Flow and Coral Morphology Control Coral Surface pH: Implications for the Effects of Ocean Acidification

Neil C S Chan,Michael Kühl,Daniel Wangpraseurt,Sean R Connolly

doi:10.3389/fmars.2016.00010

Abstract

The future impact of ocean acidification (OA) on corals is disputed in part because mathematical models used to predict these impacts do not seem to capture, or offer a framework to adequately explain, the substantial variability in acidification effects observed in empirical studies. The build-up of a diffusive boundary layer (DBL), wherein solute transport is controlled by diffusion, can lead to pronounced differences between the bulk seawater pH, and the actual pH experienced by the organism, a factor rarely considered in mathematical modelling of ocean acidification effects on corals. In the present study, we developed a simple diffusion-reaction-uptake model that was experimentally parameterized based on direct microsensor measurements of coral tissue pH and O2 within the DBL of a branching and a massive coral. The model accurately predicts tissue surface pH for different coral morphologies and under different flow velocities as a function of ambient pH. We show that, for all cases, tissue surface pH is elevated at lower flows, and thus thicker DBLs. The relative effects of OA on coral surface pH was controlled by flow and we show that under low flow velocities tissue surface pH under OA conditions (pHSWS = 7.8) can be equal to the pH under normal conditions (pHSWS = 8.2). We conclude that OA effects on corals in nature will be complex as the degree to which they are controlled by flow appears to be species specific.

Highlights

Besides harboring a substantial proportion of the ocean’s biodiversity, corals reefs are estimated to present an annual net global economic value of about $30 billion
diffusive boundary layer (DBL) Thickness and Net Photosynthesis The DBL thickness exhibited a negative power law relationship with flow for both Favites and P. damicornis, i.e., it was thick at low flow and thinned out as flow increased (Figure 1A, see Figure S1 in Data Sheet 1 for examples of O2 microprofiles, see Datasheet 2 for raw O2 microsensor data: depth from tissue surface against percentage air saturation)
It is known that a thickening of the DBL, via reduced flow velocity, causes an elevation of tissue surface pH which reduces the negative effects of ocean acidification (Cornwall et al, 2013, 2014)

Summary

Introduction

Besides harboring a substantial proportion of the ocean’s biodiversity, corals reefs are estimated to present an annual net global economic value of about $30 billion. About 33% of this enhanced atmospheric CO2 dissolves into the oceans, causing a shift in the seawater carbonate system that results in a decrease in both oceanic pH and the abundance of carbonate ions that corals use to build their skeletons. These effects of enhanced dissolution of CO2 into the oceans are commonly termed. Flow Controls Coral Surface pH ocean acidification (OA) and have been identified as a global threat for coral reefs, predicted to slow coral calcification to a point where the rate of reef erosion exceeds the rate of skeletal accretion (Silverman et al, 2009; van Hooidonk et al, 2013). Considerable controversy remains about the actual impact of ocean acidification on coral calcification (Pandolfi et al, 2011; Chan and Connolly, 2013)

Methods

Results

Conclusion