The modification of sediment nutrient fluxes by burrowing urchins

A mesocosm experiment was conducted to quantify the relationships between the presence and body size of two burrowing heart urchins (Brissopsis lyrifera and Echinocardium cordatum) and rates of sediment nutrient flux. Furthermore, the impact of seawater acidification on these relationships was determined during this 40-day exposure experiment. Using carbon dioxide (CO2) gas, seawater was acidified to pHNBS 7.6, 7.2 or 6.8. Control treatments were maintained in natural seawater (pH = 8.0). Under normocapnic conditions, burrowing urchins were seen to reduce the sediment uptake of nitrite or nitrate whilst enhancing the release of silicate and phosphate. In acidified (hypercapnic) treatments, the biological control of biogeochemical cycles by urchins was significantly affected, probably through the combined impacts of high CO2 on nitrifying bacteria, benthic algae and urchin behaviour. This study highlights the importance of considering biological interactions when predicting the consequences of seawater acidification on ecosystem function.

本研究开展了一项中宇宙实验（mesocosm experiment），以量化两种掘穴心形海胆（Brissopsis lyrifera与Echinocardium cordatum）的存在与否及其个体大小与沉积物营养盐通量速率之间的关联。此外，本项为期40天的暴露实验还探明了海水酸化对上述关联的影响。本实验通过二氧化碳（CO₂）气体将海水酸化至pH_NBS 7.6、7.2或6.8，对照组则维持于自然海水环境（pH=8.0）。在常碳（normocapnic）条件下，掘穴海胆可降低沉积物对亚硝酸盐或硝酸盐的吸收，同时促进硅酸盐与磷酸盐的释放。在酸化（hypercapnic）处理组中，海胆对生物地球化学循环的生物调控作用受到显著影响，这可能是高CO₂对硝化细菌、底栖藻类以及海胆行为产生联合作用的结果。本研究强调，在预测海水酸化对生态系统功能的影响时，纳入生物相互作用的考量至关重要。