In the midst of a raging heatwave, most people think of the ocean as a nice place to cool down. But heatwaves can strike in the ocean as well as on land. And when they do, marine organisms of all kinds – plankton, seaweed, corals, snails, fish, birds and mammals – also feel the wrath of soaring temperatures.
Our new research, published today in Nature Climate Change, makes abundantly clear the destructive force of marine heatwaves. We compared the effects on ecosystems of eight marine heatwaves from around the world, including four El Niño events (1982-83, 1986-87, 1991-92, 1997-98), three extreme heat events in the Mediterranean Sea (1999, 2003, 2006) and one in Western Australia in 2011. We found that these events can significantly damage the health of corals, kelps and seagrasses.
This is concerning, because these species form the foundation of many ecosystems, from the tropics to polar waters. Thousands of other species – not to mention a wealth of human activities – depend on them.
We identified southeastern Australia, southeast Asia, northwestern Africa, Europe and eastern Canada as the places where marine species are most at risk of extreme heat in the future.
Marine heatwaves are defined as periods of five days or more during which ocean temperatures are unusually high, compared with the long-term average for any given place. Just like their counterparts on land, marine heatwaves have been getting more frequent, hotter and longer in recent decades. Globally, there were 54% more heatwave days per year between 1987 and 2016 than in 1925–54.
Although the heatwaves we studied varied widely in their maximum intensity and duration, we found that all of them had negative impacts on a broad range of different types of marine species.
Humans also depend on these species, either directly or indirectly, because they underpin a wealth of ecological goods and services. For example, many marine ecosystems support commercial and recreational fisheries, contribute to carbon storage and nutrient cycling, offer venues for tourism and recreation, or are culturally or scientifically significant.
Marine heatwaves have had negative impacts on virtually all these “ecosystem services”. For example, seagrass meadows in the Mediterranean Sea, which store significant amounts of carbon, are harmed by extreme temperatures recorded during marine heatwaves. In the summers of both 2003 and 2006, marine heatwaves led to widespread seagrass deaths.
The marine heatwaves off the west coast of Australia in 2011 and northeast America in 2012 led to dramatic changes in the regionally important abalone and lobster fisheries, respectively. Several marine heatwaves associated with El Niño events caused widespread coral bleaching with consequences for biodiversity, fisheries, coastal erosion and tourism.
All evidence suggests that marine heatwaves are linked to human mediated climate change and will continue to intensify with ongoing global warming. The impacts can only be minimised by combining rapid, meaningful reductions in greenhouse emissions with a more adaptable and pragmatic approach to the management of marine ecosystems.
The devastating bleaching on the Great Barrier Reef in 2016 and 2017 rightly captured the world’s attention. But what’s less widely known is that another World Heritage-listed marine ecosystem in Australia, Shark Bay, was also recently devastated by extreme temperatures, when a brutal marine heatwave struck off Western Australia in 2011.
A 2018 workshop convened by the Shark Bay World Heritage Advisory Committee classified Shark Bay as being in the highest category of vulnerability to future climate change. And yet relatively little media attention and research funding has been paid to this World Heritage Site that is on the precipice.
Shark Bay, in WA’s Gascoyne region, is one of 49 marine World Heritage Sites globally, but one of only four of these sites that meets all four natural criteria for World Heritage listing. The marine ecosystem supports the local economy through tourism and fisheries benefits.
Around 100,000 tourists visit Shark Bay each year to interact with turtles, dugongs and dolphins, or to visit the world’s most extensive population of stromatolites – stump-shaped colonies of microbes that date back billions of years, almost to the dawn of life on Earth.
Commercial and recreational fishing is also extremely important for the local economy. The combined Shark Bay invertebrate fishery (crabs, prawns and scallops) is the second most valuable commercial fishery in Western Australia.
However, this iconic and valuable marine ecosystem is under serious threat. Shark Bay is especially vulnerable to future climate change, given that the temperate seagrass that underpins the entire ecosystem is already living at the upper edge of its tolerable temperature range. These seagrasses provide vital habitat for fish and marine mammals, and help the stromatolites survive by regulating the water salinity.
In particular, extreme marine heat events were classified as very likely and predicted to have catastrophic consequences in Shark Bay. By contrast, the capacity to adapt to marine heat events was rated very low, showing the challenges Shark Bay faces in the coming decades.
The region is also threatened by increasingly frequent and intense storms, and warming air temperatures.
To understand the potential impacts of climatic change on Shark Bay, we can look back to the effects of the most recent marine heatwave in the area. In 2011 Shark Bay was hit by a catastrophic marine heatwave that destroyed 900 square kilometres of seagrass – 36% of the total coverage.
Some aspects of Shark Bay’s ecosystem have never been the same since. Many areas previously covered with large, temperate seagrasses are now bare, or have been colonised by small, tropical seagrasses, which do not provide the same habitat for animals. This mirrors the transition seen on bleached coral reefs, which are taken over by turf algae. We may be witnessing the beginning of Shark Bay’s transition from a sub-tropical to a tropical marine ecosystem.
This shift would jeopardise Shark Bay’s World Heritage values. Although stromatolites have survived for almost the entire history of life on Earth, they are still vulnerable to rapid environmental change. Monitoring changes in the microbial makeup of these communities could even serve as a canary in the coalmine for global ecosystem changes.
Coral reefs rightly receive a lot of attention, particularly given the growing appreciation that climate change threatens the Great Barrier Reef and other corals around the world.
The World Heritage Committee has recognised that local efforts alone are no longer enough to save coral reefs, but this logic can be extended to other vulnerable marine ecosystems – including the World Heritage values of Shark Bay.
Safeguarding Shark Bay from climate change requires a coordinated research and management effort from government, local industry, academic institutions, not-for-profits and local Indigenous groups – before any irreversible ecosystem tipping points are reached. The need for such a strategic effort was obvious as long ago as the 2011 heatwave, but it hasn’t happened yet.
Due to the significant Aboriginal heritage in Shark Bay, including three language groups (Malgana, Nhanda and Yingkarta), it will be vital to incorporate Indigenous knowledge, so as to understand the potential social impacts.
And of course, any on-the-ground actions to protect Shark Bay need to be accompanied by dramatic reductions in greenhouse emissions. Without this, Shark Bay will be one of the many marine ecosystems to fundamentally change within our lifetimes.
The latest NIWA climate assessment shows that sea surface temperatures in coastal waters around New Zealand are well above average. Marine heatwave conditions are already occurring in parts of the Tasman Sea and the ocean around New Zealand and looking to become the new normal.
Currently, marine heatwaves are defined as periods that last for five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline. Given we are likely to experience many more such events as the oceans continue to warm, it is time to understand and categorise the intensity of marine heat.
The names Hurricane Katrina, tropical cyclone Giselle (which sank the ferry Wahine 50 years ago), tropical cyclone Winston give a malevolent personality to geophysical phenomena. Importantly they get graded into categories, so we can rapidly assess their potential impact.
An Australian team has developed a classification scheme for marine heatwaves. The team used an approach similar to that used for hurricanes and cyclones – changing conditions can be slotted into to a sequence of categories. At the moment it looks like we are in marine heat wave category one conditions, but potentially entering category two if it continues to warm.
Further south, off Tasmania’s east coast, a number of species that normally occur in tropical waters have extended their range further south. A number of fish species, lobster and octopus species have also taken up residence along the Tasmanian coast, displacing some of the species that call this coast home. Mobile species can escape the warmer temperatures, but sedentary plants and animals are hardest hit.
As important as it is to identify a marine heatwave at the time, reliable predictions of developing conditions would help fishers, aquaculture companies and local authorities – and in fact anyone living and working around the ocean.
One of the important points to keep in mind is that when we are at the beach, we are sampling only the surface temperature. The same is true of satellites – they monitor less than the top millimetre of the ocean.
Sea surface temperatures are several degrees above normal at the moment. But in deeper waters, because of the high heat content of water, even a tenth of a degree is significant. Temperature in the deeper ocean is monitored by a network of moored buoys on and off the continental shelf along the Australian coast. New Zealand has almost nothing that would be comparable.
Measuring temperature in real time
What we can look to, in the absence of moored buoys, is a fleet of ocean robots that monitor temperature in real time. Argo floats drift with ocean currents, sink to two kilometres every ten days and then collect data as they return to the surface.
These data allowed us to identify that the 2017/18 marine heatwave around New Zealand remained shallow. Most of the warmer water was in the upper 30 metres. Looking at the present summer conditions, one Argo robot off New Zealand’s west coast shows it is almost four degrees above normal in the upper 40 metres of the ocean. On the east coast, near the Chatham Islands, another float shows warmed layers to 20 metres deep. To the south, the warming goes deeper, down to almost 80 metres.
Our work using the Australian Bureau of Meteorology forecast model highlights how variable the ocean around New Zealand is. Different issues emerge in different regions, even if they are geographically close.
The research on categories of marine heatwaves shows we will have to keep shifting what we regard as a heat wave as the ocean continues to warm. None of this should come as a surprise. We have known for some time that the world’s oceans are storing most of the additional heat and the impacts of a warming ocean will be serious.
Millions of people all over the world rely on desalinated water. Closer to home, Australia has desalination plants in Melbourne, Adelaide, Perth, the Gold Coast, and many remote and regional locations.
But despite the growing size and number of desalination plants, the environmental impacts are little understood. Our six-year study, published recently in the journal Water Research, looked at the health the marine environment before, during and after the Sydney Desalination Plant was operating.
Our research tested the effect of pumping and “diffusing” highly concentrated salt water (a byproduct of desalination) back into the ocean.
Contrary to our expectation that high salt levels would impact sea creatures, we found that ecological changes were largely confined to an area within 100m of the discharge point, and reduced shortly after the plant was turned off. We also found the changes were likely a result of strong currents created by the outfall jets, rather than high salinity.
Desalination is growing
We examined six underwater locations at about 25m depth over a six-year period during which the plant was under construction, then operating, and then idle. This let us rigorously monitor impacts to and recovery of marine life from the effects of pumping large volumes of hypersaline water back into the ocean. We tested for impacts and recovery at two distances (30m and 100m) from the outfall.
This study provides the first before-and-after test of ecological impacts of desalination brine on marine communities, and a rare insight into mechanisms behind the potential impacts of a growing form of human disturbance.
About 1% of the world’s population now depends on desalinated water for daily use, supplied by almost 20,000 desalination plants that produce more than 90 million cubic meters of water per day.
Increasingly frequent and severe water shortages are projected to accelerate the growth in desalination around the world. By 2025, more than 2.8 billion people in 48 countries are likely to experience water scarcity, with desalination expected to become an increasingly crucial water source for many coastal populations.
Effect of the diffusers
The diffusers that pump concentrated salt water into the ocean at a high velocity (to increase dilution) are so effective that salinity was almost at background levels within 100m of the outfall. However, the diffusion process increased the speed of currents close to the outfall.
This strong current affects species differently, depending on how they settle and feed. Marine species with strong swimming larvae, such as barnacles, can easily settle in high flow and then benefit from faster delivery of food particles. These animals increased in number and size near the outfall. In contrast, species with slow swimming larvae, such as tubeworms, lace corals and sponges, prefer settling and feeding in low current and became less abundant near the outfall.
Therefore, the high-pressure diffusers designed to reduce hypersalinity may have inadvertently caused impacts due to flow. However, these ecological changes may be less concerning than those caused by hypersalinity, as the currents were still within the range that marine communities experience naturally.
Our findings are important, because as drought conditions around the nation worsen and domestic water supplies are coming under strain, desalination is starting to ramp up in eastern and southern Australia.
For instance, water levels at Sydney’s primary dam at Warragamba have dropped to around 65% and the desalination plant is contracted to start supplying drinking water back into the system when dam levels fall below 60%. This plant can potentially double in capacity if needed.
There is a rapid expansion of the use of desalination, with global capacity increasing by 57% between 2008 and 2013. Our results will help designers and researchers in this area ensure desalination plants minimise their effect on local coastal systems.
We see the surface of the sea: the rock pools, the waves, the horizon. But there is so much more going on underneath, hidden from view.
The sea’s surface conceals human impact as well. Today, I am writing a eulogy to the Derwent River Seastar (or starfish), that formerly inhabited the shores near the Tasman Bridge in Hobart, Tasmania. It is Australia’s first documented marine animal extinction and one of the few recorded anywhere in the world.
The Derwent River Seastar, preserved in the Tasmanian Museum and Art Gallery, Hobart. Credit: Christy Hipsley, Museums Victoria/University of Melbourne
Scientists only knew the Derwent River Seastar for about 25 years. It was first described in 1969 by Alan Dartnall, a former curator of the Tasmanian Museum and Art Gallery. It was found on and off until the early 1990s but scientists noted a decline in numbers. Targeted surveys in 1993 and 2010 failed to find a single individual.
It was listed as critically endangered by the Tasmanian and Australian governments. But now, like a long-lost missing person, it is time to call it: the Derwent River Seastar appears extinct.
It is actually quite hard to document the extinction of marine animals. There is always hope that it will turn up in some unusual spot, somewhere in that hidden world. Australia has an ambitious plan to create high-resolution maps of 50% of our marine environment by 2025. This is a formidable task. But it is a reflection of our lack of knowledge about the oceans that, 20 years after the launch of Google Maps and despite an enormous effort in the interim, much of Australia’s seafloor in 2025 will be still largely known from the occasional 19th-century depth sounding, or imprecise gravity measurements from satellites.
We do notice when big animals go. There used to be a gigantic dugong-like creature called Steller’s Sea Cow, which lived in the North Pacific Ocean until it was hunted to oblivion by 1768. There is no mistaking that loss.
But the vast majority of the estimated 1 million to 2 million marine animals are invertebrates, animals without backbones such as shells, crabs, corals and seastars. We just don’t monitor those enough to observe their decline.
We noticed the Derwent River Seastar because it was only found at a few sites near a major city. Its story is intertwined with the usual developments that happen near many large ports. The Derwent River became silty and was at times heavily polluted by industrial and residential waste. The construction of the Tasman Bridge in the early 1960s cannot have helped.
The Derwent River Seastar has been a bit of an enigma. From the start, it was mistakenly classified as belonging to group of seastars (poranids) otherwise known from deep or polar habitats. Some people wondered whether it was an introduced species as well, one that couldn’t cope with the Derwent environment.
However, we used a CT scanner at the School of Earth Sciences, University of Melbourne, to look at the internal skeleton of one of the few museum specimens. Sure enough, it has internal struts to strengthen the body, which are characteristic of a different group of seastars (asterinids) that have adapted to coastal environments and are sometimes restricted to very small areas.
CT scan showing the internal structure of the seastar. Source: Christy Hipsley, Museums Victoria/University of Melbourne
Is this seastar like a canary in a coal mine, a warning of a wave of marine extinctions? Sea levels are rising with global warming, and that is going to be a big problem for life adapted to living along the shoreline. Mangroves, salt marsh, seagrass beds, mud flats, beaches and rock platforms only form at specific water depths. They are going to need to follow rising sea levels and reform higher up the shoreline.
Coastal life can take hundreds to thousands of years to adjust to these sorts of changes. But in many places we don’t have a natural environment anymore. Humans will increasingly protect coastal property by building seawalls and other infrastructure, especially around towns and bays. This will mean far less space for marine animals and plants.
We need to start planning new places for our shore life to go – areas they can migrate to with rising sea levels. Otherwise, the Derwent River Seastar won’t be the last human-induced extinction from these environments.
Rising ocean temperatures may result in worldwide change for shallow reef ecosystems, according to research published yesterday in Science Advances.
The study, based on thousands of surveys carried out by volunteer scuba divers, gives new insights into the relationship of fish numbers to water temperatures – suggesting that warmer oceans may drive fish to significantly expand their habitat, displacing other sea creatures.
The study draws from Reef Life Survey, a 10-year citizen science project that trains volunteer scuba divers to survey marine plants and animals. Over the past ten years, more than 200 divers have surveyed 2,406 ocean sites in 44 countries, creating a uniquely comprehensive data set on ocean life.
Lead author Professor Graham Edgar, who founded Reef Life Survey, said the unprecedented scope of their survey allowed them to investigate global patterns in marine life. The abundance of life in warm regions (such as tropical rainforests and coral reefs) has long intrigued naturalists. At least 30 theories have been put forward, but most studies have been based on relatively limited surveys restricted to a single continent or group of species.
By tapping into the recreational scuba diving community, Reef Life Survey has vastly increased the amount of information researchers have to work with. Professor Edgar and his colleagues provide one-on-one training to volunteers, teaching them how to carry out comprehensive scans of plants and animals in specific areas.
Dr Adriana Vergés, a researcher at the University of New South Wales specialising in the impact of climate change on ocean ecosystems, said that the Reef Life Survey has already substantially improved our understanding of the marine environment.
“For example, Reef Life Survey data has greatly contributed to our understanding of the factors that determine the effectiveness of effectiveness of marine-protected areas worldwide. The team have made all their data publicly available and more and more research is increasingly making use of it to answer research questions,” she said.
Some of the divers have been working with Reef Life Survey for a decade, although others participate when they can. One volunteer, according to Professor Edgar, was so inspired by the project that he began a doctorate in marine biology (he graduated this year).
Warming oceans means fish on the move
One of the important insights delivered by the Reef Life Survey datatbase is the relationship between water temperature and the ratio of fish to invertebrates in an ecosystem. Essentially, the warmer the water, the more fish. Conversely, colder waters contain more invertebrates like lobster, crabs and shrimp.
Professor Stewart Frusher, director of the Centre for Marine Socioecology at the University of Tasmania (and a former colleague of Professor Edgar) told The Conversation that he believes we will see wide-scale changes in fish distribution as climate change warms the oceans.
“Species are moving into either deeper water or towards the poles. We also know that not all species are moving at the same rate, and thus new mixtures of ecosystems will occur, with the fast-moving species of one ecosystem mixing with the slower moving of another,” he said.
As species migrate or expand into newly warmed waters, according to Professor Frusher, they will compete with and prey on the species already living in that area. And while it’s uncertain exactly how disruptive this will be, we do know that small ecosystem changes can rapidly lead to larger-scale impacts.
In order to predict and manage these global changes, scientists need reliable and detailed world-wide data. Professor Frusher said that, with research funding declining, scientists do not have the resources to monitor at the scales required.
“Well-developed citizen science programs fill an important niche for improving our understanding of how the earth is responding to change,” he said.