Measuring coral calcification under ocean acidification: methodological considerations for the 45Ca-uptake and total alkalinity anomaly technique.

Stephanie Cohen,Thomas Krueger,Maoz Fine

doi:10.7717/peerj.3749

Abstract

As the oceans become less alkaline due to rising CO2 levels, deleterious consequences are expected for calcifying corals. Predicting how coral calcification will be affected by on-going ocean acidification (OA) requires an accurate assessment of CaCO3 deposition and an understanding of the relative importance that decreasing calcification and/or increasing dissolution play for the overall calcification budget of individual corals. Here, we assessed the compatibility of the 45Ca-uptake and total alkalinity (TA) anomaly techniques as measures of gross and net calcification (GC, NC), respectively, to determine coral calcification at pHT 8.1 and 7.5. Considering the differing buffering capacity of seawater at both pH values, we were also interested in how strongly coral calcification alters the seawater carbonate chemistry under prolonged incubation in sealed chambers, potentially interfering with physiological functioning. Our data indicate that NC estimates by TA are erroneously ∼5% and ∼21% higher than GC estimates from 45Ca for ambient and reduced pH, respectively. Considering also previous data, we show that the consistent discrepancy between both techniques across studies is not constant, but largely depends on the absolute value of CaCO3 deposition. Deriving rates of coral dissolution from the difference between NC and GC was not possible and we advocate a more direct approach for the future by simultaneously measuring skeletal calcium influx and efflux. Substantial changes in carbonate system parameters for incubation times beyond two hours in our experiment demonstrate the necessity to test and optimize experimental incubation setups when measuring coral calcification in closed systems, especially under OA conditions.

Highlights

Continual increases in atmospheric CO2-concentration has led to measurable changes in the carbonate chemistry of the oceanic system, summarized under the term ocean acidification (OA; Kleypas et al, 2006)
Considering only the calcification rates obtained from 2 h of incubation as physiologically useful data shows that absolute estimates of calcification differed between both methods depending on pH (Fig. 2 and Table 3)
Calcification estimates from the total alkalinity (TA) anomaly technique were on average 5% and 21% higher than the 45Ca estimate, for ambient and reduced pH, respectively

Summary

Introduction

Continual increases in atmospheric CO2-concentration has led to measurable changes in the carbonate chemistry of the oceanic system, summarized under the term ocean acidification (OA; Kleypas et al, 2006). Current worst case climate models project a further decrease of surface seawater pH by 0.3 − 0.4 pH units until the end of the 21st century (IPCC, 2013) These continuing shifts in seawater pH and aragonite saturation state affect many marine organisms that form biogenic aragonite; the modern day form of calcium carbonate in scleractinian reef-building corals (Anthony et al, 2008; McCulloch et al, 2012; Orr et al, 2005; Ries, Cohen & McCorkle, 2009). Deposited skeleton in a reef is subject to various forms of physical, chemical and biologically-mediated erosion, which causes the dissolution of reef sediments and skeleton This dissolution is part of the natural turnover of matter in the reef community and can offset 20–30% of reef calcification (Barnes, 1988; Silverman, Lazar & Erez, 2007). Recognizing the fundamental impact that OA has on calcifying organisms, there is a renewed interest for calcification studies of corals

Methods

Results

Conclusion