Key Ideas about Estuaries

10K photo of estuary. R. Chesher

Estuaries are very important for the ocean. They are places where rivers meet the sea and tides mix the fresh and salt water as they cover and uncover large areas of sand and mud-flats.

They act as a huge, living filter that removes silt and transforms excess nutrients from the water flowing into the sea from the land. They are very fertile and may produce four times more plant material than farmland.

The roots of estuary plants make a great hiding place for baby fish and shrimp, and many kinds of fish grow up in the wetlands. When they are old enough to defend themselves, they swim out into the open sea. At least 30 species of finfish, like snapper, parore, flounder and mullet spend much of their lives in estuaries. Trevally, red cod, gurnard and eels come in to estuaries to feed while freshwater eels, whitebait and trout migrate through estuaries.

As the meeting place of two ecosystems - fresh and salt water - estuaries have an abundance of species of life found nowhere else, and together, these species create a labyrinth of habitats. The plants and animals that live in estuaries have special adaptations for living in environments where temperatures, salt content, and tidal exposure vary rapidly over the course of a day.

Estuaries are important for people, too. For swimming, boating, harbours, commercial and recreational fishing, aquaculture and as a refuge for wildlife.

Mangroves are the only kinds of trees that can live permanently in sea water. Some mangrove trees can survive being completely underwater during very high tides. They close the pores in their leaves and literally hold their breath until the tide goes down again.

Estuary plants, like mangroves, eel grass, and swamp grasses trap fine silt with their roots, preventing the silt from entering into the ocean and harming sea creatures. The New Zealand Mangrove (Avicennia marina resinifera), known to the Maori as manawa, is the only tree that can live in sea water. They have special roots that poke up from the black estuarine mud, bringing oxygen to the root system, and special salt glands in the leaves to get rid of excess salt.

Estuary plants use the nutrients coming down to the ocean and store them in their leaves, branches and trunks. This is very helpful to the sea, because nutrients coming down the river sometimes come very fast (during heavy rains) or very slow (during droughts). The estuary plants are like fat deposits, holding the nutrients and slowly releasing them over a long period of time. They convert the nutrients into forms that are easier for the creatures of the ocean to use. First the trees and grasses drop their leaves into the water. Then bacteria, fungi, invertebrates and some fish eat the leaves. Then larger invertebrates and fish eat the smaller creatures. Some of the nutrients are released into the water so the microscopic plants - the phytoplankton - can use them.

Phytoplankton floats in the water and coats the surface of the mud-flats. These tiny plants are important food sources for crabs, mud snails, and many species of fish.

There are some 300 estuaries along New Zealand's coastline. Six of them, including Manukau Harbour, have more than 80,000 people living around them. Roading, stopbanking, reclamation, dredging, and removal of sand from estuaries have had a severe impact on New Zealand estuaries. They are often used as rubbish dumps.

Many estuaries are polluted by excessive nutrients from farming and sewage, from run-off, rubbish, oil, and silt. While estuaries are designed to deal with some silt, excessive amounts over long periods of time smother the creatures that live on the mud-flats and sandbanks. In addition, organic poisons (such as agricultural chemicals) adhere to the fine particles of silt so that today's silt is often toxic as well as suffocating for marine life. Excessive siltation, combined with poor water quality from urban and farm run-off is considered one of the major problems for New Zealand estuaries.

Not all signs of decay in estuaries are pollution. Following storms, decaying seaweed is a natural and beneficial feature of estuaries. On incoming tides, a brownish foam on the beach is a natural bloom of beneficial phytoplankton.

Activities for Estuaries

Resources to use

Microsoft Oceans CD.

Forests in the Sea (Poster) by John Walsby. New Zealand Geographic Magazine #15. This is one of the best resources for NZ schools on mangroves.

Margins of the Sea, Exploring New Zealand's Coastlines 1985, John Morton.

Mangroves - Not just a stick in the mud TVNZ video from Wild South.

Margins of the Sea, Exploring New Zealand's Coastlines 1985, John Morton.