Source: Radio New Zealand
Sediment in the water off the coast of Gisborne during a marine ‘darkwave’ event. Supplied / Jean Thoral
A newly-named ‘darkwave’ phenomenon – where underwater light is blocked by sediment and other murk – can wreak havoc on marine ecosystems, New Zealand-led research has found.
The short-term events – which affect the entire underwater food chain – could increase in frequency as the climate warms, because many of them are driven by storms that churn up sediment or cause run-off from land.
University of Waikato researchers analysed up to 16 years of data from the Hauraki Gulf, the East Cape and California, and found that short-term, extreme reductions in light can damage kelp forests, sea grasses, and fish and marine mammal species.
“Anything that relies on light will be impacted by marine darkwaves,” lead researcher Frankie Thoral said.
“Species that need light or photosynthesis like kelp forests or seagrass meadows will be directly affected… but also fish, marine mammals, sharks – any species that relies on marine light for hunting or behaviour like swimming.”
Sediment in the water off the Wairarapa coast during a marine ‘darkwave’ event. Supplied / Jean Thoral
Marine darkwaves have always occurred but until now have not been described or defined, Thoral said.
The research, published in Communications Earth & Environment today, found one of the most important drivers of marine darkwaves is sediment discharging into the ocean, through either human activities like farming and forestry, or from extreme weather events.
“Looking at the last 21 years, the year 2023 – so the year of Cyclone Gabrielle – really stands out in terms of the number of darkwaves,” Thoral said.
That meant there could be more darkwaves in future, as severe weather events increase in frequency and intensity.
“More intense rain events and also wave events will definitely increase the amount of sediment on the coast, and this will create really murky conditions for days to weeks.”
The data he and his colleagues analysed included darkwave events that lasted up to two months. In some events, almost no light reached the seabed.
The most intense effects were observed close to the source of sediment discharge, like river mouths, but could extend by tens of kilometres, he said.
University of Waikato researcher Dr Frankie Thoral ESNZ / Luke McPake
Chlorophyll and phytoplankton blooms were among other common causes.
“Anything that makes the water murky.”
Many parts of New Zealand are currently experiencing higher than usual sea temperatures and marine heatwaves, which can cause phytoplankton blooms.
However, Thoral said more work needed to be done on the link or interaction between marine heatwaves and darkwaves.
Having a proper definition and framework to measure darkwaves meant their effects could now be properly studied, Thoral said.
“Now we can measure them in a really consistent way and… compare them to any other place around the world.”
Sediment in the water off the coast of Taranaki during a marine ‘darkwave’ event. Supplied / Jean Thoral
Using Endeavour programme funding from the Ministry of Business, Innovation and Employment, the team was now using analysis of underwater soundscapes to find out how many and what species of fish were in the water before, during and after a darkwave event.
Darkwaves were a natural phenomenon, but could be made worse by human activities, he said.
The good news was that it was clear how to tackle that.
“We know that we can limit and prevent this sediment input, and the way we could do that is really looking at what is happening on land,” Thoral said.
“That means adapting land practices to limit erosion [through] native forest reforestation or changing practices in farming or forestry.”
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand
