Maintaining pH stability in Recirculating Aquaculture Systems (RAS) is essential, as it has a direct impact on the toxicity of ammonia, CO₂, and other metabolite compounds, as well as the efficiency of critical processes like nitrification and CO₂ removal. Alkalinity is necessary for pH stability and for the inorganic carbon supply to nitrifying bacteria. The relationship between alkalinity, nitrification rate, pH, and CO₂ concentration emphasize the need to determine the optimal alkalinity levels in RAS. However, the consequences of operating RAS under non-optimal alkalinity levels, especially in marine systems, are not well understood. This study aims to investigate the influence of alkalinity on nitrification rates and CO₂ removal efficiency in RAS with brackish water, with a specific focus on ammonia, total inorganic carbon (TIC), and CO₂ levels, as well as their removal rates. To accomplish this, the study was conducted in a pilot-scale Atlantic salmon post-smolt RAS. The RAS was operated with a moving bed biofilter. Alkalinity treatments of nominal 70, 100, and 200 mg/L as CaCO₃ were maintained by supplying sodium bicarbonate and NaOH. Each of the three treatments was operated on for 2 weeks and replicated three times. During every two weeks of treatment, five water samples on days 3, 5, 8, 10, and 15 were collected. The system received 12-hour feeding of 1.5 kg/day, with continuous lighting and a water temperature and salinity of 14.2 ± 0.16^◦C and 15.4 ± 0.53‰, respectively. The results indicated that high alkalinity levels (100 and 200 mg/L as CaCO₃) resulted in significantly lower CO₂ concentrations after the water treatment, due to higher pH. The highest alkalinity treatment (200 mg/L) showed lower CO₂ removal efficiency. Furthermore, high alkalinity levels (above 100 mg/L as CaCO₃) related to reduced total ammonia nitrogen concentrations and increased volumetric nitrite removal rate. Elevated alkalinity levels can also help prevent rapid pH fluctuations, benefiting fish health and production.). To summarize, for Atlantic salmon post-smolt RAS, operated in brackish water, maintaining alkalinity slightly above 100 mg/L as CaCO₃ appears to be the optimal choice in terms of TAN and CO₂ concentration and rate of removal or consumption.

维持循环水产养殖系统 (RAS) 中的 pH 稳定性至关重要，因为它直接影响氨、CO ₂和其他代谢化合物的毒性，以及硝化和 CO ₂去除等关键过程的效率。碱度对于 pH 稳定和硝化细菌的无机碳供应是必需的。_{碱度、硝化速率、pH 值和 CO 2}浓度之间的关系强调需要确定 RAS 中的最佳碱度水平。然而，在非最佳碱度水平下运行 RAS 的后果，尤其是在海洋系统中，尚不清楚。_{本研究旨在研究碱度对苦咸水RAS中硝化速率和CO 2}去除效率的影响，特别关注氨、总无机碳(TIC)和CO ₂水平及其去除率。为了实现这一目标，该研究在大西洋鲑鱼幼崽后 RAS 中进行了中试规模。RAS 使用移动床生物过滤器进行操作。通过供应碳酸氢钠和NaOH来维持以CaCO ₃计的标称70、100和200mg  /L的碱度处理。三种治疗均持续2周，重复3次。每两周处理期间，在第 3、5、8、10 和 15 天收集 5 个水样。该系统每天12小时喂食1.5公斤，持续照明，水温和盐度分别为14.2±0.16 ^° C和15.4±0.53‰。结果表明，由于 pH 值较高，高碱度水平（以 CaCO ₃计为 100 和 200 mg/L ）导致水处理后CO _{2浓度显着降低。}最高碱度处理(200 mg/L)显示出较低的CO ₂去除效率。此外，高碱度水平（以 CaCO ₃计高于 100 mg/L ）与总氨氮浓度降低和亚硝酸盐体积去除率增加相关。提高碱度水平还有助于防止 pH 值快速波动，有利于鱼类健康和生产。）。总而言之，对于在咸水中操作的大西洋鲑鱼幼崽后RAS，就TAN和CO ₂浓度以及去除或消耗率而言，将CaCO _{3 的}碱度维持在略高于100 mg/L似乎是最佳选择。