Ocean warming has caused Tasmanian reef ecosystems to change significantly over the past twenty years

A new Australian study has revealed that pronounced sea temperature rise in the past two decades has resulted in major changes to reef community structures.

As the Earth’s oceans continue to warm due to climate change, the structure and function of marine ecosystems are becoming increasingly threatened. This is the case especially for the temperate marine environment in Tasmania, which researchers say is warming at a rate four-times higher than the global average.

Comparing reef biodiversity over three periods from 1992-2019, the University of Tasmania and CSIRO Hobart have found that, in particular, there’s been an increase in fish biomass, a decrease in algal cover, and a reduction in the richness and abundance of macroinvertebrate species – animals without a spine.

“While reef communities were relatively stable and warming was slight between the 1990s and mid-2000s (+0.12°C mean temperature rise), increased abundances of warm affinity fishes and invertebrates accompanied warming during the most recent decade (+0.68°C rise),” the authors write.

Tasmanian oceans are warming at a higher rate

The study looked at the change in biodiversity in what’s known as the Tasmanian ocean-warming hotspot.

“Monitoring of sea temperature at the Maria Island Station in eastern Tasmania since 1944 has revealed a warming rate of 2.28°C/century, which is four-times higher than the global average,” the authors write.

“This warming has been driven by the strengthening of the East Australian Current.”

Warming oceans drive the re-distribution of marine species, which can have cascading impacts across all levels of the ecosystem.

To evaluate the effect of the changing climate on fish, invertebrate, and macroalgal communities across Tasmania, the researchers analysed a dataset spanning four coastal regions between three periods (1992–95, 2006–07, 2017–19). The data included scuba diver-based underwater visual censuses of almost 100 reef sites.

An influx of warmer water-loving fishes

Algal cover decreased as a result, especially over the last decade, with a Tasmania-wide decline in macroalgae from approximately 80% to approximately 60% between the mid-2000s and 2017-2019.

There was also a clear decline in the richness of macroinvertebrate biodiversity – 27% over the 27 years.

Evidence of thermophilisation – adaption to a warmer climate – was also observed.

Thermophilisation is recognised as one of the main fingerprints of climate change. It occurs because, as ambient temperatures increase, warm-adapted species perform better than cool-adapted ones – causing changes to ecosystem biodiversity.

There were significant rises in the Community Temperature Index – the weighted average of species’ temperature preferences – for fishes, invertebrates, and macroalgae, in some regions.

The authors write this has the “potential for cascading effects on reef habitats”.

One of the effects is an increase in fish biomass. The study found greater warming-associated increases occurred on fished reefs than on protected reefs.

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“Increases in abundance of fish on fished reefs resulted from increases in warm-affinity species, many of them herbivores that are not targeted by fishers,” the researchers write.

First author Dr German Soler, from the Marine and Biodiversity group at the University of Tasmania and, told Cosmos via email from Colombia that: “Greater changes in the marine communities were observed outside marine protected areas. Therefore, marine protected areas, with their more complete marine communities’ guilds, were more resilient to the changes associated with warming seas.”

The researchers write that this finding demonstrates that non-climate, human activity-caused stressors can “interact to pave the way for climate change effects, which can lead to rapid and dramatic reef ecosystem regime-shifts”.

Soler adds that “the monitoring of the Tasmanian ecosystems will continue into the future and will serve as a baseline of the changes that could occur in the coming years with the most possible scenario of increasing sea temperatures.”

The study has been published in Proceedings of the Royal Society B.

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