Abstract
Natural variability in pH in the diffusive boundary layer (DBL), the discrete layer of seawater between bulk seawater and the outer surface of organisms, could be an important factor determining the response of corals and coralline algae to ocean acidification (OA). Here, two corals with different morphologies and one coralline alga were maintained under two different regimes of flow velocities, pH, and light intensities in a 12 flumes experimental system for a period of 27 weeks. We used a combination of geochemical proxies, physiological and micro-probe measurements to assess how these treatments affected the conditions in the DBL and the response of organisms to OA. Overall, low flow velocity did not ameliorate the negative effect of low pH and therefore did not provide a refugia from OA. Flow velocity had species-specific effects with positive effects on calcification for two species. pH in the calcifying fluid (pHcf) was reduced by low flow in both corals at low light only. pHcf was significantly impacted by pH in the DBL for the two species capable of significantly modifying pH in the DBL. The dissolved inorganic carbon in the calcifying fluid (DICcf) was highest under low pH for the corals and low flow for the coralline, while the saturation state in the calcifying fluid and its proxy (FWHM) were generally not affected by the treatments. This study therefore demonstrates that the effects of OA will manifest most severely in a combination of lower light and lower flow habitats for sub-tropical coralline algae. These effects will also be greatest in lower flow habitats for some corals. Together with existing literature, these findings reinforce that the effects of OA are highly context dependent, and will differ greatly between habitats, and depending on species composition.
Highlights
Ocean acidification (OA) is a major threat to many marine calcifying species, acting to reduce the calcification of ecologically important species such as reef-forming corals and coralline algae[1]
There has been significant discussion of whether such diurnal changes in diffusive boundary layer (DBL) pH may ameliorate the impacts of ocean acidification (OA), whereby pH during the day and mean pH encountered by the organism are increased relative to the overlying seawater[16]
Reduced export of protons from the calcifying fluid to the external seawater is another possible negative ramification of thicker DBLs that could alter responses to OA, following the proton flux hypothesis[13]. This would manifest by reducing pH in the calcifying fluid, with the effects expected to be most pronounced under both OA and slow flow
Summary
Ocean acidification (OA) is a major threat to many marine calcifying species, acting to reduce the calcification of ecologically important species such as reef-forming corals and coralline algae[1]. One potentially important and still poorly studied controller of the direction and magnitude of responses is the seawater velocity that resident organisms are exposed to[4,5] This is expected to be most important at the discrete boundary layer between marine organisms and ambient seawater, where flow is reduced to the point such that the movement of dissolved substances between the bulk seawater and the surface of the organism is dominated by molecular diffusion[6]. Reduced export of protons from the calcifying fluid to the external seawater is another possible negative ramification of thicker DBLs that could alter responses to OA, following the proton flux hypothesis[13] This would manifest by reducing pH in the calcifying fluid (pHcf), with the effects expected to be most pronounced under both OA and slow flow. Neither Cornwall et al.[5] nor Comeau et al.[4] measured pHcf, nor did Comeau et al.[4] measure pH in the DBL of the different species in their communities
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