Estuaries and hapua - Climate Change & Nature: New Zealand

Nature-based solutions: Estuaries and hāpua

Avon-Heathcote estuary image: Google Earth

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Estuaries and hāpua

Summary

Estuaries are partially enclosed tidal water bodies, sometimes called a harbour, bay, inlet or sound, that forms where rivers and coastal wetlands meet and mix with salt water from the ocean.
Hāpua are freshwater lagoons that form on mixed sand and gravel beaches where braided rivers meet the coast (Fig. 1).
Both act as buffers between the land and ocean, absorbing floodwaters and dampening storm surges from the ocean.
Estuaries are rich habitats and essential breeding grounds and nurseries for many species of plants and animals. Their importance as sources of mahinga kai makes them taonga ecosystems.
Estuarine plants (see blue carbon) drawdown and store far larger quantities of CO₂ than land-based forests.
The National Policy Statement for Freshwater Management 2020 recognises that estuaries, hāpua, and all the waterways that feed into them, must be managed as single (hydrologically-linked) entity from the mountains to the sea.
The Canterbury Water Management Strategy recognises that estuaries and haphu/lagoons are highly mobile features that will naturally migrate inland as sea levels rise.

Home > Nature-based solutions > New Zealand native ecosystems > Estuaries and hapua

Summary

Estuaries are partially enclosed tidal water bodies, sometimes called a harbour, bay, inlet or sound, that forms where rivers and coastal wetlands meet and mix with salt water from the ocean.
Hāpua are freshwater lagoons that form on mixed sand and gravel beaches where braided rivers meet the coast (Fig. 1).
Both act as buffers between the land and ocean, absorbing floodwaters and dampening storm surges from the ocean.
Estuaries are rich habitats and essential breeding grounds and nurseries for many species of plants and animals. Their importance as sources of mahinga kai makes them taonga ecosystems.
Estuarine plants (see blue carbon) drawdown and store far larger quantities of CO₂ than land-based forests.
The National Policy Statement for Freshwater Management 2020 recognises that estuaries, hāpua and all the waterways that feed into them, must be managed as single (hydrologically-linked) entity from the mountains to the sea.
The Canterbury Water Management Strategy recognises that estuaries and haphu/lagoons are highly mobile features that will naturally migrate inland as sea levels rise.

“Estuaries receive and accumulate large amounts of whatever is emptied into the catchments that feed them – by foresters and farmers at the top of the catchment to motorists and businesses right on the foreshore, as well as everybody in between. As a result, trying to manage estuaries involves working with people who can live hundreds of kilometres apart and who do not necessarily feel themselves to be part of a catchment community, let alone connected to the estuary.” – Parliamentary Commissioner for the Environment, 2020.

Fig. 1: Rakaia River hapua visible as a long thin blue lagoon along the coast. The northern arm of the Rakaia River flows directly into the hapua, while the Southern (wider) arm also stretches along the coast at the rivermouth before opening to the sea. This configuration is highly dynamic and can change at any time during storms or floods. (Image: Phillip Capper | Wikipedia Commons)

Fig. 2: Diagram of the Hakatere Ashburton hāpua. The same structural features appear in all hāpua (Image: Sonny Whitelaw)

The problems

When you look at an estuary, at first glance it seems to be a permanent geographical feature, fixed in place like a mountain or valley. In reality, estuaries and hapua are continuously reshaped by an an endless dance dynamic dance between wind and waves, rivers, tides, and the ecosystems on the land and in the ocean.

A change in just one of these element interrupts this dance, which in turn affects every other element. Cutting down forests and destroying the wetlands across the Canterbury Plains and replacing them with intensive agriculture, and confining the once mighty braided rivers that roamed across the plains that they formed, has completely altered the nature of estuaries.

What were once pristine ecosystems that provided a huge range of life-supporting ecosystem services, including mahinga kai and carbon sequestration, are now on the receiving end of pollution and rubbish that’s carried from the land into the rivers: chemicals including nitrates and phosphates from intensive farming, discharge from sewerage treatment plants, heavy metals that run off roads and urban and industrial development, rubbish dumped on the riverbeds (see for example this article on Linwood Canal, Christchurch), and staggering quantities of soil eroded from land now stripped of the trees that once held soils in place:

“…The biggest challenge for our estuaries can be summed up in one word: mud… When trees are removed from the land, soils become loose and are easily eroded. Heavy rain transports that soil into rivers…. Like tar choking the lungs of a smoker, the sediment travels down waterways and settles in estuaries as mud. Mud smothers bottom-dwelling animals. It makes the water cloudy and changes the landscape, ultimately impacting the estuarine habitats and the organisms that live in them.’” – Sam Fraser-Baxter, NIWA

As the climate warms, estuarine ecosystems are also coming under increasing threat. In estuaries where fish farms exist, such as Marlborough Sounds, the combined impacts of excessive nutrients plus warmer and increasingly acidic waters has already led to a toxic algae blooms. This affects the health of the entire ecosystem, threatening wild fish spawning grounds and nurseries, the aquaculture industry, and the safety of mahinga kai.

Estuaries and hapua are also facing another problem: sea level rise. Areas that were once brackish will become more saline, changing the types of plants and animals that live there. And, in spite of the policies in both the Canterbury Water Management Strategy and the New Zealand Governments Coastal Policy Statement…

“…natural inland migration and adjustment of coastal estuaries and river mouths is unlikely to occur as, ‘it is unlikely that people will readily allow new areas of dunes, marshland or estuary to form behind those now present. The most probable response to sea level rise will be to protect assets and infrastructure by erecting new hard barriers to prevent erosion, planting sand dunes to stabilise them, and infilling encroaching wetlands and installing new drainage. This scenario (often termed ‘coastal squeeze’ in the international literature,) means that rising sea levels will probably remove large areas of the rich biological habitat.’” – Climate Change Adaptation in New Zealand: Future scenarios and some sectoral perspectives

The solutions

“It has proved hard enough to win acceptance for the fact that activities at one end of a catchment can impose unacceptable costs on communities far from the source of the problem. But at least pressures like nitrogen pollution or sedimentation are within the control of the wider community.

Estuarine change caused by the impact of emissions on sea level, temperature and ocean acidity will be determined by the action or inaction of a global community that shows no sign of acting on the scale required.

Faced with this unpalatable prospect, people who live near estuaries have no option but to adapt. The only question is whether they do so proactively with at least a measure of control over how to manage the process, or reactively, in which case the costs of disruption are likely to be much higher. Whether we can summon the social and political resources to adapt proactively is the critical question.

The longer it is left in the too-hard basket, the lower our chances of making a durable transition.” – Parliamentary Commissioner for the Environment, 2020.

Key actions – what adaptation might look like

Protect estuaries by protecting the quality of water in the waterways that enter them. The National Policy Statement for Freshwater Management 2020 offers multiple options to enable this. This and the ‘Essential freshwater Package‘ was met with objections from the farming sector over nitrate limits (Video 1), which endangers the ability of saltwater marshes to drawdown and store CO₂. However, an increasing number of farmers are seeking smart solutions by embracing regenerative agriculture. Many other groups and individuals are also working to protect, enhance, and restore rivers, lakes, wetlands and coastal dune systems. Every positive action will help make a difference.

Choices will need to be made: when and how to retreat

Retreat. As the Parliamentary Commissioner pointed out, people will have no option but to adapt to irreversible sea-levels rise by retreating from the coast. This includes retreating from the edges of estuaries and hāpua so that these taonga ecosystems have space to migrate inland. Choosing a controlled retreat now will save billions of taxpayer dollars from being wasted in defending the indefensible, while enabling retreating estuaries to continue to act as buffers against storm surges.

Restore degraded estuarine ecosystems. Sea grasses and saltwater marshes drawdown and store more CO₂ than forests. As they grow quickly, if given the room to move, as sea levels rise they will simply migrate inland, bringing the animals that depend on them and their carbon-storing capabilities with them.

Innovate. When ecosystem restoration is the key driver, integrated multi-trophic aquaculture can capture and store staggering quantities of CO₂ while also producing plentiful safe kai and providing habits for other species (Video 2). See also seaweed (this website) for research and field trials currently underway in Aotearoa.