It’s called a brownout event, and it’s pretty bad, but probably not quite in the way you’re thinking. It happens around the coasts of New Zealand after heavy rain: so much dirt is washed off bare land, carved from banks or carried by landslides that waterways change colour. Imagine an estuary the colour of milk chocolate meeting the blue waters off the coast.
Brownouts smother marine life. Mud-filled water makes it difficult for species to breathe, see, photosynthesise, hunt prey, find mates or otherwise make marine ecosystems function.
More brownout events lead to greater loss of marine life, until estuaries, harbours and coasts reach a tipping point – they stop working as they should.
Figuring out the impact of brownouts – what causes them, and how we prevent them – has been the work of decades for marine scientist Professor Simon Thrush, director of the George Mason Centre for the Natural Environment at the University of Auckland, as well as a crew of marine ecologists, modellers, social scientists, and environmental managers.
How to cause a brownout
In 2000, Auckland was expanding past its northern border, creating huge new suburbs. The regional council at the time was concerned about the impact of all that construction on the nearby Ōkura Estuary.
“When you expose a whole bunch of dirt for a long period of time, the risk of sediment runoff is astronomical,” says Thrush. “They wanted to understand what the implications of that would be for the estuary and coastal environment.”
Was it bad to let dirt wash into waterways? No one was certain. And so, Thrush and his colleagues chartered a helicopter to dump a small amount of dirt in the estuary, planning to measure what happened next.
“After spending tens of thousands of dollars on heli time, I was thinking, ‘I wonder if that stuff is going to be there the next day?’ Because we really had no clue as to what was going to happen. It turns out it was there for an awful lot longer than that.”
That was the start of Thrush’s research into brownouts, or “large depositional events” – and it wasn’t just Auckland that was experiencing them. They’ve increased around the country in recent years alongside extreme weather events.
“They’re very prone to happen on land that is being urbanised – which is kind of Auckland’s problem – and when roads and motorways are built,” says Thrush. “They also happen when forestry is clear-felled, when our streams lose the protection of riparian plants or when slips and slides express soil to the sea.”
What’s so bad about a bit of dirt in the sea?
Brownouts can trigger big ecological changes, says Thrush, “particularly to the microscopic plants that are a major source of food in our harbours and estuaries”.
Ocean life depends on plants: tiny phytoplankton, microscopic algae, and seaweed. They’re the foundation of the marine food chain, and they all photosynthesise just like land-based plants. If dirt clogs the water, they can’t make food from sunlight and grow. Similarly, shellfish and other filter feeders usually clean seawater as they eat, extracting pollutants – but not if they’re smothered by sediment and their filters are clogged.
North of Auckland, the Mahurangi Harbour offers a cautionary tale. “That’s had a big problem with getting muddier in the last 20 years,” says Thrush. “We’ve seen radical changes in the ecology of the harbour, a decrease in diversity, a decrease in large animals, a decrease in shellfish, alongside an increase in animals like mud crabs.”
An estuary might not be “cute and furry”, says Thrush, “but it’s doing a whole lot of things that really help us”. Estuaries act as nurseries for young marine life, help buffer against floods and storms, filter water before it reaches the sea, recycle nutrients, detoxify and bind some contaminants and store carbon. They’re home to shorebirds – and they provide access to the coast for people to immerse themselves in nature, too.
But once a certain level of muddiness is reached, species start disappearing – and it’s not possible to simply get them back again. “Recovery is kind of tricky. We could recover and restore places in the sense of improving habitat and biodiversity, but that may not necessarily be going back to what it was like in the past.”
What could prevent this happening in the future?
Now, urbanisation is less of a problem than it once was – thanks in part to the rules brought in for construction following research, including Thrush’s, in the early 2000s.
That isn’t to say that urban land is problem-free; Auckland still flushes sewage and stormwater into the sea during bad weather. Elsewhere, other land activities don’t have the same regulations or management practices to prevent sediment run-off.
Preventing brownouts requires keeping an eye on land activities across the entirety of a catchment – the total land area that drains into a harbour or estuary. “These are really major events in our landscape, and we need to reduce the risk of those kinds of things happening,” says Thrush. “It’s a really important problem for the country.”
Many of our estuaries are heading the way of Mahurangi – they’re already muddier than they should be. “You can see signs of this increasing muddiness in many of our harbours, estuaries and coastal ecosystems,” says Thrush.
Preventing brownout events means protecting nature’s infrastructure, says Thrush. Though it will involve coordination across all sectors of society, rehabilitating estuaries involves benefits for all sectors of society, too. “I mean, do you want your kids to go play in a mud flat, or do you want them to go play on a sand flat?”
The world is facing unprecedented environmental challenges. Planetary Solutions, an initiative of the Sustainability Hub at Waipapa Taumata Rau, University of Auckland, and Newsroom, explores these issues – and the practical ways we can all be part of the solution.
