Chemical changes in the diffusive boundary layer (DBL) generated by photosynthesising macroalgae are expected to play an important role in modulating the effects of ocean acidification (OA), but little is known about the effects on early life stages of marine invertebrates in modified DBLs. Larvae that settle to macroalgal surfaces and remain within the DBL will experience pH conditions markedly different from the bulk seawater. We investigated the interactive effects of seawater pH and DBL thickness on settlement and early post-settlement growth of the sea urchin Pseudechinus huttoni, testing whether coralline-algal DBLs act as an environmental buffer to OA. DBL thickness and pH levels (estimated from well-established relationships with oxygen concentration) above the crustose coralline algal surfaces varied with light availability (with photosynthesis increasing pH to as high as pH 9.0 and respiration reducing pH to as low as pH 7.4 under light and dark conditions, respectively), independent of bulk seawater pH (7.5, 7.7, and 8.1). Settlement success of P. huttoni increased over time for all treatments, irrespective of estimated pH in the DBL. Juvenile test growth was similar in all DBL manipulations, showing resilience to variable and low seawater pH. Spine development, however, displayed greater variance with spine growth being negatively affected by reduced seawater pH in the DBL only in the dark treatments. Scanning electron microscopy revealed no observable differences in structural integrity or morphology of the sea urchin spines among pH treatments. Our results suggest that early juvenile stages of P. huttoni are well adapted to variable pH regimes in the DBL of macroalgae across a range of bulk seawater pH treatments.

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扩散边界层和海洋酸化：对海胆定居和生长的影响

光合作用大型藻类产生的扩散边界层 (DBL) 中的化学变化有望在调节海洋酸化 (OA) 的影响中发挥重要作用，但对改性 DBL 中海洋无脊椎动物早期生命阶段的影响知之甚少。定居在大型藻类表面并留在 DBL 内的幼虫将经历与大量海水明显不同的 pH 条件。我们研究了海水 pH 值和 DBL 厚度对海胆 Pseudechinus huttoni 沉降和沉降后早期生长的交互影响，测试珊瑚藻 DBL 是否作为 OA 的环境缓冲剂。壳状珊瑚藻表面上方的 DBL 厚度和 pH 值（根据与氧浓度的公认关系估计）随光照变化（光合作用将 pH 值提高至 9.0，呼吸作用将 pH 值降低至 7.4，在光照和暗条件），独立于大量海水 pH 值（7.5、7.7 和 8.1）。无论 DBL 中的估计 pH 值如何，所有处理的 P. huttoni 的沉降成功率都随着时间的推移而增加。在所有 DBL 操作中，幼鱼测试的生长情况相似，显示出对可变和低海水 pH 值的适应能力。然而，脊柱发育显示出更大的差异，脊柱生长仅在黑暗处理中受到 DBL 中海水 pH 值降低的负面影响。扫描电子显微镜显示，在不同 pH 值处理中，海胆刺的结构完整性或形态没有明显差异。我们的结果表明，P. huttoni 的早期幼年阶段很好地适应了大量海水 pH 值处理范围内大型藻类 DBL 中的可变 pH 值方案。