Environmental Physics, Limnological Institute, University of Konstanz, Mainaustr. 252, D-78464 Konstanz, Germany.
Environmental Physics, Limnological Institute, University of Konstanz, Mainaustr. 252, D-78464 Konstanz, Germany.
Water Res. 2023 Sep 1;243:120423. doi: 10.1016/j.watres.2023.120423. Epub 2023 Jul 28.
Lakes and reservoirs are important sources/sinks of atmospheric CO. Primary production and respiration transforming inorganic to organic carbon and vice versa alter CO concentrations in the surface waters and thus affect CO emissions. Here we investigate the link between net-production (NEP) and CO concentrations and emissions at high temporal resolution over more than two months in a German pump storage reservoir. Continuous in-situ pH measurements in combination with few alkalinity measurements provided concentrations of CO and dissolved inorganic carbon (DIC) at high temporal resolution over more than 75 days. Time series of metabolic rates of carbon were determined with an open-water diel pH technique, which utilizes the diel changes in DIC obtained from the observed diel changes in pH and data on alkalinity. During the measuring period, average NEP was positive and CO concentrations were typically substantially under-saturated. On average, the reservoir acted as a sink for CO, whereby CO uptake was 39% larger in the evening than in the morning. Only few consecutive days with negative NEP were sufficient to turn the reservoir temporally into a source of CO. Therefore, the average CO uptake determined from continuous data can be 80% larger to 30% smaller than estimates of average uptake based on bi-weekly data. Daily mean NEP explained only 9% and 4% of the variance of daily mean DIC and CO. Note that NEP is proportional to the time derivative of DIC and therefore not expected to correlate well with DIC in general. Because CO changes nonlinearly with DIC, NEP explains less variance of CO than of DIC. Numerical experiments confirmed the arguments above and revealed that at positive average NEP the total CO uptake over several weeks is not well predicted by average NEP but depends on the detailed temporal pattern of NEP. However, if average NEP is negative, average NEP may be a good predictor of total CO emissions. Similar conclusions apply for high and low alkalinity waters, but uptake rates and temporal variability of CO emissions are smaller in high than in low alkalinity waters. Assessment of the link between NEP and CO emissions requires differentiation between lakes with different alkalinity and, because of the non-linear relationship between NEP and CO, strongly benefits from data with high temporal resolution especially during time-periods with positive net-production.
湖泊和水库是大气 CO 的重要源/汇。初级生产和呼吸作用将无机碳转化为有机碳，反之亦然，这会改变地表水的 CO 浓度，从而影响 CO 的排放。在这里，我们研究了德国一座抽水蓄能水库在超过两个月的时间内以高时间分辨率进行的净初级生产力（NEP）与 CO 浓度和排放之间的联系。连续原位 pH 测量结合少数碱度测量，在超过 75 天的时间内提供了 CO 和溶解无机碳（DIC）的高时间分辨率浓度。利用开放水域昼夜 pH 技术确定了碳代谢率的时间序列，该技术利用从观察到的 pH 昼夜变化和碱度数据中获得的昼夜 DIC 变化。在测量期间，平均 NEP 为正值，CO 浓度通常明显过饱和。平均而言，水库是 CO 的汇，其中 CO 的吸收在晚上比早上大 39%。只有少数连续几天的负 NEP 就足以使水库暂时成为 CO 的源。因此，从连续数据确定的平均 CO 吸收量可能比基于双周数据的平均吸收量大 80%到小 30%。日平均 NEP 仅解释了日平均 DIC 和 CO 日方差的 9%和 4%。请注意，NEP 与 DIC 的时间导数成正比，因此一般不应与 DIC 很好相关。由于 CO 与 DIC 呈非线性变化，因此 NEP 解释的 CO 方差小于 DIC 的方差。数值实验证实了上述论点，并表明在正平均 NEP 的情况下，数周的总 CO 吸收量不能很好地由平均 NEP 预测，而取决于 NEP 的详细时间模式。但是，如果平均 NEP 为负，则平均 NEP 可能是 CO 总排放量的良好预测因子。对于高碱度和低碱度水，类似的结论也适用，但高碱度水中的 CO 吸收速率和排放的时间变异性较小。评估 NEP 与 CO 排放之间的联系需要区分具有不同碱度的湖泊，并且由于 NEP 和 CO 之间的非线性关系，强烈受益于具有高时间分辨率的数据，特别是在净初级生产力为正的时期。
