Deterioration of an annual kelp Saccharina japonica forest and its effects on dominant herbivores, sea urchin Mesocentrotus nudus and abalone Haliotis discus hannai, in northeast Japan

A marine forest of the annual kelp Saccharina japonica was monitored at three depth sites (6, 9 and 12 m) along a 200-meter-long transect in the Pacific coast of northeastern Japan from 2014 to 2021, during which the abundance of the kelp plummeted, and the forest shifted to a crustose coralline bed. Colder seawater temperature in the winter/spring (February and March), which is regulated by the extension of the cold Oyashio Current, plays an important role in establishing the kelp forest through reducing the grazing of herbivores during the critical period of kelp germination and early growth. The Oyashio Current reached the region in 2014 and 2015 but not from 2016 to 2021, making a fundamental change in the kelp forest. Macroalgal abundance was strikingly different between these two periods. Biomass of S. japonica reached up to 38.6-45.4 kg-ww m-2 in the first two years, whereas macroalgal biomass was less than 3.2 kg-ww m-2 in the latter 6 years except for in 2016 when the biomass reached 9.0 kg-ww m-2 at the 6 m site. Gonad and somatic growth of sea urchin Mesocentrotus nudus declined during the kelp deforestation, while the density of the sea urchin moderately increased in 2017. Similarly, the density of abalone Haliotis discus hannai plummeted by about 90% and the mean shell length decreased from 83 mm in 2014 to 31 mm in 2020. Under the ocean warming scheme, ecosystems depending on the primary production of S. japonica are expected to be altered in the future, and thus urging the need of environmental monitoring.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

A long-term monitoring study conducted from 2014 to 2021 revealed a significant decrease in abundance of the annual kelp Saccharina japonica, leading to a transition from a kelp forest to a crustose coralline bed along the northeastern coast of Japan. The decline in kelp forest was attributed to colder seawater temperature in the winter/spring, which reduced herbivore grazing during kelp germination and early growth. Changes in the Oyashio Current further contributed to the transformation of the ecosystem. This study highlights the importance of environmental monitoring and the potential impacts of ocean warming on kelp-dependent ecosystems.