Pāuatahanui Inlet is an estuary — but an unusual one!
An estuary is a place where a freshwater river or stream meets the sea. Here, salty sea water mixes with fresh river water resulting in a reduced salt concentration described as ‘brackish’. A salt gradient exists between the sea water and the fresh water and this gradient in salt content or salinity is reflected in the changing nature of the plants and animals that live in the estuary.
Very few freshwater animals and plants are adapted to anything other than a very small rise in salinity. At the mouth of an estuary therefore live true sea creatures, such as pipi, intolerant of highly reduced salinity, while those at the head are freshwater forms (e.g. mud snails) intolerant of any increase in salinity. In between these extremes live marine species that are adapted, to a greater or lesser degree, to deal with the physiological demands of reduced 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 graduating to sand flats at the mouth. Estuaries are a region of deposition of the sediments carried downstream by the river.
In the estuary water flow has slowed sufficiently for the suspended sediment to drop out of the water. However, at the mouth, constant disturbance by wave action and water currents tends to move the finer sediments back into suspension leaving the courser sands behind.
Pāuatahanui Inlet is unusual in that the range of salinity from head to mouth is much smaller than in most estuaries, so that the sand flats at the mouth 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 the restriction on diversity that is demonstrated here, Pāuatahanui Inlet does have a number of distinct habitats that provide for a varied and interesting fauna and flora.
At the eastern end of the Inlet is a large area of saltmarsh covering about 100 ha. The seaward side of the marsh is subject to tidal inflow most days of the year and is dominated by sea rush (Juncus krausii). This gives way to jointed rush (Apodasmia similis) as the amount of tidal inundation decreases. Plants in this marsh must be able to survive long-term 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 the Pāuatahanui Wildlife Reserve.
Tidal Flats and Shores
At low tide extensive tidal sand flats extend out from the saltmarsh and also dominate the western end of the Inlet. Over this entire area the dominant species living in the sand are cockles (Austrovenus stutchburyi) and the worm Axiothella serrata (no common name) while several species of crabs spend low tide in burrows on the higher parts of the shore. Mud snails (Potamopyrgus antipodarum) are abundant on the surface of muddier shores around the mouth of the Horokiri Stream and in the saltmarsh. Also found in large numbers are the little, scurrying, tunnelling mud crabs (Austrohelice crassa) with the entrance to their tunnels often the only evidence of their presence at low tide. Beds of seagrass (Zostera novazelandica) occupy parts of the tidal flat and sea lettuce (a large, leaf-like, bright green alga) (Ulva lactuca) covers much of the flats during summer. Many birds feed on these flats.
Although Pāuatahanui Inlet is unusually shallow, with a maximum depth of only about 2 metres, much of its area remains under water at low tide. A large number of animal species do live in this zone but, for reasons of accessibility, few are ever seen alive by the casual human visitor. However, dead shells of the Arabian volute (Alcithoe arabica), a large carnivorous snail that can be found here, are common on the shore.
The Water Column
The waters of the Inlet support a great diversity of life, from minute plankton to the occasional visiting seal. Numerous fish species that feed on the invertebrates and plants found on the harbour floor are themselves food for several species of birds while other fish use the Inlet as a nursery, like the rig shark, or as a pathway to reach the major streams like the species of inanga that are harvested by humans during the whitebait season.