Diffusion Boundary Layers Ameliorate the Negative Effects of Ocean Acidification on the Temperate Coralline Macroalga Arthrocardia corymbosa

Abstract

Anthropogenically-modulated reductions in pH, termed ocean acidification, could pose a major threat to the physiological performance, stocks, and biodiversity of calcifiers and may devalue their ecosystem services. Recent debate has focussed on the need to develop approaches to arrest the potential negative impacts of ocean acidification on ecosystems dominated by calcareous organisms. In this study, we demonstrate the role of a discrete (i.e. diffusion) boundary layer (DBL), formed at the surface of some calcifying species under slow flows, in buffering them from the corrosive effects of low pH seawater. The coralline macroalga Arthrocardia corymbosa was grown in a multifactorial experiment with two mean pH levels (8.05 ‘ambient’ and 7.65 a worst case ‘ocean acidification’ scenario projected for 2100), each with two levels of seawater flow (fast and slow, i.e. DBL thin or thick). Coralline algae grown under slow flows with thick DBLs (i.e., unstirred with regular replenishment of seawater to their surface) maintained net growth and calcification at pH 7.65 whereas those in higher flows with thin DBLs had net dissolution. Growth under ambient seawater pH (8.05) was not significantly different in thin and thick DBL treatments. No other measured diagnostic (recruit sizes and numbers, photosynthetic metrics, %C, %N, %MgCO3) responded to the effects of reduced seawater pH. Thus, flow conditions that promote the formation of thick DBLs, may enhance the subsistence of calcifiers by creating localised hydrodynamic conditions where metabolic activity ameliorates the negative impacts of ocean acidification.

Evidence for water-mediated mechanisms in coral–algal interactions

“…When testing for the effect of flow speed on the coral -turf interactions, we found that increased flow reduces DBL thickness and oxygen extremes at the coral -algae interface. Increased flow commonly results in thinner DBLs [56,59]. Previous studies showed that high flow reduces algal competitiveness over corals and bacterial concentrations at the coral -algal interaction zone [60,61].…”

Ocean acidification accelerates dissolution of experimental coral reef communities

“…The ability to create ecologically relevant flow conditions in the flumes is likely to be especially important for establishing ecological relevance, as flow is critical in modulating mass transfer and metabolism of coral reef organisms (Atkinson and Gilmer, 1992;Carpenter and Williams, 2007;Comeau et al, 2014c). In the case of stony corals, for example, high flow speeds are suspected to enhance coral calcification by favoring proton export from coral tissue through boundary layers (Jokiel, 2011;Jokiel et al, 2014); for coralline algae, high flow speeds might increase sensitivity to OA by reducing the capacity to maintain high pH in the diffusion boundary layer adjacent to the algal thallus (Cornwall et al, 2013(Cornwall et al, , 2014.…”

The responses of brown macroalgae to environmental change from local to global scales: direct versus ecologically mediated effects

“…Biogenic fluctuations (e.g. pH, O 2 ) at different temporal and spatial scale will produce alternating phases of adverse versus benign conditions which may buffer or amplify the impact of a stressor (Cornwall et al 2014).…”