Estuaries

Estuaries

 

Estuaries are the point where streams and rivers rest momentarily as they contemplate their destiny amidst the great salty ocean. As the river lulls in its peaceful respite, it starts to unload its burden - particles washed from the headwaters, foothills and plains of the surrounding catchment. These particles accumulate within the estuary as mementos of the river’s journey. The health of the surrounding land very much affects the health of the estuary.

Estuaries are far more than just a geological process however! We New Zealanders love our estuaries! Estuaries have long been regarded as useful to humans as sheltered harbours and food gathering sites. It is no surprise that most large towns in New Zealand are built around them. Although we have recognised the benefits estuaries can provide, we have not realised our own strength in terms of our impacts upon them. With population increases and industrialisation our impacts have been ever intensifying.

 

 

 

Culture and History of our Estuaries  

Tangata Whenua Values

Estuaries and their surrounding margins are regarded by Mäori as a taonga or treasure.  Many have historical and cultural importance and may include wahi tapu (sacred sites). They are mahinga kai (food gathering) sites, and provide significant habitats for a range of culturally important animals including fish, tuna (eel), birds, reptiles, amphibians and insects. Many plants found in estuaries are used for weaving, medicine and carving.

Sheltered coastal sites were the first homes for our early Polynesian migrants. Fish, shellfish and waterfowl formed a major part of the diet, as did moa in the very early years of settlement. Several important “moa hunter” sites have been identified in estuarine environments, typically on dry shingle spits or sand dunes bordering lagoons or river mouths. Sites on the east coast of the South Island include Papatowai, Pounawea and the Wairau Bar, and in the North Island, Tairua and Opito on the Coromandel Peninsula.

 

The Ecology of Our Estuaries

 The Physical Environment

How many and how big?

New Zealand has over 300 estuaries (McLay et al. 1975) that vary in size from small coastal creeks and lagoons (as small as 1ha) to large harbours, sounds and fiords (up to 15,00ha). The majority of our estuaries are under 500ha.

How were they formed?

Our estuaries have been formed through a variety of means. These include former coastal river or glacial valleys, coastal plains, rocky shores (fiords) and tectonically active zones. Most of our estuaries were developed from valleys formed by river and glacial activity during the last glacial period. These were filled with water as the sea level rose approximately 10,000 years ago.

What happens when freshwater meets saltwater?

The mixing of fresh and sea water within an estuary creates a unique and dynamic environment. Freshwater is less dense and hence lighter than saltwater and will therefore flow above the heavier seawater. In estuaries with a large contributing river, a ‘salt wedge’ is formed.  This is a wedge-shaped bottom layer of seawater which is pushed up the estuary along the river bottom with each incoming (high) tide. The separation between the freshwater and saltwater (the interface) can break down on an outgoing (low) tide. Such conditions of mixing will create zones of brackish, or diluted seawater (a common feature of estuaries).

 

 

 

The Biological Environment

Twice-daily tides create both fluctuating salinity levels and extremes of temperature. This means those choosing to live in estuaries are some pretty tough characters.

Tough but highly productive

Estuaries are some of the most productive, if not the most productive, ecosystems in the world.  Each year they produce about as much per square metre as a tropical rainforest, four times as much as good New Zealand pasture, ten times as much as our near shore sea and twenty times as much as the deep ocean. Only a small part of this living material is consumed through direct grazing by herbivores. Over 90% of the plant matter produced breaks down into tiny particles called detritus. Detritus and the micro-organisms feeding off it, provides fuel for primary consumers such as amphipods, crabs, molluscs and fish.

Current Threats to Estuaries

 

Notwithstanding the Resource Management Act 1991, many estuaries remain vulnerable to harmful influences.

The major threats are:
•    excess silt flowing in from land clearance
•    pollution from sewage, industrial wastes and agricultural run-off
•    oil spills
•    invasion by introduced species
•    reclamation
•    extraction of sand and gravel.
•   
These activities decrease the habitat available for estuarine plants and animals, spoil recreational activity in the area, and jeopardise the role estuaries play in maintaining the health of coastal fisheries and waters.

Why value estuaries?

The estuary plays an important and complex role in the life of the coast, acting as:
•    a breeding and feeding ground for fish and birds
•    a buffer for the coast from storms and floods
•    a filter for sediments and pollutants from coastal waters
•    a record of past environments and events
•    a place for recreation (such as water sports, fishing, shellfish gathering, duck shooting, birdwatching)
•    a site for marine farms.

In the past, many New Zealanders failed to appreciate the value of estuaries, as their life in the towns and on farms seemed to be independent of the state of coastal waters. But with an expanding population demanding clean water for recreation and aquaculture, people are learning that healthy estuaries are an asset and deserve careful management.

 

Turning the Tide

 
Gretchen Robertson and Monica Peters, of the New Zealand Landcare Trust, have put together a superb resource kit for studying and monitoring estuaries – Turning the Tide: an estuaries toolkit for New Zealand Communities.

This is going to be a great help for kaitiaki wanting to look after their local estuary or actively manage an estuary within their taiāpure or mātaitai, or to track whether a rāhui is working. It's packed full of explanations for why estuaries matter and how to monitor ecological changes, mobilise your local community and basically get on with the job. There are also pointers of where else to find more detailed information to help you promote enhanced kaitiakitanga.

You can get a bound and colour copy of Turning the Tide for $25 by writing to:

Gretchen Robertson
NZ Landcare Trust
26 Hunt St.,
Andersons Bay
Dunedin

Or email her at

Gretchen@landcare.org.nz

Alternatively you can download a full copy below

 

Resources

Turning the Tide: An estuaries toolkit for New Zealand communities.

This kit provides communities with tools to build up knowledge about their estuary, host community dialogue, and take action to improve ecosystem health. Read more about the Turning the Tide Toolkit.