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People like to live near estuaries. In pre-European times, estuaries were favourite places for Māori to gather – particularly to harvest and enjoy kaimoana. Today, there are major settlements near estuaries in Whangārei, Auckland, Tauranga, Napier, Wellington, Nelson, Christchurch, Dunedin and Invercargill. People value estuaries for many reasons – from recreation and aesthetics to fishing and harvesting food from the sea. Consequently, estuaries are subjected to impact from human activities.

The two main problems affecting estuaries are sedimentation and pollution from run-off.

Sedimentation

This is probably the biggest threat to estuaries. It occurs when rain, rivers and streams wash sediment off the land and into estuaries. This can be sped up by land clearance for urban living and industrial development that exposes soils, leaving them vulnerable to being washed away. Sedimentation smothers the seabed, killing mud-dwellers that can’t burrow up quickly and disturbing the delicate balance of life in the estuary. Eventually, sedimentation will fill up an estuary and turn it into dry land.

Fine suspended sediments reduce light penetration and visual clarity of estuarine water. This can result in the loss of aquatic plants such as seagrass, which can no longer photosynthesise to make their food. Loss of light and water clarity also affects visual predators, such as birds and some fish (such as snapper) who can’t find their prey. An estuary in this state is also not aesthetically appealing for swimming and recreation.

Pollution from run-off

Estuaries can easily become polluted due to industrial waste and agricultural and horticultural run-off.

Industrial and urban waste

Heavy metals and polycyclic aromatic hydrocarbons (PAHs) come from industrial and urban zones. As pollutants, these compounds are toxic – many are identified as carcinogenic, mutagenic and teratogenic. They stress an ecosystem, lowering its resilience so that the health of the ecosystem is weakened (becomes less productive, has increased mortality and is less resistant to further stress). The compounds enter the food web through the filter-feeders (such as bivalves and worms). The pollutants then bioaccumulate, threatening the health of those higher up in the food web – a major concern for people who eat kaimoana.

Agricultural and horticultural run-off

Nutrients are essential to support the productivity of estuaries, but too many nutrients can alter the balance of the natural cycle of nutrients within the estuary. Nutrients can be carried into estuaries from neighbouring farms and orchards by groundwater and waterways, causing algal bloom or excessive growth of plants such as mangroves. In New Zealand, however, algal bloom in estuaries is often suppressed due to light reduction from suspended fine sediments (reducing the potential for photosynthesis). Other types of pollution from agricultural and horticultural run-off include pesticides and herbicides.

Other human impacts affecting estuaries

  • Oil spills – accidental oil spills from ships at sea, for example, the Rena oil spill affected marine life in Tauranga Harbour.
  • Excessive harvesting – including kina, snails, bivalves, crabs and flounder used for either eating or bait. Excessive removal of these species threatens the ecosystem.
  • Invasion by introduced species – when ships flush their ballast water while at port. Ballast water is water taken on board at the ship’s port to help balance the ship’s load. The water may contain organisms from the original port. Introduced molluscs and other organisms are a threat to the balance of our ecosystems. The Asian date mussel, for example, thrived at Onerahi Beach and smothered other marine life on the seafloor.
  • Reclaiming land by drainage – for marinas, farms, subdivisions and industrial development. Stopbanks (structures) are sometimes built to prevent land erosion, which stops estuarine habitats (for example, areas for wading birds) moving landwards as sea levels rise. This is known as coastal squeeze.
  • Building bridges – this restricts inner estuary tidal flows.
  • Extracting sand – this includes sand extraction for construction aggregates (for making concrete and so on) and maintenance dredging of navigation channels (such as in Kaipara Harbour). This activity removes organisms’ habitat and is akin to excessive harvesting of the seafloor dwellers.
  • Rubbish dumping.
  • Aquaculture – farming monocultures (one particular species) can upset the balance of an ecosystem, causing its degradation.

These impacts on estuaries contribute to the reduction of habitats for estuarine animals and plants and spoil recreational activity and the beauty of our estuarine coastline. They also jeopardise the role estuaries play in maintaining the health of coastal waters, including the marine organisms within them, which affects the fishing industry and the ability of people, such as iwi, to eat kaimoana as they have traditionally done.

Nature of science

Scientific research sometimes reveals environmental problems that are linked to human activity or lifestyles. We are learning to balance the way we live with environmental needs. This balance between environmental needs and our needs is often the subject of debate involving scientists, environmentalists, authorities and ordinary people.

    Published 12 June 2017