Climate change resulting in more frequent flooding of the Wooramel River that leads into Shark Bay is threatening the unique ‘living rock fossils' that make Shark Bay a World Heritage site, according to researchers from the Oceans Institute, at The University of Western Australia, Curtin University and the CSIRO.
Shark Bay was added to the World Heritage List in 1991 because it contains living examples of the most ancient records of life on Earth: stromatolites.
These stromatolites - rocky structures formed over millennia by blue-green algae or cyanobacteria - thrive in Shark Bay's Hamelin Pool, where an unusual undersea landscape has created an environment twice as saline as normal seawater.
"But climate change is disturbing the natural system," Oceans Institute Emeritus Winthrop Professor Diana Walker said.
"The Wooramel River, which flows into Hamelin Pool, has flooded three times in the past year, washing in a huge amount of sediment and damaging Shark Bay's seagrass meadows.
"Usually the river only floods every eight years or so."
Professor Walker, Winthrop Professor Gary Kendrick and other marine scientists are investigating how these disturbances are affecting Shark Bay's World Heritage values.
The research forms part of a Caring For Our Country project of the Australian Government-funded to the Western Australian Marine Science Institution.
Surprisingly, it is the seagrass meadows in Shark Bay that are responsible for creating Hamelin Pool's unique salty conditions that allow stromatolites to thrive.
Over the past 4,000 years, seagrasses have collected sediment to build a barrier known as the Fauré Sill around the top of Hamelin Pool, limiting ocean circulation.
"The trapped water inside the pool evaporates at a very high rate and - because it doesn't mix well with the wider ocean - this water becomes extremely salty," Professor Kendrick said.
"With climate change, the amount of sediment washing into Hamelin Pool is expected to only increase, which poses a major threat to the seagrasses.
"We've looked at growth rates and shoot lengths of seagrasses buried by sediment and we know that they are significantly affected." A large area of defoliated seagrasses was observed on Wooramel Bank to the north-east of Hamelin Pool.
The current research will help marine scientists to understand how increased sediment input and changing ocean circulation patterns will affect the Fauré Sill ecosystem, and how this will ultimately affect the stromatolites.
"Shark Bay is world-renowned for its high diversity of marine flora and fauna, and for its unique ecosystems," Professor Walker said. "Our current research will help to confirm how climate change will affect these natural assets."
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