Abed El Rahman Hassoun

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. The objectives of this communication are: i. To update our knowledge of existing observation capabilities in the Mediterranean Sea.ii. To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future.iii. To describe the present strengths and weaknesses of the Mediterranean Sea's observation system.iv. To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 128. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 88 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support.

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Nitrous oxide (N ₂ O) and methane (CH ₄ ) are potent greenhouse gases for which oceanic contributions remain uncertain, particularly in undersampled regions like the Southwest and Southeast margins of the Mediterranean Sea, where there is a major observational gap. This data paper presents a comprehensive dataset of monthly N ₂ O and CH ₄ concentrations and air-sea fluxes collected over a full seasonal cycle (April 2023–June 2024 at most sites, with one station extended to September 2024) from eight coastal stations across three distinct Mediterranean ecoregions (Alboran, Balearic, and Levantine Seas) as part of the ROADSTER collaborative project. Sampling, preservation, and analytical procedures were standardized across sites, and dissolved-gas analyses were performed in a single laboratory to ensure comparability. We detail standardized sampling and analytical methodologies, including ancillary variables (temperature, salinity, dissolved oxygen, chlorophyll a and inorganic nutrients). The complied dataset reveals distinct seasonal and spatial variability: N ₂ O concentrations exhibit a strong negative correlation with temperature, with all stations acting as moderate N ₂ O sources. Conversely, CH ₄ concentrations show greater variability and a positive correlation with temperature, with the Levantine sub-basin stations displaying episodic high-flux events (up to 35.20 µmol m ⁻² d ⁻¹ ) indicative of localized seafloor sources. This dataset bridges significant data gaps in the Mediterranean, providing a crucial baseline for regional climate modeling, understanding biogeochemical processes, and future climate change impact assessments. The dataset is publicly available at Zenodo (https://doi.org/10.5281/zenodo.19351642; de la Paz et al., 2026).

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

Ocean monitoring is a crucial component of Earth's observation systems. These systems have evolved to achieve a global scope and an exceptionally high level of complexity, encompassing not only observation programs but also data centres for collection and storage, as well as computational facilities where predictions and re-analyses of oceanic conditions are generated using coupled ocean-atmosphere circulation models. The products generated by the different components of the oceanic observation systems are of paramount importance for the knowledge of the environmental state of the oceans, their variability ranges, the impact of natural and anthropogenic stressors (such as climate change) and, in summary, the appropriate governance of the sea and the sustainable use of its resources. Given the diverse nature of the data (collected through a wide range of technologies, instruments, and institutions across multiple countries), it can be challenging to maintain a comprehensive overview of existing observation programs and datasets. MonGOOS (Mediterranean Oceanographic Network for the Global Ocean Observing System) serves as a GOOS (Global Ocean Observing System) Regional Alliance (GRA). Through its Observations Working Group (OBSWG), MonGOOS has conducted an extensive consultation among Mediterranean institutions to compile information on ongoing observation programs and the institutions operating within this basin. The results of this consultation, obtained through the collaboration of a very large number of scientists and technicians from a wide range of institutions, are reflected in this community report. These objectives of this report are: i) To update our knowledge of existing observation capabilities in the Mediterranean Sea. ii) To identify contact persons for strengthening the current MonGOOS network in order to keep this information up to date in the future. iii) To describe the present strengths and weaknesses of the Mediterranean Sea’s observation system. iv) To support European ocean observation programs such as the Copernicus Marine Service and the European Marine Observation and Data Network (EMODnet). The consultation phase began in July 2023 and continued until December 2025. It included a survey addressed to 90 researchers from 59 institutions across 18 countries as well as contacts with several EuroGOOS task teams, IOC (Intergovernmental Oceanographic Commission) sea level monitoring, as well as OceanOPS, for the compilation of metadata. During this consultation phase, the list of researchers addressed increased to 126. Finally, metadata from observation activities were obtained from 74 institutions belonging to 16 countries, 4 companies and the European Joint Research Centre (JRC). The description of all the observation capabilities in the Mediterranean Sea is not closed and should be considered as an ongoing effort to be reviewed periodically and more systematically in the future. However, we believe that this objective has been accomplished to a high degree of detail and inclusivity. The present compilation includes most of the currently active and operational observing systems, including 80 Argo floats and 16 surface drifters, 58 high-frequency radars (HFR), 220 tide gauges and 85 tsunami alerting devices, 9 periodic or sustained glider lines, 5 FerryBox lines, 11 wave buoys, 36 subsurface or deep moorings, 46 meteorological and oceanographic surface buoys, 5 seabed platforms or cabled observatories, 13 periodic transects or repeated cruises, and 17 oceanographic stations visited on a regular basis (time-series stations). The present survey revealed a strong asymmetry in the distribution of these platforms, reflecting a clear discrepancy in the capabilities of different Mediterranean countries. It also shows the difficulty for maintaining long-term observations, the fragility of some of these programs, and the need for national programs with structural and sustainable funds that are not only subject to competitive financial support

The Mediterranean Sea (MedSea) is highly sensitive to climate-driven changes in temperature, oxygen, and pH, among other variables. To better assess these long-term trends, we developed CARIMED (CARbon, tracers, and ancillary data In the MEDiterranean Sea), the first comprehensive, harmonised data synthesis product for the MedSea. CARIMED integrates hydrographic, inorganic carbon, transient tracer, and ancillary measurements from 46 research cruises spanning the period from 1976 to 2018, containing observations for the entire water column across all MedSea sub-basins. A substantial component of the data was retrieved from fragmented or locally archived historical records, thus consolidating previously inaccessible measurements. Following global synthesis approaches, CARIMED applies a quality-controlled, and bias-adjusted framework. A key adaptation was the secondary quality control (2QC) procedure, specifically tailored to the MedSea's unique hydrography, utilising sub-basin divisions and supplementary checks (including statistical consistency assessments) to resolve complex, often contradictory, inter-cruise offsets. This rigorous process minimised systematic biases, yielding a dataset with improved consistency, and highlights the urgent need for adapted standard operating procedures and reference materials to address the MedSea biogeochemical particularities. CARIMED delivers two complementary, freely available products: the aggregated original cruise data product (https://doi.org/10.20350/digitalCSIC/17785, García-Ibáñez et al., 2025) and the final bias-adjusted data synthesis product (https://doi.org/10.25921/cp5b-zq67, Álvarez et al., 2025; hosted at https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/oceans/CARIMED/, last access: 26 June 2026). This essential resource establishes a new benchmark for assessing long-term biogeochemical trends, validating regional ocean models, and supporting climate-change mitigation and adaptation strategies in this rapidly changing semi-enclosed basin.