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Estuaries in the Oreti

How is your local estuary faring?

New River Estuary

New River Estuary is a large 'tidal lagoon' type estuary (area 4,100ha). It is situated next to Invercargill, the largest urban centre in the region. The estuary receives freshwater from a large, predominantly agricultural catchment, which includes both the Oreti and Waihopai Rivers.

This shallow estuary (mean depth ~2m) is bordered by a mix of vegetation and land uses (urban, bush and grazed pasture). It has a wide range of habitats (extensive mudflats, seagrass and saltmarsh areas).

The estuary has been significantly affected by urban and rural development over the past 150 years. This includes large areas of reclaimed land, urban discharges including treated sewage and untreated stormwater, past landfill leaching, and agricultural activities and run-off further up the catchment.

Early Invercargill

In 1856 Governor Gore Browne ordered a town in the south to be laid out on a suitable site, and declared this new town (Invercargill) and Bluff as ports of entry. The area around New River Estuary was selected over other locations due to proximity to water, which made transport easier. Originally, Invercargill was planned as a port town with small boats taking goods up the Otepuni Stream as far as the current corner of Clyde and Tay Streets.

Reclamation

Reclamation of the tidal flats at the northern end of the estuary began in 1865 and continued over the next century until nearly a quarter of the original estuary area (roughly 1,650 ha) was reclaimed. To achieve this, Spartina grass was planted and rock groyne walls were built to enhance sedimentation. Clean fill and rubble from demolished buildings was added with topsoil farming the final layer. The Waihopai Arm has been most affected with approximately 1,200ha (75%) of the Arm reclaimed, greatly reducing its ability to filter, dilute, and assimilate nutrient and sediment inputs.

Old landfill

Between 1930 and 1950, the Invercargill community began dumping waste, just south of the Stead Street wharf. The piles of refuse migrated further south to Pleasure Bay and were used for further reclamation. In 1970 a causeway was built to prevent the waste from floating into the estuary. The landfill was closed in 2004 and converted into a recreation area with walking and cycling tracks. Recent observations however, have shown leachate seeping around the edges of the old landfill. Initial sampling has revealed high concentrations of Total Nitrogen, Ammoniacal Nitrogen and Total Kjeldahl Nitrogen.

Wastewater treatment

The Waihopai River flows through Invercargill city which discharges its treated wastewater and untreated stormwater to the estuary. The Invercargill Wastewater Treatment Plant has been located on the edges of the estuary since the first single septic tank was constructed in 1910. The treatment plant underwent an upgrade between 1959 and 1969 to cope with the expanding population. This consisted of new pipes, extensions, intercepting trunk sewers, pumping stations and a primary treatment plant. A secondary treatment facility was added in 1992 and maturation ponds and wetlands were built in 2004. As part of the management of the wastewater, the discharge is timed to be released with the outgoing tide.

Agricultural development

Before European settlement, the flat plains all over Southland were covered by bush - mataī, rimu, lowland beech, kānuka and mānuka, interspersed with tussock grasslands, and swamp and bog in low-lying areas. Historical clearance and altered drainage of land for farming and human occupation has since made the area more prone to erosion, with greater and faster water runoff and river flood flows, reduced area of wetlands and riparian habitat, and an increase of contaminants to waterways (e.g. sediment, nutrients and micro-organisms).

Of the nearly 400,000 hectares of land in the Ōreti and Invercargill catchments, approximately 71% (280,300 ha) is now used for farming, with a land use mix of sheep and beef (approximately 45% of the farming area), dairy and dairy support (38%), mixed livestock and livestock support (15%), deer (1.7%) and arable farming and horticulture (less than 0.3%). Approximately 20,300 ha (5%) of the catchment area is in commercial forestry.

Approximately 64,500 ha is Department of Conservation estate and 5,600 ha is Māori land.

There are 28 water permits for irrigation use and 252 permits for stock water and dairy shed washdown. Land use consents for dairying specify a maximum consented total of 221,994 dairy cows in the catchment.

Areas of concerns

Nuisance blooms of macroalgae (Ulva and Gracilaria), failure to meet faecal bacterial guidelines in regard to swimming and gathering shellfish, and sedimentation problems are common within the estuary.

As a consequence of the much reduced saltmarsh area, the estuary is more vulnerable to issues of eutrophication and sedimentation (given that saltmarsh acts to reduce nutrient and sediment impacts).

The 'New River Estuary Macroalgal Monitoring 2018' report combined with the 'New River Estuary Review of Water Quality data' show that regions of the upper estuary are under stress and showing eutrophication. There is excessive macroalgal growth including sediment quality decline and high concentrations of chlorophyll-a in the water column. Chlorophyll-a was used as an indicator of eutrophic conditions in the water column, and is a colour pigment present in many types of algae that can give an indication of how much algae is present in the water column.

Despite the presence of these issues, human use and ecological values of large parts of the estuary remain high. However, we know that action needs to be taken to improve the situation, particularly in areas where the condition is poor.

Latest results

Soft mudPoor
Nutrients in sedimentPoor
Oxygenation layerPoor
Macroalgae cover/conditionPoor
Sea grass lossPoor
Gross Eutrophic zonePoor

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