Australian scientists revealed on Monday they were shocked to discover that more than 40 percent of the Great Barrier Reef's inshore areas are dominated by seaweed.
The weed invasion was found during a study in which they swam over 500 different sections of the reef to see what was thriving.
Study co-author Dave Bellwood of James Cook University said the shift from a coral-dominated reef to a weed-dominated one was concerning and
difficult to reverse.
"We got quite a shock when we saw how much of the inner reefs were dominated by weed," Professor Bellwood said.
The weed invasion could be caused by nutrients coming into the water from the land, or by a decline in the weed-eating fish species that usually "mow" the seaweed.
Bellwood called on the Australian government to do more to protect coral fish species such as the parrot, surgeon, rabbit and bat fish.
However, he said due to a lack of historical data, scientists were not sure if the seaweed was spreading.