Marine heatwaves threaten underwater forests

Issue 
Little penguins are at risk from high water temperatures.

Western Australia's marine environment is unique. It has two world heritage areas, the largest fringing coral reef in Australia and more than a thousand kilometres of underwater forests, supporting incredible wildlife, important fisheries and tourism.

Western Australia's marine environment is unique. It has two world heritage areas, the largest fringing coral reef in Australia and more than a thousand kilometres of underwater forests, supporting incredible wildlife, important fisheries and tourism.

But these precious habitats were decimated in 2011 by a record-breaking marine heatwave, with temperatures rising more than 2°C above normal for ten weeks. The culprit: warm water pushed south by the same La Niña event that caused devastating flooding in Queensland.

Research shows the oceans are continuing to warm steadily despite a slowdown in the rate of warming at the earth's surface, increasing the likelihood of extreme heat undersea. And La Niña events are forecast to become twice as frequent thanks to climate change.

Sadly, these changes could spell the end for large swathes of Western Australia's underwater forests and much of the marine life that depends on them for food and shelter.

WA's extreme marine heatwave

La Nina refers to the positive phase of the El Nino–Southern Oscillation (ENSO) climate pattern, which brings cyclones, rain and floods to South-East Asia and Australia. While the devastating impacts of such extreme weather are clearly visible on land, many are unaware of the similarly destructive consequences under water.

The diverse, expansive marine ecosystems along the vast coastline off Western Australia are no strangers to the devastating impacts of La Nina events. In 2011, strong La Nina conditions increased the southward flow of the Leeuwin Current, which pushed warm water from the tropics into cooler temperate latitudes.

At the same time, winds were calmer than normal resulting in unusually high transfer of heat from the air into the upper layers of the ocean. The outcome was an unprecedented marine heatwave, which affected more than 2000 kilometres of the West Australian coastline from north of Ningaloo Reef to Cape Leeuwin on the south-western corner of the continent. Water temperatures soared past anything recorded in the past 140 years.

Climate variability driven by El Nino and La Nina is a natural phenomenon that has influenced Earth's ecosystems for millennia. However, the recent modelling experiments, which are the most sophisticated to date, suggest that anthropogenic climate change will intensify extreme climate variability driven by El Nino and La Nina events.

The projected increased frequency of extreme La Ninas — from an average of once every 23 years to once every 13 years — will mean that temperate marine ecosystems will have less time to recover in between periods of excessive temperatures.

Unique marine environment

The West Australian marine environment is unique by global standards. It hosts two World Heritage areas — Ningaloo Reef, Australia's largest fringing coral reef; and Shark Bay, home to extensive seagrass meadows, turtles, sharks and dugongs.

Seagrass meadows and seaweed forests also fringe more than 1000 kilometres of coastline from Ningaloo Reef to Cape Leeuwin. These underwater forests harbour some of the highest diversity of seagrasses and seaweeds in the world and hundreds of other species that are found nowhere else on Earth.

Temperate seaweed forests and seagrass meadows are like tropical coral reefs and forests on land — they provide habitat, food and shelter for a myriad of associated species, many of which have economic importance such as abalone and rock lobsters.

Many of the species found along the coastline of south-western Australia have evolved to live in cooler temperate waters. When peak summer sea temperatures soar, as they did in early 2011, many species overheat and become physiologically stressed or even die.

Following the 2011 La Nina event, mass deaths occurred in a wide range of species, including seaweeds, seagrasses, shellfish and finfish.

Plants that formed forests such as kelps and seagrasses were particularly hard hit, with knock-on effects on the wildlife that depend on these forests for food and shelter.

One forest-forming seaweed was eradicated from over 100 kilometres of coastline in just a few months as a result of the marine heat wave. Further north, unprecedented levels of coral bleaching and mortality were recorded for a range of reef-building coral species.

Rapid loss of underwater forests in response to recent oceanic warming has also been observed in other temperate regions of the world.

And tropical plant-eating fishes moving into temperate zones often help finish off what heatwaves and warming starts.

Grim future

The La Nina also affected important fisheries and tourism. For example, a regional abalone fishery in Kalbarri was decimated and stocks of scallops and blue swimmer crabs in Shark Bay have crashed.

High water temperatures and a lack of available food also killed and reduced the breeding success of little penguins, while the highly-valued western rock lobster also suffered many deaths. On the other hand, many warm water species were found further south than usual, presenting opportunities for recreational and commercial fishers alike.

In many cases, species' populations have not returned to normal in the five years since the marine heat wave, with some species showing no signs of recovery. For example, northern populations of Roe's abalone have not recovered naturally and a large-scale restocking programme is underway to re-establish the fishery. Similarly, populations of forest-forming seaweeds and kelp have not yet bounced back.

The increasing frequency and severity of marine heatwaves set to follow the doubling of La Nina events may eventually sound the death knell for Western Australia's globally significant underwater forests and will have profound socioeconomic implications.

As millions of people rely on ecological goods and services provided by temperate marine ecosystems, a better understanding of what conditions boost resilience to climate variability is needed to maximise chances of protecting and sustaining them in decades to come.

Recent research on coral reefs has shown that tropical ecosystems can recover from extreme warming events, if local conditions are favourable. The capacity of temperate reef ecosystems to recover from marine heatwaves is, however, far less understood.

The question that remains is whether our seaweed forests will have time to recover between the blows dealt by these underwater heatwaves — or shift to a new, more impoverished state.

[Thomas Wernberg is ARC Future Fellow in Marine Ecology, University of Western Australia. Dan Smale is a Research Fellow in Marine Ecology, Marine Biological Association. This article was originally published at The Conversation.]

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