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Estuaries connect land and sea. They are partly enclosed bodies of water situated at the edge of the land – a mixture of freshwater from streams and rivers and saltwater from the sea. Estuaries come in all shapes and sizes and can be called harbours, inlets, bays, lagoons, sounds, wetlands and swamps. They are the nurseries of oceans. Many fish and shellfish are spawned in estuaries. Full of nutrients and home to resilient organisms, estuaries provide rich feeding grounds for fish and birds.

An open ecosystem

The estuary is an ecosystem – a group of living and non-living things interacting with each other. The physical environment of the ecosystem is the habitat in which organisms live.

New Zealand estuaries include many different habitat types, such as sandflats, mudflats, tidal channels, shellfish beds, saltmarsh, seagrass meadows and mangrove forests. Because estuaries are highly dynamic environments subject to processes occurring on the land and in the sea, the locations, sizes and types of habitats can change relatively quickly, or form over years or decades. Each habitat type has different ecological functions as well as values for people.

The habitat must provide the organisms within it with what they need for survival such as food, water oxygen and minerals. The organisms within the estuaries, as with all ecosystems, are constantly competing, preying and/or co-operating with others to survive.

The estuary ecosystem is ‘open’ because it is also interdependent with the connecting environment. Nutrients are brought in by rivers and dispersed by tidal currents. Nutrients are also transferred through the nutrient cycle. Plants (such as phytoplanktonalgae, seagrass, salt marsh and mangroves) take up nutrients, which are then eaten by animals. When the plants and animals die and decompose, the nutrients are released again. Organisms such as fish and birds transfer nutrients as they move in and out of the estuary.

A rich biodiversity of life

Estuaries are one of the most productive ecosystems in the world and contain a rich biodiversity of life. An estuary may appear to be just an expanse of mudflats but it is teeming with life, including bacteria, snails, worms, crabs, fish, shellfish, mangroves, seagrass, and migratory and coastal birds. At least 30 species of fish use estuaries at some stage in their life cycle including snapper, flounder, mullet, sole, rockfish, kahawai, trevally, parore, red cod, gurnard, eel, salmon, whitebait and sharks. The life is diverse yet connected – interdependent for its own survival.

Food webs and interdependency

Vital to the interdependence of life in estuaries is the feeding relationship, known as the food web. The estuary food web contains:

  • phytoplankton – microscopic plants that produce food through photosynthesis and also absorb nutrients from the water
  • larger plants such as mangroves, saltmarsh and seagrass, which also produce food through photosynthesis and absorb nutrients form the water
  • zooplankton – microscopic animals that eat phytoplankton.
  • larger animals such as filter-feeding worms, crabs, snails, shellfish and fish, which feed on detritus, zooplankton and larger organisms
  • detritus – dead organic matter, which includes dead phytoplankton, zooplankton and other plants and animals.

Detritus is predominantly made up of rotting plants such as mangroves, seagrass and rushes. Mud-dwellers such as snails, worms and crabs play an important role in recycling the nutrients from the detritus within an estuary. They consume the detritus material and produce nutrient-rich waste. This causes plants to grow quickly producing more food and then detritus. Larger animals such as fish and birds consume the mud-dwellers, transferring nutrients further afield.

Estuary plants such as mangroves, saltmarsh and seagrass also provide a habitat for a range of organisms. The plants trap sediments coming into the estuary and therefore nutrients. Filter-feeders such as snails, cockles and oysters live at the base of these plants. Shellfish use siphons, gills and cilia (fine hairs) to strain out food particles suspended in water. Worms and snails filter out food particles using their tentacles. The plants also provide protection and food (the mud-dwellers) for a variety of juvenile fish. This makes for an attractive breeding habitat for many birds such as the pūkeko, bittern and marsh crake. Estuaries are also rich feeding grounds for migrating birds such as bar-tailed godwits, lesser knots and plovers.

Kuaka

The kuaka or Godwit is found in a number of New Zealand estuaries. Estuaries are important for godwits, who use the habitat to recover from their 12,000 km migration from their breeding grounds in Alaska. It is also the estuary that will provide the nourishment the godwit needs in order to then make the return migration to Alaska the following breeding season.

Learn more about godwits and their epic migration.

Nature of science

Scientists study the interdependence of organisms and their physical surroundings in an effort to understand what makes a healthy ecosystem. Scientists then use this information to inform decision-makers about how proposed developments may impact on that ecosystem.

    Published 12 June 2017