Phytoplankton are surviving and thriving in warmer-than-usual waters around Hokkaido, raising concerns that red tide could harm the marine ecosystem and fisheries of Japan’s northernmost main island, a survey showed.

The study, conducted by researchers from Nagasaki University and Yamagata University between August and September 2016, pointed to global warming as the cause of the threat.

“It is only a matter of time before red tide will cause damage in Hokkaido,” said Hideto Tsutsui, one of the researchers from Nagasaki University.

Using a research vessel, the team surveyed waters off Nagasaki Prefecture, the Sea of Japan and Hakodate Bay in Hokkaido by pumping seawater up from a depth of 4 meters once every three hours.

They checked samples from 66 locations for plankton and other creatures that normally live in warm seas.

The results showed an abundance of nutrient-depleting phytoplankton species that can cause red tide, including a plantlike flagellate known as Dictyocha messanensis, were found in Hakodate Bay and waters off Akita Prefecture or farther north.

The average water temperature on the surface of Hakodate Bay for the July-September period increased by about 0.1 degree annually between 2004 and 2015, according to data of the U.S. National Oceanic and Atmospheric Administration.

The temperature rise likely allows plankton carried by ocean currents from southern areas to survive in waters near Hokkaido, according to the scientists.

When they show up in large numbers, the microscopic organisms deplete nutrients in the water and can cause red tide.