More Frequent, More Intense: What Extreme Weather Means for the Ocean on the East Coast
As Jono swims down the East Coast, scientists say the ocean here is increasingly shaped by more frequent and intense extreme weather events.
Heavy rainfall can send huge volumes of sediment and debris into coastal waters, smothering marine habitats and stressing ecosystems already under pressure.
Research shows these systems can recover, but reducing additional human impacts during recovery periods is critical to maintaining ocean resilience.
As Jono swims down the East Coast of Te Ika-a-Māui the North Island, conditions change daily – the wind shifts, swell builds, currents turn.
Beyond what’s visible on the surface, marine researchers say the ocean in this region is adjusting to a bigger pattern: more severe and more frequent extreme weather events.
Over the past five years, Te Tairāwhiti and Hawke’s Bay have experienced repeated, intense high-rainfall events. When that scale of rain falls in a short period of time, it moves enormous volumes of water from land to sea, carrying fine sediment, nutrients, trees and debris into coastal waters.
“Sediment deposition is a natural event,” says Professor Rochelle Constantine, marine biologist at the University of Auckland / Waipapa Taumata Rau. “Coastal ecosystems typically experience natural pulses of sediment flowing from land to sea delivering important nutrients to coastal ecosystems. But the volume and frequency of some of these recent rainfall events exceed the levels those systems typically experience.”
Cyclone Gabrielle in February 2023 became one of the most studied examples. Research in the months that followed documented significant sediment movement into offshore waters. ESNZ (formerly NIWA) surveys of 36 sites in waters deeper than 15 metres found that 11 showed signs of sediment impact, including fresh mud layers and stressed seabed communities.
Some sites showed signs of recovery within months, while others remained affected and longer-term ecological changes continue to be studied.
In nearshore areas, sediment accumulated up to a metre deep in some locations, and wave action continued to resuspend fine mud, prolonging stress on marine life.
“This mud was moved to the seabed where it smothered marine life – plants and animals – leading to poor condition or loss of important species such as kelps and sponges,” Professor Constantine says.
The seabed is structured by living organisms such as sponges, shellfish, worms and other invertebrates that create habitat and food for fish and larger species. When thick sediment settles, some animals can move, but many cannot. Burrowing species may struggle to reach the surface to breathe, and visual predators can find it harder to hunt in murky water.
Storm-driven sediment can also carry larger debris. Logs and building materials transported from land can settle on the seabed and are unlikely to disperse quickly, covering habitat for extended periods.
The impacts extend beyond ecology. Coastal communities along this stretch of the North Island rely on kaimoana and a range of nearshore fisheries. The disruption affects livelihoods and food gathering.
“These extreme and more frequent events are occurring in habitats that, in some areas, have already experienced long-term fishing pressure and increased sedimentation associated with land use,” Professor Constantine explains.
Scientists describe this as cumulative stress – when multiple pressures act on the same ecosystem at once.
Modelling work in Hawke’s Bay examining bottom trawl fishing and sedimentation shows that while fishing effort can be adjusted relatively quickly, ecological recovery takes much longer.
“Even if you reduce sediment loads and fishing effort, there is likely to be a substantial lag before ecosystems restore themselves,” Professor Constantine says. “Depending on the life history of the organisms living on the seabed, it could take a decade or more before you see recovery in benthic structure. It would take many decades to recover to pre-disturbance levels and is unlikely to return to its former diversity.”
That lag matters in a changing climate.
When ecosystems are disrupted suddenly by large weather events, researchers, communities and other people whose livelihoods depend on healthy coastal waters are asking whether additional disturbance during recovery periods compounds stress.
“There needs to be better discussion about when ecosystems are disrupted so suddenly by these large weather events,” Professor Constantine says. “When is it sensible to return to activities that may further disrupt recovering habitat? We need to rethink our approaches to managing and living off the ocean.”
As Jono continues south, he is swimming through waters that are productive, dynamic and deeply valued by coastal communities, but also responding to a series of extreme weather events.
Research shows that parts of this coastline are capable of recovery. It also highlights that resilience depends on the pressures ecosystems face during and after extreme events.
In a climate where these events are becoming more frequent, reducing avoidable pressure on marine habitats is one way to support that resilience.
Because what happens beneath the surface, especially in the wake of more intense storms, shapes the future health of the ocean.
More detailed information can be found in:
Leduc, D. et al. (2024). Cyclone Impacts on Fisheries. Aquatic Environment and Biodiversity Report No. 326. Fisheries New Zealand.
https://fs.fish.govt.nz/Doc/25605/AEBR-326-Cyclone-Impacts-On-Fisheries-2024-4461.pdf.ashx
Lundquist, C. et al. (2022). Hawke’s Bay Seafloor Model Report. Sustainable Seas National Science Challenge.
https://backend.sustainableseaschallenge.co.nz/assets/dms/Reports/Hawkes-Bay-Seafloor-Model/HawkesBaySeafloorModelReport_22Aug2022.pdf
“There needs to be better discussion about when ecosystems are disrupted so suddenly by these large weather events. When is it sensible to return to activities that may further disrupt recovering habitat? We need to rethink our approaches to managing and living off the ocean.”
Professor Rochelle Constantine, ONZM
UNIVERSITY OF AUCKLAND
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