The Okura Estuary.
Most rivers empty into the sea and there is little difference at the river mouth between low and high tides.
An estuary is different. The Okura River estuary is a coastal plain estuary, which means it is formed as sea levels rose and flooded a river valley. The Okura estuary is protected from the open sea by a sand bar and the islands of the Hauraki Gulf.
At high tide seawater covers the river channel and moves upstream so that fresh and salt waters mix inland. At low tide the sea retreats revealing the previously submerged river and exposing large areas of sand and mudflats. The waters of an estuary are generally calm. The river carries nutrients and sediment from upstream, which is dropped into the calm estuary waters at high tide and some of which is washed out to sea as the tides pull back. The action of the tides is a steady filling and emptying of the estuary basin rather than the powerful action of waves dashing against the land.
This mix of salt and fresh water, ebb and flow, creates an extreme environment for all forms of life. Plants need to be able to cope with regular baths of salty water and exposure to air, sun and the weather during the twice-daily periods of ebb.
The mangrove tree is a perfectly adapted plant for tidal habitats. Their survival depends on the movement of tides creating an ecosystem filled with life. The roots hold the sand and mud in place against the tidal flow as well as trapping sediment washed down the river. Bit by bit over many years this raises the land. The seed pods float on the water and are left to germinate and grow at the high tide mark expanding the range of the trees and creating a forest.
A mangrove forest is a nursery area and safety zone for schools of small fish. They can find food amongst the roots and branches as do many types of shellfish, barnacles, snails, oysters, mussels. Marine worms, shrimps and crabs find food in the sediment trapped by the tangled maze of mangrove roots and they take their place in the food chain. Different forms of life can be found above the high tide mark amongst the exposed leaves and branches, like spiders, insects and birds.
A large number of winged insects like midges, flies, mosquitos and gnats are found in estuary habitats adding variety to the diets of fish and birds.
If pollution from industry, farms or cities washes into the river that feeds the estuary this can be disastrous for the health of the species living there. The cycle starts with the smallest forms, worms, shellfish, crabs and moves up the food chain affecting everything. Nature can recover, if the event is short and prevented from occurring again, but regular inflows of polluted water will kill everything. The whole estuary then becomes a toxic, smelly wasteland where nothing grows. Many birds depend on estuary habitats and would face extinction should this happen.
Different species target particular areas and food sources within tidal habitats and use different ways to . Some (like oystercatchers and godwits) probe the mud with their long beaks feeding at the water’s edge. Others (like pied stilts) have longer legs and can hunt in deeper water. Certain species (like dotterel, turnstones and wrybills) feed on the marine life found on the surface of the sand and under stones. Other species (like herons) catch fish by stalking the shallows or, as do shags, terns and kingfishers, diving after fish. Others (like spoonbills, teals and ducks) sift the water through their bills trapping small invertebrates and nutrients. While still other birds (like gulls and skuas) clean up the habitat by scavenging on dead fish or other carcasses left by the tide or washed down the river. Some birds like the welcome swallow fly over the water catching insects on the wing.
At the edge of the estuary the land may be swampy and then a whole new community of animals and plant life dominate. Birds like rails, pukeko, bitterns and fern birds call this habitat home.
Because an estuary is rich in life forms it attracts migratory birds that do not breed in New Zealand but come to escape the cold weather, fatten up and then fly north to avoid our winters. The Firth of Thames is one of the most significant estuary habitat for migratory birds in New Zealand especially bar-tailed godwits and lesser knots. Over 74 different bird species have been recorded here.