Facing sea level rise: Ōhiwa saltmarshes in the spotlight
- Admin
- 3 days ago
- 3 min read
Updated: 6 minutes ago
It was certainly not an April fool hoax when Dr Andrew Swales from the National Institute of Water and Atmospheric Research Taihoro Nukurangi (NIWA) talked about unstoppable sea level rise, its risks but also its opportunities, at the Bay of Plenty Regional Council Toi Moana Wetland Forum on 1 April 2025 in Whakatāne. Andrew is one of several scientists at Future Coasts Aotearoa, a five-year collaborative research programme that started in 2021. Led by NIWA the programme aims to ‘transform coastal lowland systems threatened by relative sea-level-rise into prosperous communities’. Ōhiwa Harbour is one of six sites around the country where the scientists study the likely extent and impact of sea level rise as a result of climate change.
Back in 2022 Bay of Plenty Regional Council Toi Moana contracted NIWA’s principal scientist Andrew to install 12-rod surface elevation table (RSET) devices (see Figure 1) in the Nukuhou Saltmarsh at Ōhiwa Harbour and in Athenree estuary, Tauranga Harbour. The Ōhiwa and Athenree saltmarshes are among the last unspoiled coastal wetlands in Aotearoa. A RSET is a portable mechanical levelling device for measuring the relative elevation change of wetland sediments resulting from sediment accretion, consolidation, and subsidence. It provides a non-destructive method for making highly accurate and precise measurements over long periods of time. RSET monitoring also assists in interpreting the cause of any changes in wetland vegetation and produces valuable information of the effects of sea level rise generally.
A better understanding of estuaries and saltmarshes is needed as they buffer against erosion and flooding in prone areas. Residents of the low-lying properties in Kutarere have already felt the impact of climate change through flood events in past years.
For wetlands such as the Nukuhou Saltmarsh to survive, they need to accumulate sediment at a faster rate than rising seas. The Bay of Plenty Regional Council Toi Moana wants to protect and restore the very few remaining wetlands because they are extremely valuable ecosystems, similar to tropical rainforests. As the sea rises, wetlands normally move inland, but human infrastructure - railroads, stopbanks, and roads - stops that in many places. Mangroves are another complicating factor, especially when they spread landward into saltmarsh habitat. In face of these pressures, wetlands literally become squeezed in from both the sea- and landward side.
Potentially, restoring wetlands can do more than enhancing flood resilience, the harbour’s water quality and providing habitats for important taonga species. They may become carbon storage sites. Blue carbon describes coastal and marine areas' abilities to sequester and store carbon. Figure 2 shows areas suitable for marsh restoration at Ōhiwa Harbour, based on forecasts of 0.2 m of sea level rise (in orange) and 0.6 m of sea level rise (in red), compared to current wetland elevation levels (in blue). Saltmarshes in and around Ōhiwa Harbour could be significant carbon sequestration sites.
Sources:
First devices installed in Te Moana a Toi to monitor estuaries Stuff, 27 October 2022
Josie Crawshaw and Erin Fox, 'Potential restoration sites for saltmarsh in a changing climate', Bay of Plenty Regional Council Environmental Publication 2022/14, Whakatāne
Saltmarsh carbon sink opportunity
RNZ, 16 March 2023
Future Coasts Aotearoa
More information:
Watch an introduction to Future Coasts Aotearoa on the NIWA website
Check out the Bay of Plenty’s Wetland Restoration Guide
Read Marli and Shane’s story of restoring a wetland in Wainui
