Seawater carbonate chemistry and calcification rate, calcifying fluid pH, calcifying fluid DIC, photosynthetic rates, metabolic alteration of pH in the DBL of corals and coralline algae

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.

扩散边界层（DBL）——即主体海水与生物外表面之间的离散海水层——中的pH自然变异性，可能是决定珊瑚和珊瑚藻对海洋酸化（OA）响应的关键因素。本研究将两种形态不同的珊瑚和一种珊瑚藻置于包含12个水槽的实验系统中，在两种不同的流速、pH和光照强度条件下培养27周。我们结合地球化学指标、生理学测量及微电极测量，评估这些处理如何影响DBL中的环境条件及生物对OA的响应。总体而言，低流速并未缓解低pH的负面影响，因此未为OA提供避难所。流速具有物种特异性效应，对两种物种的钙化作用产生积极影响。仅在低光照条件下，两种珊瑚的钙化液（pHcf）pH值会因低流速而降低。对于能够显著改变DBL pH的两个物种，其pHcf受DBL pH的显著影响。珊瑚在低pH条件下、珊瑚藻在低流速条件下，钙化液中的溶解无机碳（DICcf）含量最高；而钙化液的饱和状态及其替代指标（FWHM）通常不受处理影响。本研究表明，对于亚热带珊瑚藻，OA效应在低光照与低流速组合的生境中表现最为严重；对于某些珊瑚，低流速生境中的OA效应也最为显著。结合现有文献，这些发现进一步证实OA效应具有高度情境依赖性，且在不同生境间及不同物种组成下差异显著。