Physical and Biogeochemical Parameters of the 1 Mediterranean Sea during a Cruise with RV MARIA MERIAN in March 2018

The last decades have seen dramatic changes in the hydrography and biogeochemistry of the 2 Mediterranean Sea. The complex bathymetry, highly variable spatial and temporal scales of atmospheric forcing, convective and ventilation processes contribute to generate complex and 4 unsteady circulation patterns and significant variability in biogeochemical systems. Part of the 5 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 8 data from an oceanographic cruise in the Mediterranean Sea on the German research vessel March 2018. The main objective of the cruise was to 10 contribute to the understanding of long-term changes and trends in physical and biogeochemical 11 parameters, such as the anthropogenic carbon uptake and to further assess the hydrographical 12 situation after the major climatological shifts in the eastern and western part of the basin, known 13 as the Eastern and Western Mediterranean Transients. During the cruise, multidisciplinary 14 measurements were conducted on a predominantly zonal section throughout the Mediterranean 15 Sea, contributing to the Med-SHIP and GO-SHIP long-term repeat cruise section that is 16 conducted at regular intervals in the Mediterranean Sea to observe changes and impacts on 17 physical and biogeochemical variables .


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Contrary to earlier ideas that the Mediterranean Sea is always in a steady state, we now know  (Schroeder et al., 2006, Schroeder et al., 2008. This abrupt climate shift is referred to as 28 Western Mediterranean Transient (WMT) and the physical changes are comparable to the EMT, 29 both in terms of intensity and observed effects (Schroeder et al., 2008). The existence of both 30 6 transients contradicts the hypothesis of a steady state. On the other hand, it has also been proven 1 that an EMT has never been observed before (Roether et al., 2013). 2 The characteristic of the Mediterranean Sea is also such that it has the potential to sequester 3 large amounts of anthropogenic CO2, Cant, since the Mediterranean Sea has high alkalinity and 4 temperature, which can be rapidly transported to deep by the overturning circulation (e.g. 5 Schneider et al., 2010). The column inventories of Cant in the Mediterranean are among the 6 highest found in the world oceans; the Mediterranean Sea thus stores a significant portion of 7 the global anthropogenic emissions of Cant despite its relatively small volume. 8 Furthermore, marine dissolved organic carbon (DOC) represents the largest reservoir of 9 reduced carbon (662·10 15 g C) on Earth (Hansell, 2009), it therefore plays a major role in the 10 global carbon cycle. Its role in the functioning of marine ecosystems is equally crucial since 11 DOC is released at all the levels of the food web, as a byproduct of many trophic interactions 12 and/or metabolic processes and is the main source of energy for the heterotrophic prokaryotes 13 (Carlson and Hansell, 2015). Although most of DOC is produced in-situ, external sources 14 (atmosphere, rivers, sediments) may affect its concentration and distribution. Physical

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The main scientific objective of the cruise reported here was to add knowledge to the different 20 scales and magnitudes of variability and trends in circulation, hydrography, and 21 biogeochemistry of the Mediterranean Sea. Key variables were measured in strategic regions 22 in order to understand changes, the reason for occurrence, and the drivers. In this context, this 23 cruise is part of the Med-SHIP and GO-SHIP long-term repeat cruise section that is conducted 24 at regular intervals in the Mediterranean Sea to observe changes and impacts on physical and 25 biogeochemical variables. 26 The following science questions were addressed: 27 1.
What are the long-term changes and/or trends in physics and biochemistry in the 28 Mediterranean Sea, including all the sub-basins? 29

2.
How is the hydrographic situation in the Mediterranean developing further after the EMT

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The survey was carried out on the German RV Maria S. MERIAN from 2 nd of March to 3 rd of   During the thirty-three days of the cruise we carried out measurements of hydrographic and 10 biogeochemical variables along-track with the classical approach i.e. CTD, lADCP, uCTD 11 instrumentation and bottle samples on highly resolved sections across the Mediterranean Sea. 12 The high resolution of CTD stations, enhanced for the physical parameters by additional uCTD 13 measurements, allowed us to resolve the eddy field on the sections, the analysis was also   samples were taken at depth with a constant salinity gradient to ensure that no natural changes 29 in salinity affect the comparison between sample and sensor.

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The primary CTD system (specifications see table 2) initially used on board was a Seabird 31 SBE9plus + CTD s/n 0285 from the University of Hamburg connected to a SBE11 deck unit, 32 configured with a 24-position SBE-32 pylon (from GEOMAR) with 10-liter Niskin bottles. 1 Position of bottles #23 and #24 was occupied by the lADCP (specifications see table 3). 2 Initially, the CTD was set up with two sensors for temperature and conductivity, an oxygen 3 sensor, a fluorometer and an altimeter. To test the configuration and performance of the 4 instrument a station was carried out on the Cretan Sea at the start of the cruise. Unfortunately, 5 we had countless problems with instruments, sensors, cables and rosette during most of the 6 campaign which forced us to change them very often with others available on board resulting 7 in a continuous change of system configuration. Thus, all different configurations were 8 carefully considered when post-processing the CTD data.     advantage of this type of measurements is that it is not required to stop the vessel, but only to 7 maintain lower velocities (about 3 kn) during the deployments to reach greater depths. These 8 measurements were made with an Ocean Science uCTD system.

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The first uCTD deployment was done on March 5 th , between CTD 015 and 016 stations, and 10 we continued with this type of sampling between each CTD station to increase the sampling 11 resolution. Unfortunately, several deployments were cancelled due to severe weather conditions 12 and no uCTD cast was performed when the depth was shallower than 500m. Altogether 176 13 casts were taken with depths ranging from 557 to 864 m.
14 Two probes were used during the cruise with a no time limit mode configuration (apart from 15 the first cast configured to stop recording after 600 seconds, reaching 616 m depth) in order to 16 get longer records. The probe tail spools were attached to the winch through a rope loop that 17 was made new every day in the morning. Despite the probes can record several casts, data were 18 downloaded right after each cast using a SBE software in order to avoid losing the data in case 19 the probe was lost, and to free the memory. The probes were exchanged when the battery was 20 running low (around 3.8V). In three occasions, no data were recorded because the magnet was 21 taken off twice before deployment. 22 For calibration purposes, some additional casts were done right after the CTD cast in order to 23 compare the data sets. The probes were also sent down with the starboard CTD in station 130. 24 Data files were processed using a set of MATLAB® routines. After extracting the downcast 25 data, a first correction was done for removing inaccuracies in the descend rate based on the 26 work of Ullmann and Hebert (2013). Additionally, the data were aligned to the comparable 27 CTD data sets. 28 29

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During the whole campaign, underway current measurements were taken with two vessel-  pressure (pO2, the corresponding data set in Table 1b only contains pCO2) in seawater were 10 carried out by means of a Contros HydroC pCO2 analyzer for pCO2 and an Aanderaa optode 11 for oxygen. 12 The instruments were placed in a cooling box in the hangar. Seawater was drawn from the 13 ship's centrifugal pump for clean seawater that was continuously flowing through the cooling 14 box with the inlet close to the instruments. Water was pumped through a SeaBird 5 salinity and 15 temperature sensor and on to the HydroC instrument (Gerke et al., 2020).

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The system operated reliably throughout the cruise, except when data acquisition was 17 interrupted for the pCO2 instrument for 2 days directly after the ship's centrifugal pump was   (table 5). 16 In addition, during the cruise 46 duplicates were analyzed. The results are given in table 6.    11 Hansen and Koroleff (1999). Nitrite was determined through the formation of a reddish-purple  The results are shown in table 7.

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In addition, during the cruise 140 duplicates were analyzed. The results are shown in table 8.

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An internal quality check was daily performed by means of analyses of QUASIMEME samples, 23 which provided results within the already certified ranges.   since the concentration levels recorded were too low, often below the detection limit. Total Alkalinity (TA) and carbonate ion (CO3 2-) were measured at selected stations and depths 10 (table 9). In addition, discrete samples for DIC, pH and TA were analyzed specifically from 11 surface Niskin bottles to be compared with the continuous water supply feeding the pCO2   were analyzed with a MARIANDA VINDTA 3D system coupled with a UIC 5011 coulometer. 23 This analysis overall consists of extracting seawater CO2 from a known volume of sample by 24 adding phosphoric acid, followed by coulometric detection (Johnson et al., 1993). No precision is estimated to be 1 µmol kg -1 and the accuracy 2 µmol kg -1 . Seawater spectrophotometric pH was measured following Clayton and Byrne (1993) at almost 7 all depths in the chemical and isotope stations during the MSM72 cruise (Table 1). This method  (1987) further improved by Pérez et al. (2000). This technique is faster than the whole curve 33 titration, with comparable results (Mintrop et al., 2000). TA was measured at most stations and 1 depths (Table 1). Seawater samples for TA were collected after pH samples in 600 ml   were always performed between stations to determine the short-term drift in the detector. 17 Replicate measurements were taken except for near coastal stations due to high workload. To

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The CDOM data collected during the MSM72 cruise will represent an unique opportunity to:    3.14 LISST -DEEP

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The LISST-Deep instrument obtains in-situ measurements of particle size distribution, optical 32 transmission, and the optical volume scattering function (VSF) at depths down to 3,000 meters.  In general, the use of LISST -DEEP during the cruise follows the standard methods, which are 28 provided by Sequoia Inc., but with one important difference. For the estimation of the above 29 parameters the use of a background file is required for normalization purposes. This file is 30 normally produced in laboratory conditions with MilliQ 2 filtered water. However, experience 31 until now has proved that especially in the eastern Mediterranean Sea (which is characterized 1 as ultra-oligotrophic) the use of this background file leads us to an overestimation of the 2 parameters and especially of the beam attenuation coefficient. Therefore, during this cruise we 3 used a sampled in situ background file chosen as the minimum of the sum of the digital counts 4 in the 32 rings and where the LaserPower to LaserReference (Lp/Lr) ratio is maximum. 5 The main problem, which we faced, was the frequent change of the CTD main unit and the 6 different cables that we had to use for the instrument connection to the CTD. Fortunately, with 7 the most valuable help of the cruise technician we managed to deploy the LISST -DEEP as

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Discussion and conclusion will focus in this publication on the quality of the data of MSM72 13 cruise. We will concentrate here on the basic physical and biogeochemical parameters, as     The vertical distribution of dissolved oxygen along a section from the Cretan Sea to Gibraltar, 7 including part of the Cretan Passage and the southern Ionian is shown in figure 5. This section 8 shows the Oxygen Minimum Layer (<180 µmoles/kg) which occupies the layer 500-1500m.

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Increased oxygen towards the bottom indicate the ventilation of deep water in the 10 Mediterranean. The western part of the Ionian Sea appears to be better oxygenated than the 11 eastern part due to the spreading of newly ventilated dense water from the Adriatic Sea via the 12 Otranto Strait -a feature that is observed in the transient tracer section as well.  likely do to subject to mesoscale dynamics (as, for example, south of Crete).  The DOC data collected during the MSM72 cruise represents an unique opportunity to (i)   The financial support for the cruise was provided by the project of the "Deutsche 10 Forschungsgemeinschaft" U4600DFG040204. We gratefully acknowledge their support.

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The CO2 team was funded by an internal IEO grant MEDSHIP18, A.E.R. Hassoun was funded 12 by a POGO grant. The OGS team was funded by an internal grant.