There is a general scarcity of oceanic observations that concurrently examine air–sea interactions, coastal–open-ocean processes and physical–biogeochemical processes, in appropriate spatiotemporal scales and under continuous, long-term data acquisition schemes. In the Mediterranean Sea, the resulting knowledge gaps and observing challenges increase due to its oligotrophic character, especially in the eastern part of the basin. The oligotrophic open Cretan Sea's biogeochemistry is considered to be representative of a greater Mediterranean area up to 10⁶ km², and understanding its features may be useful on even larger oceanic scales, since the Mediterranean Sea has been considered a miniature model of the global ocean. The spatiotemporal coverage of biogeochemical (BGC) observations in the Cretan Sea has progressively increased over the last decades, especially since the creation of the POSEIDON observing system, which has adopted a multiplatform, multivariable approach, supporting BGC data acquisition. The current POSEIDON system's status includes open and coastal sea fixed platforms, a Ferrybox (FB) system and Bio-Argo autonomous floats that remotely deliver fluorescence as a proxy of chlorophyll-a (Chl-a), O₂, pH and pCO₂ data, as well as BGC-related physical variables. Since 2010, the list has been further expanded to other BGC (nutrients, vertical particulate matter fluxes), ecosystem and biodiversity (from viruses up to zooplankton) variables, thanks to the addition of sediment traps, frequent research vessel (R/V) visits for seawater–plankton sampling and an acoustic Doppler current profiler (ADCP) delivering information on macrozooplankton–micronekton vertical migration (in the epipelagic to mesopelagic layer). Gliders and drifters are the new (currently under integration to the existing system) platforms, supporting BGC monitoring. Land-based facilities, such as data centres, technical support infrastructure, calibration laboratory and mesocosms, support and give added value to the observatory. The data gathered from these platforms are used to improve the quality of the BGC-ecosystem model predictions, which have recently incorporated atmospheric nutrient deposition processes and assimilation of satellite Chl-a data. Besides addressing open scientific questions at regional and international levels, examples of which are presented, the observatory provides user-oriented services to marine policy makers and the society, and is a technological test bed for new and/or cost-efficient BGC sensor technology and marine equipment. It is part of European and international observing programs, playing a key role in regional data handling and participating in harmonization and best practices procedures. Future expansion plans consider the evolving scientific and society priorities, balanced with sustainable management.

Research infrastructures have been established throughout Europe in order to create robust organizations that will facilitate and enhance research and innovation processes and will advance society with innovative products and services. The Hellenic Integrated Marine Observing, Forecasting and Technology System (component of HIMIOFoTS RI) has been implemented in the framework of the National Roadmap for Research Infrastructures to form a large-scale infrastructure for the marine environment in Greece. It links together ocean observing and forecasting systems, coastal zone monitoring and management practices, as well as ocean engineering testing facilities. The overarching framework of the system supports the coordination of five organizations with expertise in the field of marine science and technology, the central management of research activities, and the common development of services and products. It comprises facilities and resources while it provides open access to research communities (academia, industry) to support the scientific advancements and innovation in their fields. The Hellenic Marine Observing, Forecasting and Technology System was further enhanced during its implementation through significant upgrades and developments in order to extend its observing capacity and the forecasting and technological abilities, while advancing the provided services and products.

Advanced marine observation infrastructures are the most significant scientific tool for the study of marine ecosystem trends and shifts. Ocean monitoring technologies, though highly demanding and expensive, are essential for the monitoring and long-term study of oceanic systems. The POSEIDON system for monitoring and forecasting the marine environment is an augmented research infrastructure, unique in the Eastern Mediterranean basin, contributing to the European Ocean Observing System implementation, focusing—among others—on biogeochemical observations and deep-sea ecosystem and geological processes. The technological evolution of the POSEIDON system through a science-coupled strategy supported by engineers and scientists, resulted in a state-of-the-art ocean observing system. There has been a continuous expansion of the infrastructure with new scientific platforms and supporting facilities. Innovative sensing technologies were introduced in the operational data acquisition and new methodologies and tools were developed to improve the system operations and efficiency. As a part of the scientific community of ocean observatories, POSEIDON contributes actively to the improving of the ocean observing. International access to engineering and field demonstrating services, data products and technology testing facilities has been offered to scientists and industry partners. POSEIDON is a widely recognizable international technology testing/demonstrating node specializing in marine technology providing high-level services.

The Cretan Sea is an intermediate and occasionally deep water formation area within the Eastern Mediterranean that accumulates and transforms water masses from the adjacent Aegean, Levantine and Ionian Seas. Six years of glider observations (2017–2023) in the Cretan Sea were analysed to study the properties, variability and dynamics of the water masses during the study period. The analysis revealed progressive warming and salinification of the intermediate and deep layers. The mean temperature increased by around 0.05–0.07 °C per year, and the salinity by approximately 0.02 per year. Furthermore, comparisons with climatological data from 2000 to 2015 show temperature departures of + 0.4 to 0.6 °C in the upper 400 m and salinity increases of up to + 0.3 at the surface. Both of these values decline with depth, highlighting the intensified warming and increased salinity near the surface and in the upper intermediate layers. Additionally, the analysis of salinity and temperature datasets revealed the formation of intermediate water annually, except in winter 2022 when an intense mixing event occurred in the Cretan Sea triggered by exceptionally cold atmospheric conditions. The mixed layer, as captured by the glider, extended below 600 m inside the Cretan basin. These newly formed waters almost reached the deep layers, significantly modifying the properties of the intermediate and deep waters, although full deep convection was not reached. The observed downward displacement of the TMW core below 1000 m is associated with the strong convective event as well as with the redistribution of the heat and salt in the intermediate and deep layers. These findings emphasize the importance of sustained, high-resolution observations in capturing both gradual trends and extreme events, and in improving our understanding of the evolving thermohaline circulation of the Eastern Mediterranean.

The Cretan Sea is an intermediate and occasionally deep water formation area within the Eastern Mediterranean that accumulates and transforms water masses from the adjacent Aegean, Levantine and Ionian Seas. Six years of glider observations (2017–2023) in the Cretan Sea were analysed to study the properties, variability and dynamics of the water masses during the study period. The analysis revealed progressive warming and salinification of the intermediate and deep layers. The mean temperature increased by around 0.05–0.07 °C per year, and the salinity by approximately 0.02 per year. Furthermore, comparisons with climatological data from 2000 to 2015 show temperature departures of + 0.4 to 0.6 °C in the upper 400 m and salinity increases of up to + 0.3 at the surface. Both of these values decline with depth, highlighting the intensified warming and increased salinity near the surface and in the upper intermediate layers. Additionally, the analysis of salinity and temperature datasets revealed the formation of intermediate water annually, except in winter 2022 when an intense mixing event occurred in the Cretan Sea triggered by exceptionally cold atmospheric conditions. The mixed layer, as captured by the glider, extended below 600 m inside the Cretan basin. These newly formed waters almost reached the deep layers, significantly modifying the properties of the intermediate and deep waters, although full deep convection was not reached. The observed downward displacement of the TMW core below 1000 m is associated with the strong convective event as well as with the redistribution of the heat and salt in the intermediate and deep layers. These findings emphasize the importance of sustained, high-resolution observations in capturing both gradual trends and extreme events, and in improving our understanding of the evolving thermohaline circulation of the Eastern Mediterranean.