Waituna Lagoon

The Waituna Lagoon is one of the best remaining examples of a natural coastal lagoon in New Zealand.

It is a highly valued, large, brackish coastal lagoon that is fed by three creeks, and drains to the sea through a managed opening. Historically the lagoon was surrounded by peat bog wetland, the drainage from which gave the lagoon its characteristic clear brown stain, low nutrient status, and low pH. It has high ecological habitat diversity, a unique macrophyte community (Ruppia dominated), internationally important birdlife, and large areas of relatively unmodified wetland and terrestrial vegetation meaning it has a number of nationally significant ecosystems. In addition, it is highly valued for its aesthetic appeal, its rich biodiversity, duck shooting, fishing (for brown trout primarily), boating, walking, and scientific appeal.

Waituna Lagoon is part of the internationally recognised 20,000ha Awarua Wetland. The lagoon and immediately surrounding wetland (an area of 3,500ha) known as the Waituna Wetland Scientific Reserve, was designated a Ramsar Wetland of International Importance in 1976, with the wider wetland complex being included in 2008. The cultural significance to the local Ngāi Tahu people was recognised under a Statutory Acknowledgement with the Ngāi Tahu claims Settlement Act 1998.  The lagoon and wetland have also been a source of food and recreation for the wider community over many generations e.g. fishermen, hunters and trampers.

The catchment at a glance:

  • Waituna Lagoon is located 40 km south east from Invercargill
  • The lagoon is 1350 hectares (ha), the catchment is approximately 20,000 hectares (ha)
  • More than 80 different species of bird have been recorded in the wetland complex
  • Hunting and fishing camps have been in the area for over 100 years
  • Waituna receives over 2000 'angler days' per year
  • From small dwellings to large farms there are approximately 130 properties in the catchment
  • There are at least 5 types of farming in the catchment (arable, forestry, sheep, beef and dairy)
  • Consented dairy cow numbers have more than doubled since 2000
  • There are 45 dairy effluent discharge consents, up from 28 in 2000
  • Environment Southland has monitored water quality at 4 sites in the lagoon since 2003

Project background

Environment Southland is part of a multi-agency response to helpensure the wellbeing of the people, the land, the ecosystems, and the life-force of the Waituna catchment and lagoon, now and for future generations through a partnership approach. We are working with local farmers and residents, local community groups, the Department of Conservation, Awarua Rūnanga, Ngāi Tahu, Southland District Council, DairyNZ, Fonterra, Beef and Lamb NZ, Federated Farmers, and Fish and Game Southland.

Over the last few years there has been significant investment by various parties to develop a greater understanding of the catchment and lagoon. While the level of knowledge has improved dramatically, some of the causes of the water quality decline, and the relationships between land use activities, lagoon openings and lagoon ecosystem health are still not fully understood. Therefore, the agencies and community are taking an incremental approach to undertake actions with known benefits now, whilst continuing to investigate the feasibility of potential actions.

While there is lots of discussion around the environmental concerns at Waituna, the investments families and businesses have made in their land in the Waituna catchment have contributed to the economic and social development of the area, some over a number of generations. Any actions will need to continue to give consideration to the wide ranging values that a wide range of people hold for the Waituna catchment and lagoon, and any legislative requirements.

Changes over time in the catchment and lagoon

Waituna Lagoon sits at the bottom of a small (approximately 20,000ha), intensively farmed catchment. Because of many years of land development in the catchment, and changes in lagoon water levels, its health and that of its tributaries is under stress.  Land development has included: drainage of wetland areas;  clearance of indigenous vegetation; and more recent land use intensification since the 1950s when the main tributaries to the lagoon were straightened, and Government schemes cleared and developed land and encouraged other people to do as well.

Waituna is just one example of a number of lagoons and estuarine systems located at the end of agricultural catchments which are under stress throughout New Zealand. As such, the primary concerns are the loss of nutrients and sediment from land use activities, thus increasing the risk of the lagoon becoming eutrophic, as well as the loss of wetlands through land development. Three large creeks and other small waterways and farm drains make up the drainage network. This network transports water, sediment or soil particles, nutrients and other material from the land within the catchment to the lagoon. Groundwater also transports nutrients to the lagoon. The management of lagoon opening events is important as it influences ecosystem health and farm management practices.

Environmental monitoring shows that the water quality in the lagoon and the creeks that flow into it is under stress. Nutrients, particularly nitrogen and phosphorus, are needed by plants and animals for growth. Nutrients are essential, but high levels are harmful. As such, the catchment and lagoon require on-going active management to improve their ecological condition. This is to reduce the risk of the lagoon experiencing a ‘regime shift’, that is, a change from having clear water and an aquatic environment dominated by aquatic macrophyte plants such as Ruppia, to one which has turbid and murky water dominated by algal slime and other suspended phytoplankton.

The Waituna Lagoon system is highly complex. The intermittent opening and closing of the lagoon to the sea is a feature that strongly influences the lagoon’s ecology and water quality. The changes in salinity and water level associated with freshwater and seawater influxes create an environment that favours some species over others, but not always at the same time. In this dynamic environment, species alter their distributions and abundances in response to changing water level, salinity, other environmental factors, and interactions with other species present.

What the project involves

Farm Visits – Environment Southland's Land Sustainability team is providing individual on-farm advice and working with community groups to increase awareness of land management issues and good environmental practices, particularly for the critical winter grazing period.

Drainage Enhancement – As part of the three-yearly drainage maintenance programme, in early 2015 clearing along the Waituna Creek was carried out to remove weed and sediment, and improve farm drainage.

Fresh Start for Freshwater Clean-up Fund In July 2012 Environment Southland began a programme of works with funding assistance from the Ministry for the Environment's Fresh Start for Freshwater Clean-up Fund.  This involved reconstruction of banks along the Waituna Creek, trialing constructed wetlands and undertaking openings of Waituna Lagoon.  Funding support was also received from DairyNZ, DOC, the DOC Fonterra Living Water Partnership, private landowners and the Southland District Council.

Bank reconstruction Approximately 14km of Waituna Creeks were rebattered, and 17 tonne of rock armouring installed.

See the map of where reconstruction works have taken place over time. (JPEG, 2MB)

Waituna Creek before reconstruction.jpg
Waituna Creek before bank reconstruction.

Waituna Creek after reconstruction.jpg
Waituna Creek after bank reconstruction.

Lagoon openings - The lagoon is mechanically opened to the sea after the water levels reach the trigger point set in the Lake Waituna Control Association's resource consent.

Openings of the lagoon have been historically undertaken for fish passage, to artificially manage the water levels in the lagoon to help drainage of agricultural land from the surrounding catchments, and also encourage flushing of nutrient rich waters and sediment out of the lagoon.  Both the Interim Recommendations and Ecological Guidelines for Waituna Lagoon prepared by the Lagoon Technical Group recommended periodic openings during winter months to flush out accumulated sediment and nutrients.  At this time of year the lagoon has a high chance of closing before summer and should also provide the most efficient flushing effect.  On this basis Environment Southland funded openings of the lagoon in July 2012 and July 2013.

Waituna lagoon opening.jpg
Waituna lagoon opening.

Constructed wetlands – The Clean-up Fund also provided funding to support the trials of constructed wetlands. Wetlands are known to act as buffers between the land and waterways, helping to protect them from erosion runoff and nutrients.

Environment Southland, in partnership with DairyNZ, commissioned NIWA to identify the most appropriate locations and types of constructed wetlands that could be implemented in the Waituna catchment to intercept nutrients and sediments en route to Waituna Lagoon. Thirty different constructed wetland options at 14 different sites were investigated across the catchment.  Cost estimates were provided, along with recommendations on wetland locations, size, and type to optimise environmental improvements for the funding available. It was decided to go ahead with a trial constructed wetland pond which flows into trial phosphorus filtration beds, one containing lime rock and the other oyster shell.

The wetland pond has been planted out with the native plant Eleocharis sphacelata or tall spike sedge to stimulate nitrate-removing bacteria.  This site has now been fully planted around the edges and within the wetland.

Waituna constructed wetlands.JPG
The constructed wetlands at Waituna.

An existing gravel pit was also one of the sites considered by NIWA. Often in Southland gravel is extracted from below the top soil for on-farm use, leaving open pits, frequently with standing water areas.  These provide a ready situation for development into constructed wetlands to remove dissolved nutrients, suspended solids and provide biodiversity benefits. This particular gravel pit had already been modified in the past to encourage flow distribution for sediment settling and nutrient transformations. Further enhancement involved riparian planting, and as more stock of Eleocharis becomes available this will be planted in the pond. A trial lime rock phosphorous filter bed was also constructed on this property.

gravel pit.jpg
A gravel pit is being developed into a constructed wetland to remove dissolved nutrients.

What you can do

We all have a responsibility to future generations to look after the environment we live and work in. To stop the lagoon changing to an algal dominated system we need to reduce the nutrients and sediment getting into the creeks and the lagoon. Listed below are actions you can take to help achieve this:

  • Exclude stock from waterways and wetlands
  • Make sure you have an up to date nutrient management plan and apply fertiliser at appropriate rates, in appropriate conditions.
  • Manage winter grazing activities carefully to prevent overland flow carrying sediment and nutrients to waterways. Good environmental practice includes; leaving a good buffer (minimum of 3m is required), break feeding towards the waterway and identifying swales that deliver runoff to waterways. Fence these swales out and leave ungrazed until the end of the season.
  • Manage a decent riparian margin (buffer zone) along waterways. These can be planted to shade water and help trap contaminants. Not sure what to plant? Get some advice from our land sustainability team.
  • Improve management of farm dairy effluent (storage, low rate and low depth application).
  • Check the soil moisture monitoring site.
  • Ensure your septic tank is well maintained and working properly. Septic tanks can contribute lots of phosphorus e.g. from dishwashing liquids, as well as other unwanted human waste to the environment.
  • Get involved with planting days.
  • Know your physiographic zone and the types of mitigations options you can put in place to help reduce your nutrient and sediment losses that are most appropriate for your zone.
  • Adhere to good management practices on your land at all times.

Contact the land sustainability team for more information. Katrina Robertson is the land sustainability officer for the Waituna catchment.

Page reviewed: 10 Jun 2016 10:20am