Contrasting Changes in Diel Variations of Net Community Calcification Support That Carbonate Dissolution Can Be More Sensitive to Ocean Acidification Than Coral Calcification

Wen-Chen Chou,Ying-Hsuan Chen,Pi-Jen Liu,Wei-Jen Huang

doi:10.3389/fmars.2020.00003

Abstract

Previous studies have found that calcification in coral reefs is generally stronger during the day, whereas dissolution is prevalent at night. On the basis of these contrasting patterns, the diel variations of net community calcification (NCC) were monitored to examine the relative sensitivity of CaCO3 production (calcification) and dissolution in coral reefs to ocean acidification (OA), using two mesocosms that replicated a typical subtropical coral reef ecosystem in southern Taiwan. The results revealed that the daytime NCC remained unchanged, whereas the nighttime NCC decreased between the control (ambient) and treatment (OA) conditions, suggesting that carbonate dissolution could be more sensitive to OA than coral calcification. The average sensitivity of the integrated daily NCC to changes in the seawater saturation state (a) was estimated to be a reduction of 54% in NCC per unit change in a, which is consistent with the global average. In summary, our results support the prevailing anticipation that OA would lead to a reduction in the overall accretion of coral reef ecosystems. However, increased CaCO3 dissolution rather than decreased coral calcification could be the dominant driving force responsible for this OA-induced reduction in NCC.

Highlights

Anthropogenic activities, such as the burning of fossil fuels, land-use changes, and cement production, have led to a rapid rise in atmospheric carbon dioxide (CO2) levels, which have increased from 280 ppm in preindustrial times to a present-day level of 400 ppm (Le Quere et al, 2016)
In order to verify if carbonate dissolution could be more sensitive to ocean acidification (OA) than biological calcification, we investigated the effects of OA on the diel variations of net community calcification (NCC) in a typical subtropical coral reef community, which was replicated and incubated in mesocosms over 5 weeks
Most previous studies investigating the effect of OA on coral reef ecosystems have shown a consistent decrease in NCC

Summary

Introduction

Anthropogenic activities, such as the burning of fossil fuels, land-use changes, and cement production, have led to a rapid rise in atmospheric carbon dioxide (CO2) levels, which have increased from 280 ppm in preindustrial times to a present-day level of 400 ppm (Le Quere et al, 2016). The CaCO3 accretion of a coral reef is determined by the physical balance between CaCO3 transport onto and off the reef, and the chemical balance between CaCO3 production (calcification) and dissolution, which is referred to as net community calcification (i.e., NCC = calcification – CaCO3 dissolution). The impact of OA on coral reef NCC, and chemical accretion, is dependent on the effect of OA on CaCO3 dissolution, which has received less attention than the effect on calcification. Several recent studies have even shown evidence that carbonate dissolution may be more sensitive to OA than coral calcification (Cyronak and Eyre, 2016; Eyre et al, 2018). Understanding the effect of OA on CaCO3 dissolution could be as important as understanding its effect on biogenic calcification

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Conclusion