# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 5 Contributions to the Ocean Observatories Development and Open Access to the System  
In addition to the system development and the continuous operations of the POSEIDON system, the scientific and engineering team participated actively and shared the technological common knowledge with the international community in an ongoing procedure to identify and adopt best practices for the design [28], the operation of ocean monitoring systems and the sensor calibration [29,30]. This effort is focused on introducing a new open-access, permanent digital repository of best practices documentation (oceanbestpractices.org) that is part of the Ocean Best Practices System (OBPS) contributing to improving the future ocean observing [31]. Through transnational calls and research projects access, provision of engineering services, data products, technology testing facilities and demonstrating services have been offered repeatedly to scientists and industry partners originating from several scientific fields. Since 2010, the POSEIDON system has hosted 15 user proposed projects, 13 internationally and 2 nationally funded, offering access to the nodes and the laboratories of the infrastructure [32]. The joint effort of the scientific user group and the POSEIDON team in experiments for scientific instrumentation calibration have resulted in advancing the scientific methodology for the precise characterization of the instrumentation deployed in the East Med's unique environmental conditions and in improving the impact of regional RIs with the adoption of a common conceptual structure [33,34]. New sensing technologies for ocean acidification were introduced in the operational data acquisition scheme to study the seasonal pH variability. In the Saronikos station, a new spectrophotometric pH-based system was deployed over the period between September 2013 and October 2014. The tested instrumentation proved capable of providing sea-surface temperature and salinity, together with highly accurate pH values [35]. Innovative cost-effective technologies were also demonstrated in operational mode, adding high-precision pH observations to the suite of measurements that are already part of the Ferrybox infrastructure, thus better characterizing the carbonate system [36]. Other scientific access activities in the POSEIDON field network include the  
Figure 16: The TAO Array Data Return for 2011–2022. testing of cost-effective instrumentation and samplers for microplastics monitoring, vertical distributions and settlement of deep-sea larvae and monitoring of organic contaminants by passive samplers. Through the HIIMIOFoTS access call, the proposal \"Underwater passive acoustic monitoring of cetaceans using the POSEIDON underwater gliders--YTIAKOUE\" was granted. The project was a cooperation between the Sea Mammal Research Unit, Scottish Ocean Institute, University of St Andrews, and the Operational Oceanography department of HCMR for the monitoring of cetaceans and environmental noise in the Cretan Sea [5]. During the CLAIM project, a passive filter system able to operate in the FB systems and other types of vessels equipped with a flow-through sea water pumping system was developed. The system was deployed in several types of vessels to provide information about marine microlitter distribution and control data points for validation of advanced micro- and macrolastic models. In the framework of the national MARRE project, the detection system KATERINA II was attached to the POSEIDON Athos station in order to provide continuous measurement and monitoring of natural and artificial radioactivity. The calibration and preparation of the KATERINA II system was achieved with the support of the MERL group, while the installation of the detection system was completed with the support of the POSEIDON engineers and HCMR's diving team [37]. Apart from the scientific and research community, access to POSEIDON was also provided to industry applicants.