Continuous Monitoring and Future Projection of Ocean Warming, Acidification, and Deoxygenation on the Subarctic Coast of Hokkaido, Japan

Masahiko Fujii,Masahide Wakita,Akitomo Yamamoto,Shintaro Takao,Yamato Fujita,Tsuneo Ono,Tomoo Akamatsu,Takuto Yamaka

doi:10.3389/fmars.2021.590020

Abstract

As the ocean absorbs excessive anthropogenic CO2 and ocean acidification proceeds, it is thought to be harder for marine calcifying organisms, such as shellfish, to form their skeletons and shells made of calcium carbonate. Recent studies have suggested that various marine organisms, both calcifiers and non-calcifiers, will be affected adversely by ocean warming and deoxygenation. However, regardless of their effects on calcifiers, the spatiotemporal variability of parameters affecting ocean acidification and deoxygenation has not been elucidated in the subarctic coasts of Japan. This study conducted the first continuous monitoring and future projection of physical and biogeochemical parameters of the subarctic coast of Hokkaido, Japan. Our results show that the seasonal change in biogeochemical parameters, with higher pH and dissolved oxygen (DO) concentration in winter than in summer, was primarily regulated by water temperature. The daily fluctuations, which were higher in the daytime than at night, were mainly affected by daytime photosynthesis by primary producers and respiration by marine organisms at night. Our projected results suggest that, without ambitious commitment to reducing CO2 and other greenhouse gas emissions, such as by following the Paris Agreement, the impact of ocean warming and acidification on calcifiers along subarctic coasts will become serious, exceeding the critical level of high temperature for 3 months in summer and being close to the critical level of low saturation state of calcium carbonate for 2 months in mid-winter, respectively, by the end of this century. The impact of deoxygenation might often be prominent assuming that the daily fluctuation in DO concentration in the future is similar to that at present. The results also suggest the importance of adaptation strategies by local coastal industries, especially fisheries, such as modifying aquaculture styles.

Highlights

Global change-driven global ocean warming, acidification, and deoxygenation are all thought to be caused primarily by excessive anthropogenic CO2 emissions
Because around 60% of the shellfish and half of the calcifiers in Japan are caught in subarctic seas in which the arag is relatively low, ocean acidification is of great concern and the social demand for adaptive fisheries, such as changing the aquaculture style as mentioned in section “Suggestion for future adaptation,” in response to global change is high
This study was the first multi-year in-situ monitoring study of a subarctic coast and produced future projections of physical and biogeochemical parameters related to global change-driven ocean warming, acidification, and deoxygenation

Summary

Introduction

Global change-driven global ocean warming, acidification, and deoxygenation are all thought to be caused primarily by excessive anthropogenic CO2 emissions. The global average oceanic pH is currently around 8.1 and is anticipated to decrease to 7.8 by the end of this century (e.g., Feely et al, 2008; Intergovernmental Panel on Climate Change (IPCC), 2014). Recent studies have suggested that ocean acidification will affect marine organisms that form shells or bodies made of calcium carbonate (calcifiers) because these chemicals are more dissolved or harder to generate in high-CO2, low-pH environments. The impact of ocean acidification on calcifiers has already been reported, such as the significant mortality of oyster (Crassostrea gigas) larvae (Feely et al, 2008; Barton et al, 2012) and damage to the carapaces of larval Dungeness crabs (Metacarcinus magister; Bednaršek et al, 2020) in the high-CO2 environment of the Pacific northwest coast of the United States of America. Similar to ocean warming, ocean acidification affects marine ecosystems widely

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Conclusion