Ocean acidification (OA) is likely to differentially affect the biology and physiology of calcifying and non-calcifying taxa, thereby potentially altering key ecological interactions (e.g., facilitation, competition, predation) in ways that are difficult to predict from single-species experiments. We used a two-factor experimental design to investigate how multispecies benthic assemblages in southern California kelp forests respond to OA and grazing by the purple sea urchin, Strongylocentrotus purpuratus. Settlement tiles accrued natural mixed assemblages of algae and invertebrates in a kelp forest off San Diego, CA for one year before being exposed to OA and grazing in a laboratory experiment for two months. Space occupying organisms were identified and pooled into six functional groups: calcified invertebrates, non-calcified invertebrates, calcified algae, fleshy algae, sediment, and bare space for subsequent analyses of community structure. Interestingly, communities that developed on separate tile racks were unique, despite being deployed close in space, and further changes in community structure in response to OA and grazing depended on this initial community state. On Rack 1, we found significant effects of both pCO₂ and grazing with elevated pCO₂ increasing cover of fleshy algae, but sea urchin grazers decreasing cover of fleshy algae. On Rack 2, we found a ~ 35% higher percent cover of sediment on tiles reared in ambient pCO₂ but observed ~27% higher cover of bare space in the high pCO₂ conditions. On Rack 3, we found an average of 45% lower percent cover of calcified sessile invertebrates at ambient pCO₂ than in high pCO₂ treatments on Rack 3. Net community calcification was 137% lower in elevated pCO₂ treatments. Kelp sporophyte densities on tiles without urchins were 74% higher than on tiles with urchins and kelp densities were highest in the elevated pCO₂ treatment. Urchin growth and grazing rates were 49% and 126% higher under ambient than high pCO₂ conditions. This study highlights consistent negative impacts of OA on community processes such as calcification and grazing rates, even though impacts on community structure were highly context-dependent.

海洋酸化（OA）可能会不同程度地影响钙化和非钙化类群的生物学和生理学，从而可能以难以从单物种实验中预测的方式改变关键的生态相互作用（例如，促进，竞争，捕食）。我们使用了两个因素的实验设计，研究了南加州海带森林中的多种底栖动物群如何响应紫色海胆（ Strongylocentrotus purpuratus）的OA和放牧。在加利福尼亚州圣地亚哥附近的一个海带森林中，沉降砖产生了藻类和无脊椎动物的自然混合组合，一年后才暴露于OA并在实验室实验中放牧了两个月。确定了占空间的生物并将其归为六个功能组：钙化无脊椎动物，非钙化无脊椎动物，钙化藻类，肉质藻类，沉积物和裸露空间，用于后续的群落结构分析。有趣的是，尽管在近距离的空间中部署，但在单独的瓷砖架子上开发的社区却是独特的，并且响应于OA和放牧，社区结构的进一步变化取决于此初始社区状态。在机架1上，我们发现p CO ₂和高p CO放牧均具有显着影响₂增加肉质藻类的覆盖率，但海胆放牧者减少肉质藻类的覆盖率。在机架2上，我们发现在环境p CO _2下饲养的瓷砖上沉积物的覆盖率高出〜35 ％，但在高p CO ₂条件下，裸露空间的覆盖率却高出约27％。上机架3，我们发现平均钙化固着在室温下无脊椎动物45％百分比盖的p CO ₂比在高p CO ₂处理对机架3净社区钙化为137％在升高的降低p CO ₂治疗。不带海胆的瓷砖的海藻孢子密度比带海胆的瓷砖的海藻孢子密度高74％，而海带密度在升高的p CO ₂处理中最高。与高p CO ₂条件相比，在环境下，海胆的生长和放牧率分别提高了49％和126％。这项研究强调了OA对社区过程（如钙化和放牧率）的持续负面影响，即使对社区结构的影响高度依赖于上下文。