Estuaries

Sea level has slowly risen over the last 15 000 years remaining stable over the last 6000 years. As the sea rose it drowned river valleys and filled glacial troughs. Once formed, estuaries make good sediment traps, filling with sediment from both the land and the sea. Sediment from the land include muds and clays delivered by rivers while sediments from the sea are usually clean sands pushed into the estuary by waves and tidal currents. Sediment can also come from shoreline erosion, windblown sediment and shell production.

Estuaries have many shapes and sizes depending on how they were formed. In New Zealand about three hundred and one estuarine systems have been identified ranging from a few hectares to 15000 hectares. 164 are bar built estuaries, 56 are drowned river valleys, 65 are lagoons and 16 are fjords. We have an example of every type of estuary. This variety is due to varied geology, rainfall, land use, coastal wave climate and different basin shapes and the degree of infilling.

NIWA

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Learn more about estuary classifications

Bar-built estuaries

Bar-built estuaries form when a shallow lagoon or bay is protected from the ocean by a sand bar, sand delta or barrier island. Examples of these are found along the west coast of the North Island, eastern Coromandel and the Avon Heathcoate Estuary in Christchurch.

Tectonic estuaries

Tectonic estuaries are caused by the folding or faulting of land surfaces. These estuaries are found along major fault lines, like the Manukau Harbour in Auckland.

Coastal plain estuaries

Coastal lain estuaries are formed by the sea level rising and filling an existing river valley. Examples of this are the Okura estuary, North Auckland.

Fjords and rias

Fjords and rias are U-shaped valleys formed by glacial action. Fjords are found in areas with long histories of glacier activity, seen along the west coast of the South Island. A fjord has a sill and a ria does not.

Examples of estuaries

• Barrier enclosed lagoons e.g. Tairua
• River mouth estuaries e.g. Mokau
• Coastal embayments e.g. Coromandel Harbour
• Drowned river valleys e.g. Raglan
• A semi enclosed bay e.g. Firth of Thames

• These are areas where both ocean and land contribute to a unique ecosystem.
• A basic feature is the instability of an estuary due to the ebb and flood of the tide.
• Plant and animal wastes are washed away, sediment is shifted and fresh and salt water are mixed.
• Estuaries provide a calm refuge from the open sea for millions of plants and animals.
• The diversity of habitats enclosed in estuaries supports enormous abundance and diversity of species e.g. fish, shellfish, lobsters, marine worms, reeds, seagrasses, mangroves, algae, and phytoplankton.
• Visiting species include birds which roost and feed, pelagic fish to spawn and use as nurseries.
• Estuaries are among the most productive environments on earth.
• 4 times more productive in plant matter than a rye grass pasture and 20 times more productive than the open sea. Extremely rich in organic matter and nutrients.
• Photosynthesis occurs throughout the water column and on the sediment surface – very productive.
• The margins of the estuary contain the food webs important producers e.g. algae, eelgrass, rushes and mangroves providing a huge amount of organic matter. Marshes and mangroves produce up to ten tones of plant detritus per hectare per year – considered organic factories.
• Sediments are important as they store organic matter and are the site of microbial activity.
• Micro organisms decompose complex organic compounds into useable forms – ammonia, nitrates and phosphates. This becomes the basic food for primary consumers like crabs, shellfish, snails, and marine worms. These burrowing invertebrates – especially sediment feeders continually turn over the deposited material.
• Filter feeders such as cockles and pipis continually add faecal deposits to the sediment.
• Visiting animals from land, sea and fresh water use the estuary for feeding, breeding, spawning and as nurseries for their young. Food is abundant and easy to access because of the shallow water attracting many types of birds including gulls, ducks and wading birds.
• 20 species of bird visit the Avon-Heathcote estuary in Christchurch.
• At least 30 species of commercial fish use New Zealand estuaries at some stage in their life cycle. Include snapper, flounder, mullet, rock fish, sole, kahawai, trevally, parore, red cod and gurnard. Freshwater eels, salmon, and whitebait migrate through estuaries at least twice in their life cycle.
• Many species of shark use estuaries as pupping grounds. They bear their young in the estuary. These young use the estuary for food and shelter until they are ready to head out to the ocean.
• Plants and animals have adapted specially for the different habitats of this unique ecosystem.

Estuaries enclose a diverse range of habitats from subtidal areas to intertidal areas. These include:

• sheltered upper estuary mangroves, seagrass beds and marshes
• highly energetic beaches on the ocean side of the estuary
• rocky reefs
• wave built bars in estuary mouths
• deep estuarine channels where swift tidal currents flow
• shallow open salt water and fresh water
• river deltas
• tidal pools
• muddy fringing marshes
• mid-estuary sand banks
• intertidal flats
• estuarine beaches

Mangroves provide coastal protection, stabilize mudflats and provide important habitats for a range of animals. They are on of the worlds most productive vegetation types. The leaves they shed form a nutrient compost providing a valuable food source for many organisms.

Mangrove communities include shellfish, worms, crabs, shrimp and other crustaceans living on and in the sediment around the trees. Barnacles, snails and oysters live on the trunks and aerial roots and spiders , insects and birds live amongst the leaves and branches.

Mangroves are one of the starting points in estuarine food webs. Mangroves have become protected habitats in many countries including New Zealand showing a change in people’s perception.

Nutrients are taken up by plants or recycled by sediments. Nutrients are controlled by inputs from land, plant biomass and tidal flow. Estuaries are important sites of nutrient recycling in the coastal environment. Estuaries have a continuous source of nutrients from their freshwater inputs. These freshwater inputs often have additional nutrients compared to the natural situation because they receive wastes from cities towns and farms. The balance of nutrients in an estuary depends on the amount of input from the land, how much is taken up by plants or re-cycled in the sediments within the estuary and how much is exported or imported to and from the ocean in the tidal water.

Nutrients within the estuary are used by plants such as algae in the water, seagrasses, seaweeds and mangroves and they enter other parts of the food web when plants are grazed on by invertebrates. When the plants, or parts of them die, nutrients are re-cycled within the estuary by the actions of invertebrates and bacteria.

Some nutrients are essential to support the productivity of estuaries. Too many nutrients however, can alter the balance of nutrient cycling and lead to undesirable plant growths such as blooms of phytoplankton or seaweeds.

Plants are grazed and filtered by invertebrates like snails, cockles and oysters. These are eaten by juvenile and other small fish which may be hunted by larger fish like snapper, kingfish. rays and even sharks.