidal cycle also means that only small quantities of mud get deposited on the flats at the head of the Inlet (most of it being transported to the deeper basins of the Inlet). 
These two factors combine to restrict the Inlet fauna and flora to species that can survive in high salinity water and on the relatively hard and clean sand banks. The most abundant and widespread of these species is the cockle. Despite this restriction on diversity the Inlet has a number of distinct habitats that provide for a varied and interesting fauna and flora.

SALT MARSH

TIDAL FLATS AND SHORES

At the eastern end of the Inlet is a large area of salt marsh. The seaward side of the marsh is subject to tidal inflow most days of the year and is dominated by sea rush. This gives way to jointed rush as the amount of tidal inundation decreases. Plants in this marsh must be able to survive constant immersion of their roots in sea water. The outer part of the marsh provides a sheltered feeding ground for wading birds, and a breeding ground for some (e.g. pied stilt). The marsh contains threatened fish species and endangered vegetation. Most of the marsh lies within a wildlife reserve.

At low tide extensive tidal sand flats extend out from the salt marsh and also dominate the western end of the Inlet. Over this entire area the dominant species living in the sand are cockles and the worm Axiothella (no common name) while several species of crabs spend low tide in burrows on the higher parts of the shore. Mud snails are abundant on the surface of muddier shores around the mouth of the Horokiri Stream and in the salt marsh. Beds of seagrass occupy parts of the tidal flat and sea lettuce (a large, leaf-like, bright green alga) covers much of the flats during summer. Many birds feed on these flats.

SUB-TIDAL BASINS

THE WATER COLUMN

The Inlet is unusual in that although it is shallow – greatest depth is only about 2 metres – a large proportion of its area remains below water at low tide. There is a large number of animal species living in this zone but obviously few are ever seen alive by human visitors. However, dead shells of the Arabian volute - a large carnivorous snail - are common on the shore.

The waters of the Inlet support a great diversity of life, from minute plankton to the occasional visiting seal. There are numerous fish species that provide food for several species of birds.

THE IMPORTANCE OF THE INLET

The Inlet is well recognised for its high ecological, aesthetic, recreational and cultural values. It is classified as a 'Site of Special Wildlife Interest' by the Department of Conservation and recognised in the Regional Coastal Plan as an Area of Significant Conservation Value.

FURTHER READING

Healy, W.B. (1980) Pauatahanui Inlet - an environmental study. DSIR information Series 141. NZ Department of Scientific and Industrial Research.

(This book is out of print but is available at public libraries.)