The Guardian - Joanna Khan
Receding kelp forests, jellyfish blooms and disruption to fisheries are just some of climate change’s impacts on the ocean
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| Common kelp (pictured) has taken over from the giant kelp forests that used to dominate Tasmania’s coastline.
Photograph: Matt Doggett |
As bushfires raged across Tasmania, Victoria and New Zealand, and north Queensland faced a
massive cleanup after unexpected flooding, a different extreme weather event was silently forming in the Tasman Sea over summer.
For the
second year in a row,
a stubborn high-pressure system over the Tasman Sea was warming the
surface of the ocean to above-average temperatures, forming a marine
heatwave, wreaking destruction and providing a glimpse of the
new ecological order
in the marine Anthropocene. Globally marine heatwaves are becoming more
frequent and prolonged and affecting biodiversity, according to
new research published in Nature Climate Change this week.
In the summer of 2017-18, the intense marine heatwave was combined
with a land-based heatwave, together covering four million sq km.
Scientists found
the extreme weather event caused unprecedented loss of glacial ice in
the New Zealand Southern Alps, changes to wine-grape harvests, and major
disruption of marine ecosystems including kelp habitat loss, new
species invasions and fisheries season changes.
This year the
National Institute of Water and Atmospheric Research in New Zealand reported that sea surface temperatures in the Tasman were again above average.
Like coral reefs and tropical rainforests, the ocean suffers the slow
torture of climate change peppered with high-intensity hits from
extreme weather.
A window into the future
Marine heatwaves are generally out of sight and out of mind until one
gets so bad it becomes impossible to ignore, says CSIRO research
scientist Alistair Hobday.
A marine heatwave happens when the ocean temperature is much warmer
than usual for the time of year from sunlight heating the surface water
or warm water being brought via ocean currents – or both.
Climate change is causing marine heatwaves
to happen more frequently and with more intensity. There may not be
scorched earth or destroyed homes left in its wake, but a marine
heatwave impacts our future in different ways – and
serves as a warning.
“Marine heatwaves provide a window into what our oceans will look like in the future, which is why it’s important to
keep track of them,” Hobday says.
In 2015, New Zealand experienced its longest and most
intense marine heatwave on record. For some it was good news – “It brought down valuable tropical species like kingfish and snapper,” Hobday says.
For the aquaculture industry, however, it brought disease outbreaks
in oyster farms, disruptions to salmon farming and abalone deaths along
the coast.
Clinging on to kelp
When an extreme marine heatwave lingered over the
Shark Bay world heritage area
in 2011, seagrass and kelp forests died en masse. Some kelp species
became regionally extinct over hundreds of kilometres, says marine
ecologist Cayne Layton from the University of Tasmania. “That [kelp
loss] was a direct effect of the heat, but also due to herbivorous fish
following the warm water and moving in to munch on the kelp.”
Like coral, kelp provides habitat structure, shelter and food for an
entire ecosystem – without it the ecosystem would cease to exist. But,
also like coral, the planet is losing kelp forests to climate change at
an alarming rate.
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| A healthy kelp forest (left) versus a degraded one (right). Photograph: NOAA / Matthew Doggett |
Forests of giant kelp, known as
Macrocystis pyrifera, used
to dominate Tasmania’s coastline. But in recent years, 95% have been
lost and largely replaced by common kelp, says Layton.
“It’s
the equivalent of losing a forest on land and having it replaced by
shrubbery,” he says. “It might still have some diversity, but you’ve
undoubtedly lost something important.”
Although common kelp might take over in the short term, a marine
heatwave or a plague of hungry sea urchins could quickly knock that out
too, allowing even hardier turf algae to move in.
From the forest to the shrub, now to a lawn – albeit a weedy one.
Species that, like kelp, can’t keep pace with the fast rate of anthropogenic warming, can be quickly be
written off by an extreme event such as a marine heatwave or tropical cyclone.
Weeds of the sea
Some more physically and physiologically nimble species are set to
thrive in a warmer ocean. “[In a warmer ocean] you might expect to see
more ‘weedy species’ that can adapt more quickly to change,” says marine
ecologist Zoe Doubleday from the University of South Australia.
Weedy isn’t a technical classification of plants or animals, but rather a type of lifestyle. Animals and plants considered
weedy species
typically have short lifespans and a flexible lifestyle that allows
them to take advantage of sparse or patchy resources, Doubleday says.
“They can change their shape, what they eat and when they mature and
reproduce to take advantage of novel conditions created by climate
change,” she says.
“For example jellyfish, algae and cephalopods – they’re very
different types of organisms, but they have some similar
characteristics. They’re short-lived, tolerant, adaptable – just like a
terrestrial weed that you might see in your garden.”
Receding kelp forests
are already being replaced by seaweed turfs, bigger numbers of some
cephalopods and jellyfish blooms. “We already eat weedy species, and
cephalopods are becoming increasingly important in our fisheries, and
they might be better than fish in terms of sustainability in the
future,” Doubleday says.
“Rather than just suppressing the weeds, maybe we could start
predicting exactly which species are going to thrive and how we could
use them.”
So while jellyfish may not be on the menu now, soon there may be few other options.
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