Heat wave triggers largest sponge death ever
The 2022 heatwave in New Zealand lasted 259 days. Sea water temperatures rose by more than 4.4 degrees. The impact was devastating: Many sponges fell victim to the high temperatures and are now missing from the ecosystem.
Not only corals can bleach, but sponges are also affected. In 2022, researchers discovered a massive sponge bleaching event in New Zealand's Fiordland: Around 66 million marine sponges were affected.
The population of the settled sponge species Cymbastella lamellata along the approximately 1,000-kilometer-long coastline was estimated to have been reduced by almost half, according to a study by the researchers. "This was not only the largest sponge bleaching ever observed worldwide, but also the largest sponge mortality ever recorded," said marine biologist James Bell from Te Herenga Waka - Victoria University of Wellington, the study's lead author.
Up to 4.4 degrees warmer than usual
The bleaching occurred during a heatwave that lasted 259 days, causing sea water temperatures to rise by up to 4.4 degrees Celsius above average. As a result, the shell-like animals changed color: from a dark brown to a light white. Around 20 percent of the sponges along the coast were completely bleached, 75 percent were partially bleached, and only 5 percent were unaffected.
"Sponges form symbioses with a wide variety of organisms, from diatoms to crustaceans and zoanthids to a very large diversity of microorganisms," explains Bell. The relationships with microbes are particularly important, as it is assumed that they help the sponge to survive, especially under stressful conditions.
Diatoms give color
Similar to coral bleaching, the sponge loses diatoms from its tissue that give it its color under normal conditions. Whether the bleaching also directly leads to the death of the sponges is not yet certain, explains Bell. Possibly, the sponges were eaten by fish because they became tastier or could be seen more easily by fish.
The researchers also found that some sponges were able to recover from the bleaching. "This gives us hope that sponges may be able to adapt to the warming of the water," says Bell. Because heatwaves in the sea are becoming more frequent and intense due to climate change, which has significant impacts on marine life, write the scientists.
The Commission has expressed concerns over the impact of climate change on marine life, particularly in light of the 2022 New Zealand heatwave. The Commission is advocating for more research to understand the long-term effects of such extreme events on sponge populations and their ability to adapt to changing temperatures.
