Biogeochemical data from the HIPE project in Lakes Edward and George (East African Rift)

The geo-referenced and timestamped data-set consists of 7 files:

“db_cruise_CTD” contains the CTD profiles obtained during the cruises

“db_cruise_GHGs” contains CO2, CH4, N2O dissolved concentrations, chlorophyll-a concentrations, inorganic nutrients (NO3-, N2O-, NH4+, PO43-) and d13C-CH4 from the 4 cruises

“db_monitoring” contains CO2, CH4, N2O dissolved concentrations, chlorophyll-a concentrations, and POC from the monitoring at two stations (January 2017 to December 2019)

“db_uw” contains the continuous of CO2 and CH4 (plus EXO-II data) on 19/03/2019

“meteo_Mweya” contains the meteorological data acquired from June 2016 to March 2019

“db_monitoring_CTD” contains the CTD profiles from the deep station of the monitoring.

“mooring” contains the temperature data from a mooring at a station 10 m deep (March 2019)

Data were acquired in Lake Edward, Kazinga Channel and Lake George on four occasions (20/10-07/11/2016, 23/03-08/04/2017, 18/01-02/02/2018, 21/03-30/03/2019). From January 2017 to December 2019, a shallow station (3 m bottom depth) and a deeper station (22 m bottom depth) were regularly sampled, every 21 d in 2017 and 2018, and every 30 d in 2019. A mooring was deployed at a station at 10m bottom depth in Lake Edward (-0.2459°N 29.8635°E) equipped with RBR Solo temperature sensors at 6 depths from surface to 1m above the sediment (0.2, 1.0, 2.0, 5.0, 7.5 and 9.0 m depth) from 21/03/2019 (13:00 local time (LT)) to 23/03/2019 (13:50 LT).

Solar radiation, ultraviolet radiation, wind speed (cup anemometer), wind direction (wind vane), rain (mechanical rain collector), air temperature, barometric pressure data were acquired with a Davis Instruments weather station (Vantage Pro2 fitted with standard manufacturer sensors) in Mweya on top of a building of the Uganda Wildlife Authority, 4m above ground (-0.190384°N  29.899103°E) . Data were measured every 5 seconds, averaged and logged every 10 minutes.

During the March 2019 cruise, continuous measurements (1 min interval) of partial pressure of CO2 (pCO2) and of partial pressure of CH4 (pCH4) were made with an equilibrator designed for turbid waters consisting of a tube filled with glass marbles (Frankignoulle et al. 2001) coupled to a Los Gatos Research off-axis integrated cavity output spectroscopy analyzer (Ultraportable Greenhouse Gas Analyzer with extended range for CH4). In parallel water temperature, specific conductivity, pH, dissolved oxygen saturation level (%O2), turbidity, chlorophyll-a (Chl-a), and fluorescent dissolved organic matter (FDOM) were measured with an YSI EXO-II multi-parameter probe, position with a Garmin geographical position system (Map 60S) portable probe, and depth with a Humminbird Helix 5 echo-sounder. Surface water was pumped to the equilibrator and the multi-parameter probe (on deck) with a 12V-powered water pump (LVM105) attached to the side of the boat at a fixed depth of about 0.5 m depth.

Discrete sampling was done from the side of the boat with a 5.0 L Niskin bottle (General Oceanics). During the first cruise, vertical profiles of water temperature, specific conductivity, pH, %O2 and Chl-a were measured with a Hydrolab DS5 multi-parameter probe, while during the other three cruises and also during the monitoring, turbidity and FDOM were measured additionally with a YSI EXO-II multi-parameter probe. Both multi-parameter probes were calibrated according to manufacturer’s specifications, in air for %O2 and with standard solutions for other variables: commercial pH buffers (4.00, 7.00, 10.00), a 1000 µS cm-1 standard for conductivity. pCO2 was measured directly after water sampling with a Li-Cor Li-840 infra-red gas analyser (IRGA) based on the headspace technique with 4 polypropylene 60 ml syringes (Borges et al. 2015). The Li-Cor 840 IRGA was calibrated before and after each cruise with ultrapure N2 and a suite of gas standards (Air Liquide Belgium) with CO2 mixing ratios of 388, 813, 3788 and 8300 ppm. The overall precision of pCO2 measurements was ±2.0%.

Samples for CH4 and N2O were collected from the Niskin bottle with a silicone tube in 60 ml borosilicate serum bottles (Wheaton), poisoned with 200 µL of a saturated solution of HgCl2 and sealed with a butyl stopper and crimped with an aluminium cap. Measurements were made with the headspace technique (Weiss 1981) and a gas chromatograph (GC) (SRI 8610C) with a flame ionisation detector for CH4 and electron capture detector for N2O calibrated with CO2:CH4:N2O:N2 gas mixtures (Air Liquide Belgium) with mixing ratios of 1, 10 and 30 ppm for CH4, 404, 1018, 3961 ppm for CO2, and 0.2, 2.0 and 6.0 ppm for N2O. The precision of measurement based on duplicate samples was ±3.9% for CH4 and ±3.2% for N2O.

Samples for the stable isotope composition of CH4 (δ13C-CH4) were collected and preserved as described above for the CH4 concentration. The δ13C-CH4 was determined with a custom developed interface, whereby a 20 ml He headspace was first created, and CH4 was flushed out through a double-hole needle, non-CH4 volatile organic compounds were trapped in liquid N2, CO2 was removed with a soda lime trap, H2O was removed with a magnesium perchlorate trap, and the CH4 was quantitatively oxidized to CO2 in an online combustion column similar to that of an elemental analyzer. The resulting CO2 was subsequently pre-concentrated by immersion of a stainless steel loop in liquid N2, passed through a micropacked GC column (Restek HayeSep Q, 2m length, 0.75mm internal diameter), and finally measured on a Thermo DeltaV Advantage isotope ratio mass spectrometer (IRMS). Calibration was performed with CO2 generated from certified reference standards (IAEA-CO-1 or NBS-19, and LSVEC) and injected in the line after the CO2 trap. Reproducibility of measurement based on duplicate injections of samples was typically better than ±0.5 ‰.

Water was filtered on Whatman glass fibre filters (GF/F grade, 0.7 µm porosity) for particulate organic carbon (POC) and Chl-a (47 mm diameter). Filters for POC were stored dry and filters for Chl-a were stored frozen at -20°C. Filters for POC analysis were decarbonated with HCl fumes for 4h and dried before encapsulation into silver cups; POC concentration was analysed on an EA-IRMS (Thermo FlashHT with DeltaV Advantage), with a reproducibility better than ±5%. Data were calibrated with certified (IAEA-600: caffeine) and in-house standards (leucine and muscle tissue of Pacific tuna) that were previously calibrated versus certified standards. The Chl-a samples were analysed by HPLC according to Descy et al. (2005), with a reproducibility of ±0.5% and a detection limit of 0.01 µg L-1.

The water filtered through GF/F Whatman glass fibre filters was collected and further filtered through polyethersulfone syringe encapsulated filters (0.2 µm porosity) for nitrate (NO3-), nitrite (NO2-) and ammonium (NH4+) and were stored frozen (-20°C) in 50 mL polypropylene vials. NO3- and NO2- were determined with the sulfanilamide colorimetric with the vanadium reduction method (APHA, 1998), and NH4+ with the dichloroisocyanurate-salicylate-nitroprussiate colorimetric method (SCA, 1981). Detection limits were 0.3, 0.01, and 0.15 µmol L-1 for NH4+, NO2- and NO3-, respectively. Precisions were ±0.02 µmol L-1, ±0.02 µmol L-1, and ±0.1 µmol L-1 for NH4+, NO2- and NO3-, respectively.

References

APHA, 1998. Standard methods for the examination of water and wastewater, American Public Health Association.

Borges, A. V., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Geeraert, N., Omengo, F. O., Guérin, F., Lambert, T., Morana, C., Okuku, E., and Bouillon, S.: Globally significant greenhouse gas emissions from African inland waters, Nature Geosci., 8, 637-642, doi:10.1038/NGEO2486, 2015.

Descy, J.-P., Hardy, M.-A., Sténuite, S., Pirlot, S., Leporcq, B., Kimirei, I., Sekadende, B., Mwaitega, S. R., and Sinyenza, D., 2005. Phytoplankton pigments and community composition in Lake Tanganyika. Freshw. Biol., 50, 668-684.

Frankignoulle, M., Borges, A., Biondo R., 2001. A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments. Water Res., 35, 1344-1347.

Standing committee of Analysts: Ammonia in waters. Methods for the examination of waters and associated materials. 16 pp., 1981.

Weiss, R.F., 1981. Determinations of carbon dioxide and methane by dual catalyst flame ionization chromatography and nitrous oxide by electron capture chromatography. J. Chromatogr. Sci., 19, 611-616.

本带地理坐标与时间戳的数据集共包含7个文件：

"db_cruise_CTD" 收录了科考航次获取的CTD剖面（CTD profile）数据。

"db_cruise_GHGs" 收录了4次科考航次获取的溶解态CO₂、CH₄、N₂O浓度，叶绿素a（chlorophyll-a）浓度，无机营养盐（NO₃⁻、NO₂⁻、NH₄⁺、PO₄³⁻）以及δ¹³C-CH₄数据。

"db_monitoring" 收录了2017年1月至2019年12月期间，两个站位的监测数据，包括溶解态CO₂、CH₄、N₂O浓度、叶绿素a浓度及颗粒有机碳（particulate organic carbon, POC）。

"db_uw" 收录了2019年3月19日的CO₂与CH₄连续监测数据（附带EXO-II传感器数据）。

"meteo_Mweya" 收录了2016年6月至2019年3月的气象观测数据。

"db_monitoring_CTD" 收录了监测深水站位的CTD剖面数据。

"mooring" 收录了爱德华湖10m水深站位的锚系温度数据（2019年3月）。

数据采集区域涵盖爱德华湖、卡津加水道与乔治湖，共开展4次科考航次，时间分别为2016年10月20日-11月7日、2017年3月23日-4月8日、2018年1月18日-2月2日、2019年3月21日-3月30日。2017年至2019年12月期间，研究人员对1个浅水区站位（水底深度3m）与1个深水区站位（水底深度22m）开展定期采样：2017、2018年每21天采样1次，2019年每30天采样1次。2019年3月21日（当地时间13:00）至2019年3月23日（当地时间13:50），在爱德华湖水底深度10m的站位（坐标：-0.2459°N，29.8635°E）部署了锚系装置，该装置搭载RBR Solo温度传感器，布设深度覆盖从表层至沉积物上方1m处，具体深度为0.2、1.0、2.0、5.0、7.5及9.0m。

2016年6月至2019年3月期间，在乌干达野生动物管理局位于姆韦亚的一栋建筑楼顶（海拔地面以上4m，坐标：-0.190384°N，29.899103°E），搭载Davis Instruments Vantage Pro2气象站（配置原厂标准传感器），采集太阳辐射、紫外线辐射、风速（杯式风速仪）、风向（风向标）、降雨量（机械式雨量收集器）、气温与气压数据。数据采样间隔为5秒，每10分钟进行一次平均并记录。

2019年3月航次期间，采用专为高浊度水体设计的平衡器（内部填充玻璃珠，参照Frankignoulle等，2001）搭配Los Gatos Research公司的离轴积分腔输出光谱分析仪（Ultraportable Greenhouse Gas Analyzer，具备CH₄量程扩展功能），以1分钟的间隔连续测定水体中CO₂分压（pCO₂）与CH₄分压（pCH₄）。同步采用YSI EXO-II多参数探头测定水温、比电导率、pH、溶解氧饱和度（%O₂）、浊度、叶绿素a（Chl-a）及荧光性溶解有机物（fluorescent dissolved organic matter, FDOM）；采用Garmin Map 60S便携式GPS设备定位，使用Humminbird Helix 5回声测深仪获取水深数据。表层海水通过搭载于船舷、水深约0.5m处的12V电动水泵（LVM105）泵送至甲板上的平衡器与多参数探头。

离散水样采用5.0L Niskin采水器（General Oceanics）从船舷采集。首次科考航次期间，采用Hydrolab DS5多参数探头测定水温、比电导率、pH、%O₂及Chl-a的垂直剖面；其余3次航次及常规监测期间，额外采用YSI EXO-II多参数探头测定浊度与FDOM。两台多参数探头均按照厂商说明进行校准：%O₂采用空气校准，其余变量采用标准溶液校准，包括商用pH缓冲液（4.00、7.00、10.00）及1000 µS cm⁻¹电导率标准液。pCO₂采用顶空法（参照Borges等，2015），使用4个60ml聚丙烯注射器，通过Li-Cor Li-840红外气体分析仪（IRGA）在采样后立即测定。每次航次前后均采用超纯N₂及Air Liquide Belgium公司的系列气体标准品（CO₂体积混合比分别为388、813、3788、8300 ppm）对Li-Cor 840 IRGA进行校准，pCO₂测定的总精度为±2.0%。

CH₄与N₂O水样采用硅橡胶管从Niskin采水器中转移至60ml硼硅酸盐血清瓶（Wheaton）中，加入200µL饱和HgCl₂溶液进行固定，随后用丁基橡胶塞密封并以铝盖压封。采用顶空法（参照Weiss，1981）结合气相色谱（GC，SRI 8610C）进行测定：CH₄采用火焰离子化检测器，N₂O采用电子捕获检测器；校准气为Air Liquide Belgium公司配制的CO₂:CH₄:N₂O:N₂混合气体，其中CH₄体积混合比为1、10、30 ppm，CO₂为404、1018、3961 ppm，N₂O为0.2、2.0、6.0 ppm。平行样测定的精度为：CH₄±3.9%，N₂O±3.2%。

CH₄的稳定同位素组成（δ¹³C-CH₄）水样的采集与保存方法与CH₄浓度测定水样一致。采用定制化接口进行测定：首先创建20ml He顶空，通过双孔针将CH₄吹扫出来，非CH₄挥发性有机物在液氮中被捕集，CO₂通过苏打石灰阱去除，H₂O通过高氯酸镁阱去除；随后CH₄在在线燃烧柱（类似元素分析仪）中被定量氧化为CO₂。生成的CO₂首先通过将不锈钢环路浸入液氮进行预浓缩，随后通过微孔填充GC柱（Restek HayeSep Q，长2m，内径0.75mm），最终在Thermo DeltaV Advantage同位素比率质谱仪（IRMS）上进行测定。校准采用经认证的参考标准品（IAEA-CO-1或NBS-19及LSVEC）生成的CO₂，在CO₂阱后注入分析流路。平行进样的测定重现性通常优于±0.5‰。

采用Whatman GF/F级玻璃纤维滤膜（孔径0.7µm，直径47mm）过滤水样，用于颗粒有机碳（POC）与叶绿素a（Chl-a）分析。POC滤膜干燥保存，Chl-a滤膜置于-20℃冷冻保存。POC滤膜分析前需用HCl烟雾脱碳4小时并干燥，随后封装至银杯；POC浓度采用EA-IRMS（Thermo FlashHT搭配DeltaV Advantage）进行分析，重现性优于±5%。校准采用经认证的标准品（IAEA-600：咖啡因）及室内标准品（亮氨酸及太平洋金枪鱼肌肉组织，此前已通过认证标准品校准）。Chl-a样品采用高效液相色谱（High Performance Liquid Chromatography, HPLC）按照Descy等（2005）的方法进行分析，重现性为±0.5%，检出限为0.01 µg L⁻¹。

通过Whatman GF/F玻璃纤维滤膜过滤后的水样，进一步采用聚醚砜针头式滤器（孔径0.2µm）过滤，用于测定硝酸盐（NO₃⁻）、亚硝酸盐（NO₂⁻）及铵盐（NH₄⁺），滤液收集于50ml聚丙烯瓶中并置于-20℃冷冻保存。NO₃⁻与NO₂⁻采用磺胺类比色法结合钒还原法（参照APHA，1998）测定，NH₄⁺采用二氯异氰尿酸钠-水杨酸钠-硝普钠比色法（参照SCA，1981）测定。NH₄⁺、NO₂⁻及NO₃⁻的检出限分别为0.3、0.01及0.15 µmol L⁻¹；精密度分别为±0.02 µmol L⁻¹、±0.02 µmol L⁻¹及±0.1 µmol L⁻¹。

参考文献

APHA, 1998. 《水和废水检测标准方法》，美国公共卫生协会。

Borges, A. V., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Geeraert, N., Omengo, F. O., Guérin, F., Lambert, T., Morana, C., Okuku, E., and Bouillon, S.: 非洲内陆水体的全球显著温室气体排放，《自然·地球科学》，8, 637-642, doi:10.1038/NGEO2486, 2015.

Descy, J.-P., Hardy, M.-A., Sténuite, S., Pirlot, S., Leporcq, B., Kimirei, I., Sekadende, B., Mwaitega, S. R., and Sinyenza, D., 2005. 坦噶尼喀湖的浮游植物色素与群落组成，《淡水生物学》，50, 668-684.

Frankignoulle, M., Borges, A., Biondo R., 2001. 高动态高浊度环境中CO₂监测用新型平衡器设计，《水研究》，35, 1344-1347.

Standing committee of Analysts: 水体氨的测定，《水及相关物料检测方法》，16页，1981.

Weiss, R.F., 1981. 双催化剂火焰离子化色谱法测定CO₂与CH₄及电子捕获色谱法测定N₂O，《色谱科学杂志》，19, 611-616.