The recent animated blockbuster Finding Dory (the sequel to Finding Nemo) featured a kelp forest. With a variety of animals swimming in and out, up and down, around and over the swaying fronds, the forest indicated a murky but safe haven for the absent-minded Dory and her friends.
Indeed, these underwater forests are of great ecological significance. In Australia, kelp is the main food source for the marine food web in the temperate regions of the southern coasts. Their forests deliver many social and economic benefits, hosting a vast array of fish and other marine animals, such as abalone.
The value of Australia’s temperate ecosystems for fishing and tourism is estimated at $10 billion annually. One of the main inhabitants of the southwest kelp forests, the Western Rock Lobster, is the country’s most prized fishery, last valued in 2014 at $359 million a year.
However, a marine heatwave in 2011 has killed a massive chunk of kelp forest in Western Australia. The resulting impacts are devastating.
Kelp forest halved
According to a new study published in Science, 100 km of the Great Southern Reef on Australia’s west coast south of Kalbarri is now devoid of kelp. Stretching further south to Jurien Bay, the population of kelp is half of what it was in 2010. Worryingly, the underwater forests have showed no signs of recovery since the start of their demise five years ago.
CSIRO marine ecologists Russ Babcock and Mat Vanderklift, and oceanographer Francois Dufois are co-authors of the study. They played a major part in monitoring the death of these kelp forests. Dr Vanderklift says such a huge loss of kelp has opened a window for seaweed-eating tropical fish to thrive. “Today, even though the climate is mostly favourable for the kelp to return, they remain absent,” he says.
And their absence is likely to be permanent. “Kelp spores would have to get back to the area somehow, and even then they would have to avoid being eaten. All in all, a very difficult task,” says Dr Vanderklift.
“The total loss of kelp is more than 900 square kilometres,” explains Dr Babcock. “Kelp is a large habitat forming alga that supports biodiversity by providing shelter for a myriad of other plants and animals. As the very base of food webs in the region, their absence also has a huge impact on productivity.”
Furthermore, this change in the structure of the coastal ecosystem won’t only affect marine organisms—many people depend on the kelps for their livelihoods.
The ocean warming of recent years along the west coast was intensified during one of the strongest La Niña cycles on record in 2011. La Niña brings higher than average water temperatures to Australia’s west coast—as El Niño does to the east coast.
The problem was further exacerbated by the unique Leeuwin Current, which flows south, carrying with it the warmer waters and tropical species of the Ningaloo tropics. Temperatures along some parts of the western coastline were up to five degrees warmer than average during this marine heatwave. As kelp is a cold-water alga, it simply could not tolerate the extreme change in temperature.
The study was based on a 15-year survey, which commenced in 2000, stretching 2000 km along the west Australian coast. The data CSIRO collected before the marine heatwave formed a large part of the pre-impact baselines from Cape Naturaliste in the south to the tropics in Ningaloo.
Ecosystems at risk
The results of the study are a solid indication that
The demise of coral reefs worldwide was especially apparent with the recent global bleaching event. Concern for the Great Barrier Reef has come from the likes of Sir David Attenborough, U.S. President Barack Obama and even the voice of Dory herself, Ellen DeGeneres.
Live coral cover on some parts of Ningaloo dropped from 80 to 6 percent following the 2011 marine heatwave. These isolated reefs are still in the process of recovering. Seagrasses in Shark Bay were also hit hard, as reported in ECOS in 2013.
More impacts expected
As oceans continue to warm, and La Niña events increase in frequency, more intense effects are expected to occur in the eastern Indian Ocean.
“Consequently the prognosis for kelp forests in Western Australia and along the rest of Australia’s southern coasts is poor,” says Dr Babcock. “At a local scale there is potential to make management decisions that take into account a changed level of kelp production, and to adjust allowable fisheries catches accordingly. In terms of conservation, areas of remaining kelp could be given additional protection.”
“Ultimately, however, global warming has to be addressed as the root cause of these changes.”
This work has been enabled by Western Australia Government initiatives including the Western Australian Marine Science Institution and the Strategic Research Fund for the Marine Environment.