UWA Oceans Institute

Grim future for Western Australia's marine forests

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OCEANS ONLINE     ISSUE 1. MARCH 2015

Oceans Institute researcher, Professor Thomas Wernberg and Adjunct Professor Dan Smale predict a not-so-bright future for Western Australian seagrass meadows as frequency of extreme warming events sets to double.

Western Australia’s marine environment is unique. 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. 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.

Recent 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, according to research released in late January. While the devastating impacts of such extreme weather is clearly visible on land, many are likely unaware of the similarly devastating impacts under water.

The diverse, expansive marine ecosystems along the vast coastline off Western Australia are no strangers to the devastating impacts of La Niña events. In 2011, strong La Nina conditions increased the flow of the warm southward flowing 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 Western Australian coastline from north of Ningaloo Reef to the Capes. Water temperatures soared past anything recorded for at least 140 years and remained in excess of 2°C above the long-term average for more than 10 weeks.

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. In addition, extensive seagrass meadows and kelp forests fringe more than 1000km of coastline and these harbour some of the highest diversity of seagrasses and seaweeds in the world. Kelp forests and seagrass meadows are akin to tropical coral reefs and terrestrial forests, in that they provide 3-dimensional habitat, food and shelter for myriads of associated species, many of which are found nowhere else in the world and many others that have economic importance (e.g. abalone, rock lobster).

Many of the species found along the coastline of southwestern Australia have evolved to live in cooler temperate waters. When 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 mortality events of a wide range of species, including seaweeds, seagrasses, shellfish and finfish, were observed along the west Australian coastline. Cool-water forest-forming species such as kelps and seagrasses were particularly hard hit, which has knock-on consequences for other organisms that depend on these forests for food and shelter. One forest-forming seaweed was eradicated from over 100 km 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 species.

The effects of La Niña had socioeconomic implications too. 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 impacted breeding success in Little Penguins, while the highly-valued Western Rock Lobster also suffered high mortality. 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 4 years since the marine heat wave, with some species showing no signs of recovery.

Climate variability such as La Niña 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 climate change will intensify extreme climate variability driven by La Niña events. This means that temperate marine ecosystems will have less time to recover in between periods of excessive temperatures. This ‘death by a thousand cuts’ could sound the death knell for west Australia’s globally significant underwater forests, and will have profound ecological and socioeconomic implications. As many Western Australians 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 these valuable habitats in decades to come.

 

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