Extreme heat: High temperatures and invasive algae threaten Croatia's largest coral reef.

For over thirty years, marine biologist Petar Kruzic has dived every August into the depths of the Great Lake in Mljet National Park, Croatia, to gather data on the growing challenges facing the largest known Mediterranean coral reef. Covering an area of 650 square meters, the reef is home to a variety of marine species, including groupers, amberjacks, and barracudas.

Using data loggers, Kruzic and his team recorded a sharp increase in prolonged high temperatures, with reef water now reaching 29°C for a month and a half, compared to just 2-3 days in the early 2000s. "Polyp mortality and also necrosis—direct tissue necrosis—due to high temperatures," Kruzic said.

Invasive species such as Caulerpa racemosa, a fast-growing algae that thrives in warmer waters, pose an additional threat, often overfishing and choking out coral colonies.