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ABC Midwest & Wheatbelt

New research is helping scientists understand the impacts of underwater blackouts. (Supplied: Liam Ridgley)

A global study has measured the impact of periods of significantly reduced light in the ocean, known as darkwaves.

Over a decade since an extreme heatwave wiped out 37 per cent of Shark Bay's seagrass meadows, researchers fear Cyclone Mitchell could cause similar damage.

Scientists hope the research will allow them to identify coastal areas at risk of darkwaves and mitigate their future impact.

Link copiedShareShare articleA scientist says heightened storm activity is increasing marine ecosystems' risk of "underwater blackouts", starving them of light necessary for survival.

Known as "darkwaves", the term describes periods where natural light is blocked from reaching the ocean floor by phenomena like sediment plumes, algal blooms, erosion or human activity.

Using long-term data from California and New Zealand over two months, a team of international scientists documented areas where light levels in the ocean had drastically dropped.

This can trigger marine ecosystems such as kelp forests or sea grass meadows to be wiped out and fish breeding grounds to be starved of nutrients.

Severe weather including cyclones and heavy rain can affect the amount of light reaching marine ecosystems.  (ABC News: Monish Nand)

Similar problems occurred in Western Australia's Gascoyne 15 years ago, when intense rain caused sediment plumes from the river to spread out in a thick layer across Shark Bay, 840 kilometres north of Perth.

Flooding and an extreme marine heatwave in 2011 created a perfect storm for an algal bloom, which resulted in about 1,200km of seagrass meadows, which are a vital ecosystem for dugongs, being wiped out.

Satellite imagery of Monkey Mia, Shark Bay, in 2006 (left) and 2011 (right).  (Supplied: Liam Ridgley)

Shark Bay Seagrass Restoration project leader Liam Ridgley said the meadows could not sustain warming ocean temperatures.

"We had a massive warming event where the water stayed at 30 degrees for 90 days … the seagrass couldn't handle this," Mr Ridgley said.

"A massive inland rainfall event allowed the Gascoyne River to flood and introduce sediment into the bay which created a low light event.

"That had drastic impacts on the visibility of the water, it actually turned black for about four years due to a massive algal bloom."

Over a decade later, Mr Ridgley continues to help restore the seagrass meadows by planting seedlings which sit at the bay floor in sandbags.

Seagrass seedlings in sandbags are ready to be planted in the ocean.  (Supplied: Liam Ridgley)

As recovery efforts continue, Mr Ridgley said watching Cyclone Mitchell develop along WA's Pilbara coast was making him anxious.

"It's almost a bit of deja vu [similar] to what happened in 2011 except Shark Bay's water temperatures are a bit more stabilised at the moment," he said.

"If that cyclone tracks inland and creates a massive flooding event of the Gascoyne River again, introducing plumes of nutrients into the bay … it's of great concern.

"We don't get very big tides at this time of the year so it will be quite a struggle to flush the bay out."

Liam Ridgley says there's a growing understanding of the importance of seagrass meadows. (ABC Midwest & Wheatbelt: Piper Duffy)

University of Western Australia research fellow Shinae Montie said darkwaves could last weeks or even months, but the impacts were far reaching.

"Fish will lose their feeding grounds, breeding habitats will disappear, the cascading effects can last far longer than the actual darkwave event itself," Ms Montie said.

"We're talking threats to commercial fisheries, tourism, coastal protection … real economic impacts."

As part of a global study which created a framework to measure darkwaves, satellite imagery was also used of ex-Tropical Cyclone Seroja, which caused devastation across the Midwest in 2021, and ex-Cyclone Joyce in 2018.

Satellite images from 2018 (left) and 2021 (right) show sediment plumes spreading from the Gascoyne River after cyclones.  (Supplied: UWA)

It's been a particularly active cyclone season in WA this year, with Pilbara residents urged to prepare their homes as Cyclone Mitchell approaches.

Ms Montie said heightened storm activity could trigger more darkwave events off the coast.

"Climate change is essentially increasing extreme weather globally and it's more intense cyclones as well as heavier rainfall and bigger floods that we're likely to see become more frequent and severe," she said.

"That can then of course cause these darkwaves to last weeks or even months."

Shinae Montie says the research framework will help scientists identify other areas at risk of low light events.  (Supplied: UWA)

Ms Montie said scientists expected darkwaves to "become more frequent" but the newly created framework scientists could use to help measure the events would help identify at-risk zones.

"The good news is we can do something about this; darkwaves are also linked to land management [such as] river sediment, agricultural run-off and land clearing," she said.

"By identifying which catchments create the worst darkwaves we can target our conservation efforts better.

"Better land use planning, erosion control and catchment restoration can all reduce the severity of these events when the next big storm hits."

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