Study of 1921 Shark Bay cyclone sheds light on present dangers

A historical account of fish being stranded up to nine kilometres inland by a cyclone in Shark Bay a century ago has been used with numerical modelling to help assess current risks from extreme weather to areas on the edge of tropical cyclone zones.

The cyclone struck the area, 750km north of Perth, in 1921 killing two people, flooding the shores of the outer gulf and causing extensive harm to the pearling industry, seagrass meadows and freshwater wells which remained saline for decades.

The research paper, The utility of historical records for hazard analysis in an area of marginal cyclone influence published in Communications Earth and Environment, reconstructed the cyclone partly through extensive surveys of State and national archives including accounts by a pearling inspector. It also involved using a groundbreaking approach, Quantified Historical Data Framework, to do a structured analysis of archival information to evaluate the authenticity, consistency and relevance of reports of the extreme weather event.

Dr Jenny Shaw, Research Director for the Western Australian Marine Science Institution, worked on the paper in collaboration with researchers from the Nanyang Technological University in Singapore, The University of Western Australia, University of Sydney, Griffith University, Baird Australia and Minderoo Foundation.

Lead author Associate Professor Adam Switzer from Nanyang Technological University’s Earth Observatory of Singapore said the study engaged a diverse team of historians, geoscientists, marine biologists and engineers, showing the need for multidisciplinary collaboration to tackle complex issues such as the risks of cyclones and storm surges in a warming world.

The study noted tropical cyclones were likely to shift poleward in a warming climate and some researchers had suggested the trend was particularly obvious in the Southern Hemisphere.

“This approach provides researchers worldwide with a valuable framework for leveraging historical documents to derive actionable modeling parameters, benefiting areas that face similar challenges,” Associate Professor Switzer said.

Dr Shaw said the study of the cyclone, which struck what is now a World Heritage site, found it had a major ecological impact in a similar way to TC Yasi in Queensland in 2011.

“A recurrence would likely have considerable long-term knock-on effects to ecosystem functions and services, particularly as the seagrasses of Shark Bay contribute to several World Heritage values,” Dr Shaw said.

One of the accounts of the cyclone, found during a search of archives, was by Pearling Inspector Wally Edwards, who witnessed the cyclone and its aftermath.

He described seeing ship groundings, inundated coastal wells, flooding, an altered coastal landscape and sharks and fish stranded inland.

Longer term impacts included livestock losses, declines in the shells used in the pearling industry and reports that dugongs were seen less frequently in the years after the cyclone.

The researchers said a similar cyclone, assessed to be between a category four and five, would inundate the town of Denham flooding critical infrastructure and damaging important industries.

Dr Joseph Christensen, from UWA’s School of Humanities, said the study uncovered detailed information about the historic event.

“By employing the Quantified Historical Data Framework, we were able to establish a detailed and quantified understanding of the 1921 storm surge, including its timing, nature and ecological consequences,” Dr Christensen said.

The authors said the approach had important implications for land use planning, emergency management and environmental management of Shark Bay and other sites of marginal cyclone influence.

Seagrass put to test to find best species for withstanding climate change impact

Researchers looking at the possible impact of climate change on seagrass have tested the tolerance of the plants to rising temperatures after collecting samples at locations spanning 600 kilometres.

Nicole Said, a research associate from Edith Cowan University who is part of the WAMSI Westport Marine Science Program project, said six seagrass species were collected within Cockburn Sound and one, Posidonia sinuosa was collected along Western Australia’s coast from Geraldton to Geographe Bay.

The samples, which represent species that are all found in Cockburn Sound, were then put in chambers and subjected to incremental increases in water temperature from 15 to 43 degrees over 12-hours.

Oxygen changes in the water were measured to calculate the plant’s photosynthetic rate or the rate at which light energy was converted into chemical energy during photosynthesis. The experiments allowed researchers to understand at what temperature the plants thrived or were stressed.

“It appears from the species that we looked at in Cockburn Sound, the one most at risk from rising temperatures was Zostera nigricaulis which is commonly known as eel grass,” Ms Said stated.

Halophila ovalis, a species found in temperate to tropical areas and commonly known as paddle weed, spoon grass or dugong grass, was most able to withstand the higher temperatures.

Other species tested were Amphibolis griffithii, Posidonia sinuosa (the most widespread species in Cockburn Sound), Posidonia australis and Amphibolis antarctica, which are larger plants than the other two species assessed.

The research team found a heatwave in Perth that produced temperatures between three and four degrees higher than average summer temperatures would be likely to have a negative impact on the larger species which are generally able to withstand pressures for a greater duration than smaller species, but once damaged take longer to recover.

Heatwaves are predicted to become more frequent and more intense under climate change. An extreme marine heatwave in 2010 and 2011 saw a large area of seagrass in Shark Bay destroyed.

“With increasing ocean temperatures and an increase in marine heatwave events, seagrass species living close to their thermal limits are at risk from rising temperatures. There is limited temperature threshold information for seagrass species, which is critical information and can forewarn both present and future vulnerability to ocean warming.”

“There are other researchers around the world looking at climate resilience, but we have been missing this key baseline data to look at the physiology of seagrasses and how they may respond to these climate scenarios.”

ECU School of Science Associate Professor Kathryn McMahon, who co-leads the research on seagrass resilience said the findings were significant.

“These findings are really exciting as they indicate there are differences among seagrass species and population along our WA coast to ocean warming,” Associate Professor McMahon said.

“We can harness these differences and take actions to try and build resilience into our spectacular seagrass meadows.”