An overview of the catchment - values, threats, opportunities:
Pauatahanui is one of two arms of Porirua harbour. Porirua Harbour largely determines the character of this region. Visually it is united by views of water flanked by shallow shores behind which are patterned hills - we recognise this in the Porirua City's emblems. Our first settlers, Maori and Pakeha, sought the sheltered waters and the kai moana that were accessible from it. Today we fish, collect shellfish, swim, sail, boat, water ski on the water, or just look at it from the shore or from the homes of those lucky enough to live within view. We have a physical and biological environment which is pleasant to live in.
Tides feed the Inlet twice daily, and streams bring freshwater continuously. Pauatahanui Inlet is fed by five streams, and also urban stormwater drains that enter it directly. All the land drained by these waters forms the catchment. What happens in headwaters of the streams and drains affects all below, right from the ridge tops down to the mud flats, harbour mouth and open sea. The quality of this in-flowing water determines the quality of the Inlet and the quality of our use and enjoyment of it.
This environment is a complex system - part of its beauty is its complexity. Its ecological stability is dependent on its complexity. Food webs link single celled plants, bacteria, sea grass, sea lettuce, rotting leaves, rushes and sedges, diatoms, planktonic crustacea, cockles, crabs, worms, pipi, wedge shells, snails, eels, white bait, mullet, flounder, sea horses, spotties, rays, spotted dog fish, herons, oyster catchers, swans, ducks, gulls ...... They are interdependent, and we are dependent on them if we wish to maintain this Inlet as a living place we enjoy and are proud of.
Wider importance:
Pauatahanui Inlet is the largest area of estuary in the lower North Island. It has local and New Zealand wide importance for both fish and bird life. The productivity of all estuaries makes them of crucial importance in the reproduction and nurturing of fish and birds. Fish spawn and grow to maturity in estuaries. Birds nest in estuary marshes; others make for estuaries to fatten before migrating to other nesting sites such as South Island river flats, or to Australia, the Pacific or the Arctic.
The importance of the estuaries to reproductive cycles of fish gives them considerable economic and recreational value. And in this Pauatahanui Inlet is no exception.
Pauatahanui mud-flats and salt marsh - estuary ecology:
Almost half the area of Pauatahanui Inlet is exposed at low tide. Floods bring mud and sand that make the shallow flats, and minerals from bush soils and farm lands come down streams. Estuary flats are therefore rich in minerals and organic matter - rich sources of food for plants, animals and decomposers. Along with warm shallow mineral-rich sea water this makes estuary ecosystems the most productive habitats on earth. There is rapid turn over of food material.
Pauatahanui Inlet has approximately one square kilometre of mud flat. The mud flats produce four times more plant material per hectare than the best New Zealand pastures. Salt marshes produce additional material and they are extending since the creation of the Forest and Bird Wildlife Reserve.
Features of estuaries:
- Productivity of estuarine ecosystems: the most productive of any ecosystems in the world
- Much of the food material in estuaries is invisible: it consists of fast growing bacteria, unicellular algae and animals. Some live in mud, some are plankton
- Decay processes and scavengers are important contributors to productivity
- Dissolved organic matter (a product of decay) is important in food chains
- Food webs of estuaries are complex and diverse. Small worms are the most numerous of visible organisms in Pauatahanui Inlet. Cockles are the most important by volume. There are 10 species of crab. The Inlet sees about 50 species of bird a year (30 permanent inhabitants), and about 40 species of fish.
- Birds and fish reproduce in estuaries - there is abundant food, shelter, shallow waters quickly warmed by the sun
- Birds and fish migrate through estuaries - they are either the food rich nesting or spawning grounds, or they may be staging posts and fattening stations during a long journey.
Problems - Environmental Damage:
Most of the problems of the Inlet arise from conditions in the catchment, rather than the waters of the Inlet itself. Some environmental damage is the inevitable consequence of the wholesale change over the last 200 years from a catchment dominated by forest to one dominated by pasture. The natural process of erosion followed by deposition down stream and in the estuary has been speeded ten fold by the removal of the vegetation cover.
Urban development on the western and southern shores has again accelerated erosion. Urban development is extending.
Tidal flow is an important physical factor in estuaries. In the 1970s it was calculated that tidal flow completely flushed Pauatahanui Inlet in three days. As the Inlet silts up the total volume of tidal water is reduced, and this reduces the flushing action and the volume of food material brought in by the tides. It will be particularly important to ensure that the effective width of the entrance to Pauatahanui Inlet is not further constricted by bridge building or other construction works - efforts should be made to widen the channel when further construction work is in progress.
Urban stormwater drains are not waste disposal systems - they are designed to carry rain water alone. Pollutants that enter drains flow to streams and the Inlet. Pollutants poison waterways. They kill insects, snails and worms in streams, encourage weeds to grow, reduce oxygen levels in the water, suffocate shellfish, fish and other water life in the harbour , and spoil the recreation values of the streams and the harbour.
The Guardians of Pauatahanui Inlet in 1998 conducted an "Action Pauatahanui- drains to streams to Inlet" campaign to make the public more aware of this source of pollution.
Currently there are unacceptable levels of faecal coliform and enterococcal pollution in cockle flesh. These are indicator organisms of material from human or animal faeces that the shellfish have filtered from the water.
There are raised levels of Polycyclic Aromatic Hydrocarbons (mainly from incomplete combustion of motor vehicle fuels).
Analysis of heavy metals in sediment muds shows slightly raised levels zinc, lead, copper and cadmium. Heavy metals are absorbed by clays in estuary muds and build up year by year.
Organochlorines, mainly from pesticides, are released on farms and horticultural sites. They are long lasting in muds and in the bodies of organisms. They are fat soluble rather than water soluble: hence they are not excreted by normal excretory processes which depend on the flushing effect of water. They accumulate progressively, and can be toxic and carcinogenic. There are raised levels of organochlorines at the Pauatahanui stream mouth - probably from DDT used more than twenty years ago.
Cockle Studies - Indications of Decline:
The most abundant shore animal in the Inlet is the cockle. They are filter feeders and draw water through their gills to get the food and oxygen they need, and so clarify the water. In 1976 it was calculated that a third of the volume of incoming tides passed through the gills of cockles daily.
Cockles at that time made up 80 % of the biomass of mud flat organisms, ie. of the major food source of fish and shore birds.
There have been four surveys of cockle numbers in 1976, 1992, 1995 and 1998. Those in 1992, 1995 and 1998 were carried out by the Guardians.
Summary of cockle abundance in Pauatahanui Inlet in 4 surveys
These surveys show there has been an alarming decrease in cockle numbers, and the trend is continuing. Cockle density in 1995 was one third of the density in 1976. There was a slight increase in numbers in 1998, due almost entirely to an increase in juvenile cockles less than a year old (less than 10mm in width). Adult cockles in 1998 were close to the same as in 1995. While this increase in juveniles in the 97-98 year is in itself good, mortality rates in juveniles are much greater than they are in the older
larger cockles. We hope a large proportion of them will survive - another survey in 2001 will investigate this.
Increased silt washed onto the shore from subdivisions and road works may be a major factor in the decrease in cockle numbers. Increased organic and mineral pollution may also be inhibiting cockle growth. At the same time there has been a marked increase in numbers of cockles taken from the Inlet by humans this decade. Further investigation of this factor is needed too.
Transmission Gully: disaster or opportunity?
Most of the proposed route for the transmission Gully motorway is in the Pauatahanui Inlet catchment. Construction works and removal of vegetation cover have the potential to do irreparable harm to the Inlet. Planning must be aimed at making any motorway construction an environmental plus. Opportunities must be seized to not merely mitigate the effects of earthworks that are necessary to road construction, but to create ponds and wetlands that could act as settling areas and reduce rates of run-off and storm peaks in the streams. Revegetation of the road line and of riparian
strips along sides of streams above and below the road would reduce the effects of flash floods, and the potential for immense damage from eroded silt and clay. These will also increase habitat diversity and increase visual appeal of the landscape.
There is also an opportunity to make the road line a biological corridor linking bush and pine forests that the road traverses.
The 'Neighbourhood Biology' concept and community involvement in environmental development:
- This hinges on the democratic belief that control of change is possible: people can make a difference.
- Public concerns and action by individuals has led to several initiatives that are helping to safeguard the future of the Inlet:
- Public concerns resulted in the coordinated scientific studies called the Pauatahanui Environment Programme in the mid seventies. The initiative of the Royal Forest and Bird Protection Society led to the development of the Pauatahanui Wildlife Reserve in the eighties. Currently fifty or more Forest and Bird volunteers work for the development of this reserve most months of the year.
- The Guardians of Pauatahanui Inlet foster public awareness of Inlet issues, encourage research, and act as advocates in public forums. The Guardians have approximately 100 individual, family and corporate members.
What should and could be done?
- We should not consider ourselves to be separate from the ecosystems in which we live and work - people are integral parts of ecosystems.
- Pauatahanui Inlet needs a comprehensive action plan that will coordinate activities of several responsible bodies that at present act independently, and sometimes ineffectually. These bodies include Wellington Regional Council, Porirua City Council, Ministry of Fisheries, Ministry of Agriculture and Forestry, Department of Conservation, Ministry for the Environment, and the Forest and Bird Reserve Management Committee. Last year, 1999, the Porirua City Council called meetings to form a coordinating steering group to produce an action plan for Pauatahanui Inlet. This Pauatahanui Inlet Advisory Group has representatives from the City Council, Wellington Regional Council, Department of Conservation, Ministry of Fisheries, Forest and Bird Protection Society and Guardians of Pauatahanui Inlet.
Environmental action for schools:
- Trace the development of cockle juveniles throughout a year by sieving and counting at the same transect at say two or three monthly intervals
- Investigate effects of variation in substrate on cockle numbers by using nested sieves to separate gravel, sand, silt and clay, and relating proportions of these to cockle numbers at the same transects.
- Observe evidence of colonising of mud flat be salt marsh plants to measure the extension of the salt marsh
- In a group remove exotic weeds from area of salt marsh or reserve
- Assist the Wellington Regional Council with riparian planting, or the Trees for Survival programme
- Monitor the freshwater stream life of catchment streams, with the view to watching for changes that may be caused by roading, subdivision, or changed agricultural or farming practices.
There are many more possibilities.
Neil Bellingham neilbell@ihug.co.nz
October 1999
