Ocean warming and marine heatwaves are causing the collapse and tropicalisation of kelp forests in the Japanese Izu PeninsulaExtreme Events

Ben Harvey¹, Mariko Shiel², Shigeki Wada³, Takeo Hama ¹, Sylvain Agostini¹, Joshua Heitzman¹, Hiroki Takenaka¹, Guinther Mitushasi¹, Layla Iijima¹, Daniel Smale⁴, Nathan King⁴

¹University of Tsukuba, Japan - ²University of Plymouth, UK - ³Hiroshima University Japan - ⁴Marine Biological Association of the United Kingdom, UK

Ocean climate variability has intensified over the last century, increasing the frequency, intensity, and duration of extreme temperature events known as marine heatwaves. Regions with significant sea surface temperature variability are projected to warm faster than the global average, making them hotspots for marine heatwave activity. The Kuroshio Current, a western boundary current of the North Pacific subtropical gyre, is experiencing one of the highest long-term warming rates in the North Pacific. As a result, the northwest Pacific, including Japan, is anticipated to face disproportionate impacts from marine heatwaves in the coming decades. Our study investigates the ongoing tropicalisation of the kelp forest community in Shimoda, Japan, by examining the effects of long-term gradual warming, marine heatwaves driven by recent shifts in the Kuroshio Current, and the range expansion of tropical herbivorous fish into the region. By combining historical surveys and remote-sensing environmental data, we document a significant decline in kelp forests over decadal scales, culminating in the total loss of the kelp forest in the past few years. Controlled aquarium-based experiments further demonstrate that kelp exhibit physiological stress and reduced growth and survival under elevated temperatures (reached during recent summers and marine heatwave events). These extreme events often surpass the thermal tolerance of kelp, causing abrupt population declines, while recovery appears to be hindered by the physiological stress and herbivory from range-extending tropical herbivorous fish. This study highlights that gradual warming, marine heatwaves, and novel grazing interactions collectively shape the changes observed in temperate kelp forest ecosystems. These findings stress the importance of incorporating these factors into conservation and management strategies to safeguard these vital marine habitats in other locations.