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Case Study:

Dual impacts of oceanic and freshwater changes: implications for coastal and estuarine ecosystems


General Context (from MBIE contract)

Climate change will impact important primary sectors such as dairy, horticulture and arable cropping, as well as ecological functioning of native ecosystem remnants. Expected increases in land-use competition and intensification will compound these impacts. This modelling will explore the potential implications of the dynamics among conservation, primary production, and urban land uses. It will also explore the associated implications for native biodiversity in fragmented landscapes, long-term availability of high quality soil resources for agricultural production, and effects of land-use intensification on water resource management.


Most past New Zealand climate change research has focused on open coast systems where land and freshwater effects were ignored. There is basically only generalized material around on possible impacts of climate change on estuarine environments and freshwater-brackish transitions. This case study will consider the potential Hazards to do with climate change (such as effects of sea-level rise, temperature, river flows, and sedimentation on water supplies and other infrastructure), This will focus on estuarine systems and tidally-affected sections of rivers in New Zealand.

This case study will also examine changes in coastal Habitat distribution, i.e., models of mangrove and seagrass distribution response to sea-level rise (including tides, wave exposure plus basic biological constraints, drowning of intertidal reefs).  New Zealand mangrove forests affect coastal and estuarine ecosystems in a myriad of ways, functioning as long-term sinks for stormwater contaminants (e.g., fine sediments, nutrients), support biodiversity and detrital food-webs through primary production, and may provide nurseries for some estuarine and coastal fish species. NZ mangrove forests also have the capacity to mitigate coastal erosion and inundation hazards associated with local wind and swell waves and storm tides.  Because mangrove forests occupy a narrow elevation range and must maintain their position within the intertidal zone they are sensitive to changing sea level. Thus, the long-term fate of mangrove forests depends on tidal-flat surface elevation increasing at a rate equal to or exceeding the rate of sea-level rise (SLR). Over geological time scales, mangrove-forest response to eustatic sea-level fluctuations depends on local relative SLR (i.e., vertical land movement), climate-change effects on mangrove productivity, sediment supply from rivers, storminess and oceanographic processes (e.g., storm surge, wave climate).

Study Area Selected

After consultations within the project and with potential stakeholders we will be modelling both Hazards and Habitats in the Firth of Thames and associated contributing catchments.

Potential Impacts of Climate Change

Associated impacts of climate change on estuaries and coasts include:

  • Ground-water salinization of coastal aquifers
  • Changes in tide range
  • Enhanced upstream saline intrusion affecting potability of water supplies and suitability for irrigation
  • Changes in estuarine ecosystems (e.g., through drowning of intertidal reefs)
  • Changing temperature and aquatic plant habitat effects on kaimoana
  • Changes in estuarine sedimentation and consequent effects on ecosystem health
  • Changing underwater light regimes
  • Impacts on ICOLLs (Intermittently Closed and Open Lakes and Lagoon) and river mouths
  • Land drainage and stormwater management in low-lying land
  • Shoreline erosion
  • Microbial water quality changes (effects on public health)
  • Drainage & stormwater systems
  • Bank erosion
  • River flood control measures (e.g., willow management, outflanked by sea level rise)
  • Changing effect on river cuts or partial diversions (e.g., Kaituna, Heathcote), bridges & road/rail infrastructure

Two broad areas of impacts in the estuarine and coastal areas may be foreseen:

  • Physical hazards (from sea level rise and flooding)
  • Changes in coastal habitat distribution (e.g., mangroves/seagrasses/reefs, from sea level rise, sedimentation and temperature effects)

Potential Implications of Climate Change

Implications range from requirement to change infrastructure (roads, bridges, water supplies, fencing) to concerns about continued availability of kai moana—shoreline food gathering by Iwi and others.

Highlights of the Synthesis Report: RA2 Coastal Case Study

Firth of Thames water levels and depths

  • Sea level rise (SLR) will result in increased flooding of shore communities.
  • Water depths in the Firth over time are projected to be relatively independent of RCP values (with rate of sea level rise roughly in tandem with sedimentation), but will nevertheless be maximal for RCP 8.5 (high greenhouse gas emissions).
  • Areas of wetting and drying in the Firth may increase over time, particularly if more intense land uses evolve, making navigation more difficult.
  • Conversely, there would be less wetting and drying were widespread reforestation to be adopted in the contributing catchments.

Firth of Thames mangrove distributions projections

  • Under present landuses, up to 100% increase in mangrove habitat is projected in suitable areas, with a decrease of habitat in subtidal areas.
  • For a 20% reduction in sediment supply (e.g., because of reforestation) some reduction in suitable habitat can be expected, especially for areas presently marginal for mangrove production.
  • For a 20% increase in sediment supply (e.g., because of intensified landuses) a general increase in suitable habitat is projected.

Waihou and Piako River flows

  • Projected changes in river flows are strongly linked to projected changes in precipitation.
  • The median change in the annual mean flow is relatively small, with generally negative changes in winter and summer (decreased mean flow) and positive changes in spring and autumn (increased mean flow).
  • Uncertainty in flow projections is high, corresponding to similar uncertainty in future precipitation projections for this region.

Waihou River salinity intrusion and flooding

  • SLR is the main contributor to changes in saline intrusion and flooding.
  • Salinity intrusion up the Waihou River is projected to extend up to a further 5 km inland (for a 1 metre SLR over the next century), and is relatively unaffected by changes in low  river flows.
  • Flooding in the upper part of the river (>50 km from the Firth) is largely unaffected by SLR.
  • The largest impact of SLR and flood interaction will occur in the lower Waihou River (between Pekapeka and Puke Bridge), with up to a 0.5 m increase in combined flood level at Pekapeka for a 1 m SLR.

Economic and land use scenario analysis

  • Agricultural commodity prices vary significantly over time due to changes in global socio-economic conditions, population growth, and climate change policy.
  • Primary production is estimated to increase regardless of which Representative Concentration Pathway (RCP) is modelled. Forestry yields increase the most due to the influence of an increase in atmospheric CO2.
  • A scenario of increased sediment loads is driven by deforestation and a shift to pastoral land uses.
  • A scenario of a decline in sediment loads can be attributed to high carbon prices shifting marginal sheep and beef land to pine plantations, scrub, and fallow land.

The results giving rise to these highlights are given in the main body of the report; various technical explanations are given in the Appendix.

To download a full copy of the report, click here.

Leadership of case study: Graham McBride, NIWA

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