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 environmentswhere 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.
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.
