Pauatahanui Estuary Earthcache:
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. The walnut-sized mud snail, Amphibola crenata hold a critical place in the wetland food web. As they chew on the organic material that forms their food, they stimulate the growth of bacteria that return nutrients into the water.
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.
Pauatahanui 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 nearly 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.
This is a two stage Earthcache:
Please read the following and submit your answers and wait for permission before logging.
Stage 1:
The first set of co-ordinates take you to a sign. Read the sign and answer the following :
1. What is fragile in the estuary?
2. The Estuary is the largest relatively unmodified saltmarsh, where ?
3. The Saltmarshes, scrublands and manuka protect what?
Stage 2:
The second set of co-ordinates take you to a hide. Parking is available at the end of Grays Road, just before it meets Paekakariki Hill Road, and follow the walking track to the duck hide. Once at the hide, to the left is a track. Follow the track to the edge of the estuary, take a look at the surroundings and answer the following:
1. Describe what happens when you put your hand in the water and touch the estuary floor.
2. Where do you envisage the sediment from this estuary is derived from?
3. Describe the marine life you see in the waters of the estuary.
4. If you would like you could take a picture of your GPS (or yourself) through the middle viewing window of the hide or at the front of the hide and post with your log once permission is obtained.
Finally, from the information contained on this page and undertaking a little research of your own answer the following:
1. Does the water at this part of the estuary contain high or low levels of saline?
2. What causes water circulation?
3. What is an Amphibola Crenata?
Please note, logged finds without answers correctly submitted will be removed.
I hope you enjoy completing this Earthcache and learn a little more of our local geology.
If you would like to be involved in the maintenance and protection of this estuary through our local community contact the Guardians of Pauatahanui Inlet [GOPI] www.gopi.org.nz
