A study published in Global Change Biology has documented the largest sponge bleaching event ever recorded. The research, led by Professor James Bell of Victoria University of Wellington, reveals that a 259-day marine heatwave in 2022 caused mass bleaching of over 50 million Cymbastella lamellata sponges along 1,000 kilometers of New Zealand's Fiordland coastline.
The intensity of the heatwave, which pushed water temperatures up to 4.4°C above average, turned the normally chocolate-brown sponges bright white. Follow-up surveys nearly a year later showed that almost 50% of the sponges at sampling sites had perished, marking this as not only the largest bleaching event but also the most significant sponge mortality event on record.
The Cymbastella lamellata sponges play a crucial role in Fiordland's marine ecosystem, providing habitat and recycling nutrients that support marine life. The research team discovered that these sponges are key contributors to dissolved organic carbon production in the water column, potentially fueling the microbial food chain and supporting higher-level organisms.
The mass mortality event is estimated to have reduced this important fuel source by approximately 850 tons. However, the study also revealed a potential silver lining. Some sponges showed the ability to recover from bleaching, with changes detected in their symbiotic microbial communities. These shifts suggest a rapid evolutionary response to warming waters, offering hope for the species' ability to adapt to changing conditions.
The research team noted that drought conditions and unusually low rainfall coincided with the marine heatwave, potentially allowing more light to penetrate the typically dark Fiordland waters. This combination of factors may have exacerbated the bleaching and mortality rates of the sponges.
Dr. Robert Smith, a physical oceanographer from the University of Otago involved in the study, emphasized the importance of considering the impacts of compounding extreme events on marine ecosystems. As climate change continues to intensify, understanding how coastal temperature and freshwater extremes interact and affect marine life will be crucial for developing effective conservation and management strategies.
