The devastating bleaching on the Great Barrier Reef in 2016 and 2017 rightly captured the world’s attention. But what’s less widely known is that another World Heritage-listed marine ecosystem in Australia, Shark Bay, was also recently devastated by extreme temperatures, when a marine heatwave struck off Western Australia in 2011. In 2011, Shark Bay was hit by a catastrophic marine heatwave that destroyed 900 square kilometres of seagrass – 36% of the total coverage.

Some aspects of Shark Bay’s ecosystem have never been the same since. Many areas previously covered with large, temperate seagrasses are now bare, or have been colonised by small, tropical seagrasses, which do not provide the same habitat for animals.

This mirrors the transition seen on bleached coral reefs, which are taken over by turf algae. We may be witnessing the beginning of Shark Bay’s transition from a sub-tropical to a tropical marine ecosystem.

Shark Bay is especially vulnerable to future climate change, given that the temperate seagrass that underpins the entire ecosystem is already living at the upper edge of its tolerable temperature range. These seagrasses provide vital habitat for fish and marine mammals, and help the stromatolites survive by regulating the water salinity.

Stromatolites are a living window to the past. Matthew Fraser

The region is also threatened by increasingly frequent and intense storms, and warming air temperatures.

Because of its temperate-tropical overlap, Shark Bay has a diverse group of about 12 seagrass species. Researchers surveyed 63 sites in Shark Bay between 2012 and 2014 to assess seagrass recovery and changes. Before the heat wave, many sites were dominated by the temperate seagrass known as “wireweed” (Amphibolis antarctica). Surviving A. antarctica beds appeared stable but didn’t reclaim much turf. Instead, the tropical seagrass Halodule uninervis, began filling the gaps. H. uninvervis was spotted at 2 percent of sites in 2012, but had expanded to almost 30 percent of them by 2014.

“Losing that cover is really huge; it’s like going from a bushland in Africa to a well-mowed lawn. “After the die-off, we also saw water clarity go down a ton,” Dr Nowicki said. Decaying seagrass have nourished a bloom of microscopic algae. Major seagrass ecosystems around the world have declined by about 7 percent per year since 1990, reminiscent of declines in coral reefs and other critical ecosystems.

In Shark Bay, beds of slow-growing A. antarctica seagrass may struggle to recover further, the study suggests. Shark Bay, located where temperate and tropical ecosystems overlap, is among the warmest areas that A. antarctica can occupy, and extreme warm temperatures are predicted to become more common with climate change.