Coastal development and sea-level rise lead to a decrease in tidal flats on the world's shores, according to researchers who have first mapped ecosystems.
University of New South Wales (UNSW) researchers and The University of Queensland used machine learning to analyze more than 700,000 satellite imagery to map the extent and variation of mudflats around the world.
The study published in Nature found a decline in tidal flats ecosystems in some countries, from 1
Tidal flats are mudflats, sand plains or wide reef platforms, which are important coastal ecosystems. They serve as buffers for storms and sea-level rise and provide habitat for many species, including migratory and fish farms.
Almost 50% of the world's tidal area is concentrated in only eight countries: Indonesia, China, Australia, USA, Canada, India, Brazil and Myanmar.
Nicholas Murray, the lead author of the study and senior researcher at the Center for Ecosystem Research at the University of New South Wales, said that tidal flats were often at least partially covered on the water, making them difficult to monitor in the past.
"This is a big ecosystem," he said. "It's all over the world and very vulnerable to threats, but we do not know where they are, limiting the scope for surveillance."
The research team worked with Google and used its computing resources to analyze each satellite image. The world's coastlines have already been collected.
They found that tidal flats as an ecosystem are globally as extensive as mangroves and that coastal development and sea-level rise have caused their decline.
In parts of China In Western Europe they found tidal flats up to 18 km wide. In Australia, they can be found throughout the country, including places like Moreton Bay in Queensland and along the Gulf of Carpentaria.
For 17% of the world, sufficient data were available to measure the decline from 1984 to 2016.  At these locations, located mainly in China, the US, and the Middle East, they found a 16% decline in tidal flats.
For a further 61% of the world there was enough data to analyze the changes from 1999 to 2016 and research showed a decrease of 3.1% during this period.
Murray said airports, aquaculture and other infrastructure built on tidal planes in countries like China are a major threat. Reduced sediment flows from rivers around the world also led to less sediment than tidal flats being deposited.
Murray said that dams are one of the main reasons for reduced river sediment flows. He said that a further analysis of the continuing impact of the other major threat – sea-level rise – was needed.
"This study has really provided the data to make these connections," he said. "This means that you can really understand the effects of sea-level rise and coastal development."
The researchers suspect that the study could be used to advance protected areas for tidal flats, which for historical reasons were not always well protected. They lie between land and sea.
The map is publicly available and Murray said he laid the foundations for a continuous surveillance system.
"The easiest way to think about it is that we can do that for decades, watch the deforestation," he said. "We can do that now for shallow-water ecosystems.
" We can identify places where shallow-water ecosystems are lost and the main drivers of those losses that will allow us to respond with conservation measures. "