Intercomparison of four methods to estimate coral calcification under various environmental conditions

Miguel Gómez Batista,Jean-Pierre Gattuso,Peter W Swarzenski,François Oberhänsli,Simon Pouil,Frédéric Gazeau,Marc Metian,Eric Tambutté,Carlos M Alonso Hernández

doi:10.5194/bg-17-887-2020

Abstract

Abstract. Coral reefs are constructed by calcifiers that precipitate calcium carbonate to build their shells or skeletons through the process of calcification. Accurately assessing coral calcification rates is crucial to determine the health of these ecosystems and their response to major environmental changes such as ocean warming and acidification. Several approaches have been used to assess rates of coral calcification, but there is a real need to compare these approaches in order to ascertain that high-quality and intercomparable results can be produced. Here, we assessed four methods (total alkalinity anomaly, calcium anomaly, 45Ca incorporation, and 13C incorporation) to determine coral calcification of the reef-building coral Stylophora pistillata. Given the importance of environmental conditions for this process, the study was performed under two starting pH levels (ambient: 8.05 and low: 7.2) and two light (light and dark) conditions. Under all conditions, calcification rates estimated using the alkalinity and calcium anomaly techniques as well as 45Ca incorporation were highly correlated. Such a strong correlation between the alkalinity anomaly and 45Ca incorporation techniques has not been observed in previous studies and most probably results from improvements described in the present paper. The only method which provided calcification rates significantly different from the other three techniques was 13C incorporation. Calcification rates based on this method were consistently higher than those measured using the other techniques. Although reasons for these discrepancies remain unclear, the use of this technique for assessing calcification rates in corals is not recommended without further investigations.

Highlights

Calcification is the fundamental biological process by which organisms precipitate calcium carbonate
Evolution of pH in control beakers showed that the observed increase in beakers with corals was due to the additive effects of biological control and exchanges at the interface in the light, and mostly due to CO2 exchange with air during the much longer incubations performed in the dark
Calcification rates were lower in dark than in light conditions

Summary

Introduction

Calcification is the fundamental biological process by which organisms precipitate calcium carbonate. The ocean has absorbed large amounts of anthropogenic CO2 since the start of the industrial revolution and is currently sequestering about 22 % of CO2 emissions (average 2008– 2017; Le Quéré et al, 2018). This massive input of CO2 in the ocean impacts seawater chemistry with a decrease in seawater pH and carbonate ion concentrations [CO23−] and an increase in CO2 and bicarbonate concentrations [HCO−3 ]. These fundamental changes to the carbonate system are referred to as “ocean acidification” (OA; Gattuso and Hansson, 2011). Models project that the average surface water pH will drop by 0.06 to 0.32 pH units by the end of the century (IPCC, 2014)

Methods

Results

Conclusion