Variability and Trends in Physical and Biogeochemical Parameters of the Mediterranean Sea during a Cruise with RV MARIA S. MERIAN in March 2018

Earth Syst. Sci. Data, 12, 2747–2763, 2020 https://doi.org/10.5194/essd-12-2747-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Physical and biogeochemical parameters of the Mediterranean Sea during a cruise with RV Maria S. Merian in March 2018 Dagmar Hainbucher1, Marta Álvarez4, Blanca Astray Uceda4, Giancarlo Bachi5, Vanessa Cardin3, Paolo Celentano6, Spyros Chaikalis7, Maria del Mar Chaves Montero3,9, Giuseppe Civitarese3, Noelia M. Fajar4, Francois Fripiat10, Lennart Gerke2, Alexandra Gogou7, Elisa F. Guallart4, Birte Gülk1, Abed El Rahman Hassoun8, Nico Lange2, Andrea Rochner1, Chiara Santinelli5, Tobias Steinhoff2, Toste Tanhua2, Lidia Urbini3, Dimitrios Velaoras7, Fabian Wolf2, and Andreas Welsch1 1Institut für Meereskunde, CEN, Universität Hamburg, Bundesstraße 53, 20146 Hamburg, Germany 2GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Wischhofstr. 1–3, 24148 Kiel, Germany 3Dept. Of Oceanography, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico, Trieste, Italy 4Instituto Español de Oceanografía (IEO), Centro de A Coruña, Spain 5Istituto di Biofisica, CNR, Pisa, Italy 6Istituto di Scienze Marine, Venezia, Italy 7Hellenic Centre for Marine Research, Athens, Greece 8National Council for Scientific Research in Lebanon, National Center for Marine Sciences, Beirut, Lebanon 9Centro Euro-Mediterraneo sui Cambiamenti Climatici CMCC, Bologna, Italy 10Max Planck Institute for Chemistry, Mainz, Germany Correspondence: Dagmar Hainbucher ([email protected]) Received: 2 April 2020 – Discussion started: 7 July 2020 Revised: 17 September 2020 – Accepted: 29 September 2020 – Published: 12 November 2020 Abstract. The last few decades have seen dramatic changes in the hydrography and biogeochemistry of the Mediterranean Sea. The complex bathymetry and highly variable spatial and temporal scales of atmospheric forcing, convective and ventilation processes contribute to generate complex and unsteady circulation patterns and significant variability in biogeochemical systems. Part of the variability of this system can be influenced by anthropogenic contributions. Consequently, it is necessary to document details and to understand trends in place to better relate the observed processes and to possibly predict the consequences of these changes. In this context we report data from an oceanographic cruise in the Mediterranean Sea on the German research vessel Maria S. Merian (MSM72) in March 2018. The main objective of the cruise was to contribute to the understanding of long-term changes and trends in physical and biogeochemical parameters, such as the anthropogenic carbon uptake and to further assess the hydrographical situation after the major climatological shifts in the eastern and western part of the basin, known as the Eastern and Western Mediterranean Transients. During the cruise, multidisciplinary measurements were conducted on a predominantly zonal section throughout the Mediterranean Sea, contributing to the Med-SHIP and GO-SHIP long-term repeat cruise section that is conducted at regular intervals in the Mediterranean Sea to observe changes and impacts on physical and biogeochemical variables. The data can be accessed at https://doi.org/10.1594/PANGAEA.905902 (Hainbucher et al., 2019), https://doi.org/10.1594/PANGAEA.913512 (Hainbucher, 2020a) https://doi.org/10.1594/PANGAEA.913608, (Hainbucher, 2020b) https://doi.org/10.1594/PANGAEA.913505, (Hainbucher, 2020c) https://doi.org/10.1594/PANGAEA.905887 (Tanhua et al., 2019) and https://doi.org/10.25921/z7en-hn85 (Tanhua et al, 2020). Published by Copernicus Publications. 2748 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 Data coverage and parameter measured Repository Reference. Table 1a and b and list of available data sets (Table 1c). A link to the summary page of the cruise MSM72 can be found in the PANGAEA database under https://www. pangaea.de/?q=msm72&f.campaign%5B%5D=MSM72 (last access: 10 November 2020). Coverage. 34–41◦N, 6◦W–28◦E Location name. The Mediterranean Sea Date/Time start. 2 March 2018 Date/Time end. 3 April 2018 1 Introduction Contrary to earlier ideas that the Mediterranean Sea is always in a steady state, we now know in the light of new research that the Mediterranean Sea is not, and it is potentially sensitive to climatic changes (Malanotte-Rizzoli, 2014). Proving this are the drastic changes that the eastern Mediterranean (EMed) has undergone in the past. The largest climatic event, named the Eastern Mediterranean Transient (EMT), occurred in the EMed between the late 1980s and early 1990s, where deep-water formation switched from the Adriatic Sea to the Aegean Sea. This episode modified the thermohaline characteristics of the outflow through the Sicilian Strait, advecting anomalously salty and warm Levantine Intermediate Water (LIW) to the western Mediterranean Sea (WMed) and leading to a significant increase in temperature and salt in the intermediate and deep layers of the WMed. Additionally, strong deep convection induced by extreme atmospheric events during winter in 2004–2006 (low precipitation, cold, persistent winds) also enhanced salt and temperature in the entire basin up to about 1600m (Schroeder et al., 2006, 2008). This abrupt climate shift is referred to as Western Mediterranean Transient (WMT) and the physical changes are comparable to the EMT, both in terms of intensity and observed effects (Schroeder et al., 2008). The existence of both transients contradicts the hypothesis of a steady state. On the other hand, it has also been proven that an EMT has never been observed before (Roether et al., 2013). The characteristic of the Mediterranean Sea is also such that it has the potential to sequester large amounts of anthropogenic CO2, Cant, since the Mediterranean Sea has high alkalinity and temperature, which can be rapidly transported to deep by the overturning circulation (e.g., Schneider et al., 2010). The column inventories of Cant in the Mediterranean are among the highest found in the world oceans; the Mediterranean Sea thus stores a significant portion of the global anthropogenic emissions of Cant despite its relatively small volume. Furthermore, marine dissolved organic carbon (DOC) represents the largest reservoir of reduced carbon (662× 1015 g C) on Earth (Hansell, 2009), it therefore plays a major role in the global carbon cycle. Its role in the functioning of marine ecosystems is equally crucial since DOC is released at all the levels of the food web as a byproduct of many trophic interactions and/or metabolic processes and is the main source of energy for the heterotrophic prokaryotes (Carlson and Hansell, 2015). Although most of DOC is produced in situ, external sources (atmosphere, rivers, sediments) may affect its concentration and distribution. Physical processes, such as deep-water formation, thermohaline circulation, vertical stratification and mesoscale activities have been reported to be the main drivers of DOC distribution in the Mediterranean Sea (Santinelli, 2015; Santinelli et al., 2015, 2013). The main scientific objective of the cruise reported here was to add knowledge to the different scales and magnitudes of variability and trends in circulation, hydrography, and biogeochemistry of the Mediterranean Sea. Key variables were measured in strategic regions in order to understand changes, the reason for occurrence, and the drivers. In this context, this cruise is part of the Med-SHIP and GO-SHIP long-term repeat cruise section that is conducted at regular intervals in the Mediterranean Sea to observe changes and impacts on physical and biogeochemical variables. The following science questions are addressed in this work. 1. What are the long-term changes and/or trends in physics and biochemistry in the Mediterranean Sea, including all the sub-basins? 2. How is the hydrographic situation in the Mediterranean developing further following the EMT and WMT? Is there still a tendency of the system to return to the preEMT situation and is there a similar trend in the WMed? 3. How are eddies distributed in the EMed and WMed during the cruise? Do they differ in the sub-basins? To what extent is heat and salt transferred into the vertical by eddies in the WMed and EMed during the cruise period? 4. What is the uptake rate of the anthropogenic carbon in the Mediterranean and is this changing over time? 5. What is the extent of the variability and trends in the inventory of biogeochemical variables (including oxygen, nutrients and dissolved organic carbon)? 6. What are the baseline values of rarely measured essential ocean variables (EOVs) such as dissolved organic carbon (DOC)? Earth Syst. Sci. Data, 12, 2747–2763, 2020 https://doi.org/10.5194/essd-12-2747-2020 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 2749 Table 1. (a) List of physical parameters from Maria S. Merian cruise MSM72 as seen in the PANGAEA database. The primary investigator was Dagmar Hainbucher. (b) List of biogeochemical parameters from Maria S. Merian cruise MSM72 as seen in the CCHDO database. The primary investigator was Toste Tanhua. (c) List of available data sets. (a) Parameter name Short name Unit Method Comments Date/time Date/time Geocode Latitude Latitude Geocode Longitude Longitude Geocode Pressure, water Press dbar CTD, SEA_BIRD SBE 911plus Temperature, water Temp ◦C CTD, SEA_BIRD SBE 911plus Salinity Sal CTD, SEA_BIRD SBE 911plus Oxygen O2µmolkg−1CTD with attached oxygen sensor (SBE43) calibrated, corrected using Winkler titration Pressure, water Press dbar Underway CTD (uCTD), Oceanscience Temperature, water Temp ◦C Underway CTD (uCTD), Oceanscience Salinity Sal Underway CTD (uCTD), Oceanscience Depth, water Depth m Current velocity east–west UC ms−1Shipboard acoustic doppler current profiling (SADCP) Current velocity north–south VC ms−1Shipboard acoustic doppler current profiling (SADCP) Depth, water Depth m Current velocity east–west UC ms−1Lowered acoustic doppler current profiling (lADCP) Current velocity north–south VC ms−1Lowered acoustic doppler current profiling (lADCP) (b) Variable Unit Dissolved oxygen (O2) µmolkg−1 Sulfurhexafluorid (SF6) fmolkg−1 CCl2F2(CFC-12) pmolkg−1 Nitrate (NO− 3) µmolkg−1 Nitrite (NO− 2) µmolkg−1 Phosphate (PO2− 4) µmolkg−1 Silicate (Si) µmolkg−1 Dissolved inorganic carbon (DIC) µmolkg−1 Total alkalinity (TA) µmolkg−1 pH Total scale at 25 ◦C Carbonate (CO2− 3) µmolkg−1 δ13C of DIC ‰ Total dissolved nitrogen (TDN) µmolkg−1 Total dissolve phosphorus (TDP) µmolkg−1 CHClF2(HCFC-22) pmolkg−1 C2H3Cl2F (HCFC-141b) pmolkg−1 C2H3ClF2(HCFC-142b) pmolkg−1 CH2FCF3(HFC-134a) pmolkg−1 C2HF5(HFC-125) pmolkg−1 CHF3(HFC-23) pmolkg−1 https://doi.org/10.5194/essd-12-2747-2020 Earth Syst. Sci. Data, 12, 2747–2763, 2020 2750 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 Table 1. Continued. (c) Database Data set https://doi.org/10.1594/PANGAEA.905902 CTD (Hainbucher et al., 2019) https://doi.org/10.1594/PANGAEA.913512 uCTD (Hainbucher, 2020a) https://doi.org/10.1594/PANGAEA.913608 ADCP (Hainbucher, 2020b) https://doi.org/10.1594/PANGAEA.913505 lADCP (Hainbucher, 2020c) https://doi.org/10.1594/PANGAEA.905887 chemical data (Tanhua et al., 2019) https://doi.org/10.25921/z7en-hn85 pCO2(Tanhua et al., 2020) https://cchdo.ucsd.edu/cruise/06M220180 (additional) CTD and chemical data 2 Data provenance The survey was carried out on the German RV Maria S. Merian from 2 March to 3 April 2018. The cruise started on Heraklion, Greece, and ended in Cádiz, Spain. The main focus of the cruise was on an east–west transect across the western and eastern Mediterranean Sea (Fig. 1) starting east of Crete and ending near the Strait of Gibraltar, which is a repeating hydrographic line in GO-SHIP (MED1). Difficulties with diplomatic authorizations for Marine Scientific Research (MSR) in the area east of Crete made it impossible for us to carry out our measurements as initially planned, and thus no data were obtained east of the Kasos Strait. During the 33 d of the cruise we carried out measurements of hydrographic and biogeochemical variables along track with the classical approach, i.e., CTD, lADCP, uCTD instrumentation and bottle samples on highly resolved sections across the Mediterranean Sea. The high resolution of CTD stations, enhanced for the physical parameters by additional uCTD measurements, allowed us to resolve the eddy field on the sections; the analysis was also supported and complemented by satellite data. Most sections and CTD-positions follow previous sampling strategies (cruise M84 and others along the GO-SHIP line MED-01, i.e., Tanhua et al., 2013) to allow long-term trend analyses. Along the different sections, CTD stations including sampling of chemical parameters were conducted approximately every 30nm, CTD without sampling about every 15–20 nm and with even smaller spacing in the straits. In addition, underway CTD measurements and ADCP measurements were performed between CTD stations. The water sampling program included measurements of all level 1 variables as defined by GO-SHIP (i.e., oxygen, macronutrients, transient tracers and the carbonate system, http://www.go-ship.org/DatReq.html, last access: 10 November 2020) and measurements of the biogeochemical EOVs 13C, nitrous oxide (N2O) and dissolved organic carbon (DOC). These data were used to quantify trends and variability of ventilation and biogeochemical cycles, in particular uptake of anthropogenic carbon. Sections were additionally conducted through the important passages of the Otranto Strait, Kasos Strait, Antikythera Strait, Sicilian Strait and Strait of Gibraltar, in order to characterize the incoming and outgoing flows. CTD stations in the eastern Ionian Sea were carried out to quantify the flow of the Levantine Surface Water (LSW) into the Adriatic Sea and to track the outflow of the Adriatic Deep Water (AdDW) into the Ionian Sea. 3 Methods 3.1 CTD rosette All together 136 CTD casts were performed, from which 18 were catalogued as isotopic (a full suite of observations is given in Table 1a and b), 65 as chemical (i.e., GO-SHIP level 1 variables) and 59 as physical (i.e., only sampling for salinity). Due to the water amount needed, two casts were performed on most of the isotopic stations, the first cast was a full profile and the second a shallow one. During the physical stations, water samples at three levels were taken for salinity analysis. The samples were then analyzed on board using a Guildline Autosal Salinometer. A total of 162 samples in 59 stations were taken during the cruise with an offset with respect to standard water varying from 0.0002 to 0.0030 depending on the laboratory temperature. The samples were taken at depth with a constant salinity gradient to ensure that no natural changes in salinity affect the comparison between sample and sensor. The primary CTD system (for specifications, see Table 2) initially used on board was a Seabird SBE9plus+CTD s/n 0285 from the University of Hamburg connected to a SBE11 deck unit, configured with a 24-position SBE-32 pylon (from GEOMAR) with 10 L Niskin bottles. The position of bottle no. 23 and no. 24 was occupied by the lADCP (for specifications, see Table 3). Initially, the CTD was set up with two sensors for temperature and conductivity, an oxygen sensor, a fluorometer, and an altimeter. To test the configuration and performance of the instrument a station was carried out on the Cretan Sea at the start of the cruise. Unfortunately, we had countless problems with instruments, sensors, cables Earth Syst. Sci. Data, 12, 2747–2763, 2020 https://doi.org/10.5194/essd-12-2747-2020 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 2751 Figure 1. Station map. Yellow dots are CTD without any chemical sampling, red dots are CTD with chemical sampling, cyan dots are CTD with chemical and additional sampling of isotopes, yellow squares are the deployment of drifter and floats, blue lines are fine resolved uCTD and ADCP tracks, and black lines are tracks with uCTD casts between CTD stations. (a) Detail of the central map of the western Mediterranean Sea. (b) Detail of the central map of the eastern Mediterranean Sea. (c) Detail of the central map of the Otranto Strait and northern Ionian Sea. (d) Detail of the central map of the Tyrrhenian Sea and Sicilian Strait. and the rosette during most of the campaign that forced us to change them very often with others available on board, resulting in a continuous change of system configuration. Thus, all different configurations were carefully considered when post-processing the CTD data. Temperature, salinity and pressure data were postprocessed by applying Seabird software and MATLAB®routines. At this stage, spikes were removed and 1dbar averages calculated. A first attempt to assess the performance of the conductivity sensors installed on the CTD rosette was done by comparing the salinity data with the bottle samples analyzed with the salinometer. The different hardware setups and configurations are taken carefully into account during postprocessing. Overall accuracies are within the expected range of salinity (0.003). 3.2 Underway CTD Underway CTD measurements (uCTD; for specifications, see Table 4) provide high-resolution profiles of temperature, conductivity and depth, which allow us to characterize the upper-ocean properties and identify the position and characteristics of mesoscale structures. The advantage of this type of measurement is that it is not required to stop the vessel, https://doi.org/10.5194/essd-12-2747-2020 Earth Syst. Sci. Data, 12, 2747–2763, 2020 2752 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 Table 2. Used CTD instrument and sensors. Owner of instruments are either the University of Hamburg, Germany (IfM-HH); the National Institute of Oceanography and Geophysics (OGS), Italy; or the property of the vessel Merian (MSM). Instrument/sensor Serial number Calibration (owner) date SBE 911plus/917plus CTD 285 (IfM-HH) 3 Dec 2014 806 (MSM) 27 Jan 2016 807 (MSM) 8 Sep 2015 Temperature 1: SBE-3-02/F 1717 (OGS) 22 Nov 2017 5716 (MSM) 15 Jul 2017 Conductivity 1: SBE-4-02/2 3442 (OGS) 22 Nov 2017 4152 (MSM) 14 Jul 2017 Temperature 2: SBE-3-02/F 1294 (IfM-HH) 11 Apr 2017 5719 (MSM) 15 Jul 2017 Conductivity 2: SBE-4-02/2 1106 (IfM-HH) 12 Apr 2017 4156 (MSM) 14 Jul 2017 Oxygen 1: SBE 43 3392 (OGS) 19 Dec 2017 2417 (MSM) 16 Aug 2017 0951 (MSM) 1 Dec 2017 Oxygen 2: SBE 43 1761 (IfM-HH) 11 Apr 2017 2418 (MSM) 15 Aug 2017 0881 (MSM) 23 Dec 2017 Fluorometer WETLAB 1755 (MSM) 18 Apr 2017 1754 (MSM) 21 Dec 2017 SeaPoint (used on 1 station) SCF2874 unknown SPAR 10 Mar 2016 PAR Chelsea 17 Oct 2016 it is only necessary to maintain lower velocities (about 3kn) during the deployments to reach greater depths. These measurements were made with an Ocean Science uCTD system. The first uCTD deployment was done on 5 March, between CTD 015 and 016 stations, and we continued with this type of sampling between each CTD station to increase the sampling resolution. Unfortunately, several deployments were canceled due to severe weather conditions and no uCTD cast was performed when the depth was shallower than 500 m. All together, 176 casts were taken with depths ranging from 557 to 864 m. Two probes were used during the cruise with a no-timelimit mode configuration (apart from the first cast configured to stop recording after 600 s, reaching 616 m depth) in order to get longer records. The probe tail spools were attached to the winch through a rope loop that was made new every day in the morning. Despite the probes being able to record several casts, data were downloaded right after each cast using a SBE software in order to avoid losing the data in case the probe was lost and to free up the memory. The probes were exchanged when the battery was running low (around 3.8 V). On three occasions no data were recorded because the magnet was taken off twice before deployment. For calibration purposes, some additional casts were done right after the CTD cast in order to compare the data sets. The probes were also sent down with the starboard CTD in station 130. Data files were processed using a set of MATLAB®routines. After extracting the downcast data, the first correction was done to remove inaccuracies in the descent rate, based on the work of Ullmann and Hebert (2014). Additionally, the data were aligned to the comparable CTD data sets. 3.3 lADCP measurements Ocean currents were studied by means of vertical profiles made with a lADCP-2 system (Workhorse RD Instruments type, Table 3) which included two ADCPs operating at a frequency of 300kHz, one looking upward and the other one looking downward. The system was placed in the rosette occupying the position of Niskin bottles 23 and 24. During the cruise, the lADCP batteries were changed twice: the first time on 17 March in Station 58 and the second time on 27 March in Station 105. Except for three stations (station 73, 74, 80) with water depths less than 500m, lADCP measurements were done at all CTDs. For these stations, the currents were observed by the ship-mounted ADCP. At isotope stations, lADCP profiles were only recorded from the deep cast. The gained data were processed with LDEO MATLAB®lADCP processing system version 10.15 (Turnherr, 2014). This software uses the raw lADCP data, processed CTD data and navigational data from the CTD. The resulting data are the uand vvelocities at the depth. The bin size was set to 8 m. 3.4 Shipborne ADCP During the whole campaign, underway current measurements were taken with two vessel-mounted Ocean Surveyors (ADCP) manufactured by RDI. The first, with a working frequency of 75 kHz, covered approximately the top 500–700 m of the water column. The number of bins was set to 100 with bin size of 8m. The second, with a working frequency of 38 kHz, has a depth range of about 1600 m, set with the same bin number as the previous one and bin size of 16 m. Both instruments run in narrowband mode and were controlled by computers using the conventional RDI VMDAS software under a Microsoft Windows system with a pinging set to fast as possible. No interferences with other used acoustical instruments were observed. The ADCP data was post-processed afterwards with the CODAS3 Software System (https://currents.soest.hawaii. edu/docs/adcp_doc/, last access: 10 November 2020), which allows extracting data, assigning coordinates, and editing and correcting velocity data. Moreover, the data were corrected for errors in the value of sound velocity in water, and misalignment of the instrument with respect to the axis of the Earth Syst. Sci. Data, 12, 2747–2763, 2020 https://doi.org/10.5194/essd-12-2747-2020 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 2753 Table 3. Used uCTD sensors. Probe 1 Device type Serial number (owner) 0289 90745 uCTD/SBE49 FastCat CTD 702-0289 (IfM-HH) 0183 90745 uCTD/SBE 49 FastCat CTD 702-0183 (IfM-HH) Table 4. Used lADCP. Device Type Serial number (owner) WHM300 Master s/n no. 22762 (IfM-HH) WHM300 Slave s/n no. 22763 (IfM-HH) ship (about −2.8◦for 75 kHz ADCP and about −0.15◦for 38 kHz ADCP). 3.5 Underway CO2and O2measurements Underway (UW) measurements of partial pressure of CO2 (pCO2), and dissolved oxygen partial pressure (pCO2the corresponding data set in Table 1b only contains pCO2) in seawater were carried out by means of a Contros HydroC pCO2analyzer for pCO2and an Aanderaa optode for oxygen. The instruments were placed in a cooling box in the hangar. Seawater was drawn from the ship’s centrifugal pump for clean seawater that was continuously flowing through the cooling box with the inlet close to the instruments. Water was pumped through a SeaBird 5 salinity and temperature sensor and onto the HydroC instrument (Gerke, 2020). The system operated reliably throughout the cruise, except when data acquisition was interrupted for the pCO2instrument for 2 d directly after the ship’s centrifugal pump was switched off. This led to a 5 d period without data between 5 and 10 March. During the cruise, 13 samples were taken from the cooling box for discrete measurements of pH and total alkalinity. The UW measurements started on 2 March at 20:20 and stopped on 1 April 2018, at 14:00 UTC. The underway oxygen measurements were calibrated by comparing them to the Winkler measurements taken for surface samples at the chemical CTD stations. 3.6 Dissolved oxygen Dissolved oxygen in seawater was not only measured with the CTD, but samples were also taken at every station and depth along the cruise and reported in µmolkg−1. GO-SHIP guidelines recommend Winkler measurements on all samples, in addition to sensor measurements on the CTD package, and we largely followed those recommendations. Unfortunately, we had to mark large numbers of oxygen values determined with the CTD as questionable due to the several technical problems with the CTDs and sensors. Usually, samples were taken at standard depths, but, especially at the surface and at the bottom, the depths were varied according to the requirements of the other biogeochemical parameters. Oxygen was measured following the automatic Winkler potentiometric method, modified following Langdon (2010). Titrations were done within the sampling calibrated flasks using an Automatic Titrator Mettler Toledo T50 with a platinum combined electrode. Reagents of blank and thiosulfate standardization were done daily by means of potassium iodate standard 1.667 mmol by OSIL, UK. About 1400 samples were analyzed on board. The precision of dissolved oxygen measurements was determined on five replicates at the beginning and at the end of the cruise (Table 5). In addition, during the cruise 46 duplicates were analyzed. The results are given in Table 6. 3.7 Nutrients (nitrite, nitrate, phosphate and silicate), total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) 3.7.1 Nutrients Analyses were performed at 40 ◦C on a four-channel, Quaatro SEAL Analytical Continuous Flow Analyzer s/n 8014549; https://www.seal-analytical.com/Products/ SegmentedFlowAnalyzers/QuAAtro39AutoAnalyzer/tabid/ 814/language/en-US/Default.aspx (last access: 10 November 2020), according to Hansen and Koroleff (1999). Nitrite was determined through the formation of a reddishpurple azo dye and measured at 520nm (SEAL Method no. Q-030-04 Rev. 2). Nitrate was reduced to nitrite in a copperized cadmium reduction coil and then determined as described for nitrite (SEAL Method no. Q-035-04 Rev. 4). The determination of phosphate was based on the reduced blue phospho-molybdenum complex and then measured at 880 nm (SEAL Method no. Q-031-04 Rev. 1). Silicate was determined by means of acidic reduction of silicomolybdate to molybdenum blue and then measured at 820nm (SEAL Method no. Q-038-04 Rev. 0). About 1400 nutrient samples were analyzed on board. The onboard precision of nutrient measurements was determined on five replicates at the beginning and at the end of the cruise. The results are shown in Table 7. In addition, during the cruise 140 duplicates were analyzed. The results are shown in Table 8. https://doi.org/10.5194/essd-12-2747-2020 Earth Syst. Sci. Data, 12, 2747–2763, 2020 2754 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 Table 5. Precision of dissolved oxygen (SD is standard deviation, and CV is coefficient of variation). Parameter Beginning of the cruise End of the cruise Mean µM SD µM CV% Mean µM STD µM CV% Dissolved oxygen 196.07 0.13 0.07 198.84 0.14 0.07 Table 6. Results of duplicates. Parameter Range Mean absolute Mean Relative (µM) differencea(µM) percentage differenceb Dissolved oxygen 179–240 0.18 0.09 aAD = | duplicate no. 1 −duplicate no. 2|.bRPD % =absolute difference ·100 /mean (dupl. no. 1, no. 2). An internal quality check was daily performed by means of analyses of QUASIMEME samples, which provided results within the already certified ranges. 3.7.2 TDN and TDP About 550 samples for total dissolved nitrogen and total dissolved phosphorus (TDN and TDP) on land-based laboratory analyses were collected and frozen at −20 ◦C after filtration on pre-combusted GF/F filter. The dissolved organic components, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) were subsequently calculated by subtracting their mineral constituents (NO3+NO2) and PO4, respectively. 3.8 Discrete CO2system measurements Discrete CO2variables were measured on board, i.e., dissolved inorganic carbon (DIC), pH, total alkalinity (TA) and carbonate ion (CO2− 3); these variables were measured at selected stations and depths (Table 9). In addition, discrete samples for DIC, pH and TA were analyzed specifically from surface Niskin bottles to be compared with the continuous water supply feeding the pCO2system in determined stations. For further details, especially about the on-board procedure for the measurement of samples, see Hainbucher et al. (2018). 3.8.1 DIC Samples for DIC were collected following transient tracers and dissolved oxygen in 500mL borosilicate bottles following standard procedure. No poison was added. Samples were left at room temperature in the dark until analysis a maximum of 48 h after collection. DIC samples were analyzed with a MARIANDA VINDTA 3D system coupled with a UIC 5011 coulometer. This analysis overall consists of extracting seawater CO2from a known volume of sample by adding phosphoric acid, followed by coulometric detection (Johnson et al., 1993). No calibration unit was available for the system. A new coulometric cell was prepared for every batch of analysis and the accuracy of the DIC measurements was assessed by using Certified Reference Material (CRM no. 158 and no. 170, provided by Andrew G. Dickson, UCSD). The calibration factor obtained from the CRM was used for adjusting the final DIC of each sample measured in the corresponding batch of analysis. In addition, substandard seawater (stabilized seawater from the Cretan Sea 700 m salinity minimum, stored in the dark in a 30L container) was analyzed at the beginning and end of the batch analysis as a secondary quality control. The precision of the DIC measurements was checked by (1) double analysis from the same sample and (2) replicate analysis from four to five samples collected from the same Niskin bottle. The precision is estimated to be 1 µmolkg−1, and the accuracy is assumed to be 2µmolkg−1. 3.8.2 pH Seawater spectrophotometric pH was measured following Clayton and Byrne (1993) at almost all depths in the chemical and isotope stations during the MSM72 cruise (Table 1). This method consists on adding a volume of indicator solution to the seawater sample, so that measuring the absorbance of the sample at different wavelengths and obtaining the ratio between two of the wavelength’s absorbance is proportional to the sample pH. The indicator was a 2mM solution of unpurified m-cresol purple (Sigma Aldrich®) prepared in seawater and maintained in the dark with no air contact (absorbance ratio of 1.30). Samples were taken following standard procedures immediately after DIC and directly into cylindrical 10cm path length optical glass cells. The cells were thermostatized at 25±0.2◦C for 1 h before analysis. Absorbance measurements were obtained in the thermostated chamber of a double-beam UV 2600 Shimadzu spectrophotometer. The equipment was checked before the cruise for the absorbance and wavelength accuracy using holmium standards. The pH values on the total scale Earth Syst. Sci. Data, 12, 2747–2763, 2020 https://doi.org/10.5194/essd-12-2747-2020 D. Hainbucher et al.: Physical and biogeochemical parameters of the Mediterranean Sea in March 2018 2755 Table 7. On-board precision of nutrient measurements. Parameter Beginning of the cruise End of the cruise Mean µM STD µM CV% Mean µM STD µM CV % Nitrite (1) 0.01 0.01 100 0.03 0.01 56.5 Nitrite +nitrate 4.94 0.01 0.2 9.01 0.02 0.2 Phosphate 0.18 0.01 5.5 0.41 0.01 3.1 Silicate 8.34 0.03 0.3 9.55 0.04 0.5 Table 8. Analysis of duplicates. Parameter Range Mean absolute Mean relative (µM) Differencea(µM) percentage differenceb Nitritec0–0.19 0.01 48.77 Nitrite +nitrate 0.33–9.86 0.02 0.42 Phosphate 0–0.47 0.01 5.13 Silicate 0.93–11.00 0.04 0.72 aAD = | duplicate no. 1 −duplicate no. 2|;bRPD % =absolute difference ·100 /mean (dupl. no. 1, no. 2). cNitrite statistics was given just for completeness, since the concentration levels recorded were too low and often below the detection limit. Table 9. Total number of CO2system samples analyzed during the MSM72 cruise. The total number of fired bottles was 1723. DIC pH TA CO2− 3Surface Samples 479 1160 949 391 22 were calculated and referred at 25◦C by using the formula by Clayton and Byrne (1993). The injection of the indicator in the sample slightly changes the sample pH. Following standard operating procedures, double additions of the indicator were performed over a pH gradient in order to obtain the corresponding correction (Hainbucher et al., 2018). The pH accuracy was controlled measuring TRIS buffer solution samples (batch no. 72, provided by Andrew G. Dickson, UCSD). TRIS samples were stabilized at three different temperatures covering the pH range found during the MSM72 cruise. Differences between measured and theoretical TRIS pH varied between 0.009 to 0.005. The pH precision was checked by replicate analysis from cells collected at the same Niskin from surface and deep waters. The precision is estimated to be 0.0004 pH units, and the accuracy was estimated to be 0.005 pH units. During the cruise, some samples were also analyzed with purified m-cresol purple provided by Robert H. Byrne (USC). 3.8.3 TA TA was analyzed following a double end point potentiometric technique by Pérez and Fraga (1987) further improved by Pérez et al. (2000). This technique is faster than the whole curve titration, with comparable results (Mintrop et al., 2000). TA was measured at most stations and depths (Table 1). Seawater samples for TA were collected after pH samples in 600 mL borosilicate bottles following standard procedures. Samples were left at room temperature in the dark until analysis at a maximum of 48h after collection. TA was measured by titration with 0.1 N hydrochloric acid dispensed with an automatic potentiometric titrator, Titrando Metrohm®, provided with a combination glass electrode coupled with a temperature probe. The electrode was standardized using a 4.41 pH ftatalate buffer made in CO2free seawater. The TA accuracy was assessed with CO2CRM (batch no. 170, provided by Andrew G. Dickson, UCSD) In addition to the CRM calibration, a drift control was conducted by analyzing substandard seawater (big volume of seawater stored in the dark, as for DIC) at the beginning and at the end of the analysis session. Each sample was measured twice and the mean value is reported, with the mean standard deviation of all duplicate differences being 0.6 µmolkg−1. In addition, typical reproducibility analysis were performed from samples collected from the same Niskin bottle at different stations along the cruise. The TA precision is estimated to be 1 µmolkg−1and the accuracy 2µmolkg−1. 3.8.4 CO2− 3 The CO2− 3ion concentration was determined spectrophotometrically following Byrne and Yao (2008) incorporating the recent improvements by Patsavas et al. (2015), at selected stations and depths (Table 1). Samples for CO2− 3were collected after TA following the same procedure as for pH but within cylindrical optical quartz 10cm path length cuvettes. 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