Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, United States of America.
Department of Marine Sciences, University of Puerto Rico Mayagüez, Mayagüez, Puerto Rico, United States of America.
PLoS One. 2021 Dec 29;16(12):e0261210. doi: 10.1371/journal.pone.0261210. eCollection 2021.
Salinity normalization of total alkalinity (TA) and dissolved inorganic carbon (DIC) data is commonly used to account for conservative mixing processes when inferring net metabolic modification of seawater by coral reefs. Salinity (S), TA, and DIC can be accurately and precisely measured, but salinity normalization of TA (nTA) and DIC (nDIC) can generate considerable and unrecognized uncertainties in coral reef metabolic rate estimates. While salinity normalization errors apply to nTA, nDIC, and other ions of interest in coral reefs, here, we focus on nTA due to its application as a proxy for net coral reef calcification and the importance for reefs to maintain calcium carbonate production under environmental change. We used global datasets of coral reef TA, S, and modeled groundwater discharge to assess the effect of different volumetric ratios of multiple freshwater TA inputs (i.e., groundwater, river, surface runoff, and precipitation) on nTA. Coral reef freshwater endmember TA ranged from -2 up to 3032 μmol/kg in hypothetical reef locations with freshwater inputs dominated by riverine, surface runoff, or precipitation mixing with groundwater. The upper bound of freshwater TA in these scenarios can result in an uncertainty in reef TA of up to 90 μmol/kg per unit S normalization if the freshwater endmember is erroneously assumed to have 0 μmol/kg alkalinity. The uncertainty associated with S normalization can, under some circumstances, even shift the interpretation of whether reefs are net calcifying to net dissolving, or vice versa. Moreover, the choice of reference salinity for normalization implicitly makes assumptions about whether biogeochemical processes occur before or after mixing between different water masses, which can add uncertainties of ±1.4% nTA per unit S normalization. Additional considerations in identifying potential freshwater sources of TA and their relative volumetric impact on seawater are required to reduce uncertainties associated with S normalization of coral reef carbonate chemistry data in some environments. However, at a minimum, researchers should minimize the range of salinities over which the normalization is applied, precisely measure salinity, and normalize TA values to a carefully selected reference salinity that takes local factors into account.
总碱度（TA）和溶解无机碳（DIC）数据的盐度归一化通常用于解释珊瑚礁对海水净代谢修饰时保守混合过程。盐度（S）、TA 和 DIC 可以准确而精确地测量，但 TA（nTA）和 DIC（nDIC）的盐度归一化会在珊瑚礁代谢率估算中产生相当大且未被认识到的不确定性。虽然盐度归一化误差适用于 nTA、nDIC 和珊瑚礁中其他感兴趣的离子，但在这里，我们主要关注 nTA，因为它被用作珊瑚礁净钙化的替代物，以及珊瑚礁在环境变化下维持碳酸钙生产的重要性。我们使用全球珊瑚礁 TA、S 和模拟地下水排泄数据集来评估多种淡水 TA 输入（即地下水、河流、地表径流和降水）的不同体积比对 nTA 的影响。在以河流、地表径流或降水与地下水混合为主的假设珊瑚礁位置，珊瑚礁淡水端元 TA 范围从-2 到 3032 μmol/kg。如果错误地假设淡水端元 TA 为 0 μmol/kg，则这些情况下淡水 TA 的上限可能导致珊瑚礁 TA 的不确定性高达每单位 S 归一化 90 μmol/kg。在某些情况下，S 归一化的不确定性甚至可以改变对珊瑚礁是否净钙化还是净溶解的解释。此外，归一化参考盐度的选择隐含地假定生物地球化学过程是在不同水体混合之前还是之后发生，这可能会增加每单位 S 归一化 nTA 不确定性±1.4%。在一些环境中，需要对 TA 的潜在淡水来源及其对海水的相对体积影响进行更多考虑，以降低珊瑚礁碳酸盐化学数据的 S 归一化不确定性。但是，至少研究人员应该尽量减少归一化应用的盐度范围，精确测量盐度，并将 TA 值归一化为仔细选择的参考盐度，以考虑当地因素。
