First FerryBox-based CO2 measurements in the Eastern Mediterranean

The Eastern Mediterranean has a paucity of CO2 and pH data, which does not allow the area to be conclusively characterised as a net source or sink of CO2, highlighting the need for additional observations. The POSEIDON FerryBox (PFB) allows to fill part of this data gap. The PFB is installed on a ferry route between Heraklion and Piraeus. To our knowledge, this is the only FerryBox system in operation in the Eastern Mediterranean to date (2024). This automated flow-through system includes sensors for temperature, salinity, dissolved oxygen, fluorescence (as a proxy for Chlorophyll) and turbidity, with data provided in near real time. In 2023, the system was upgraded to include a water sampler and a pCO2 analyser. The PFB CO2 observations are complementary to other carbonate sensors observations (CO2, pH) from a fixed platform (HCB) and from regular R/V missions at the same site. The figure shows the measured pCO2 from the PFB on 30-31/08/23 going from Piraeus to Heraklion (black line), and estimated pCO2 from PFB temperature data (purple line) and from PFB temperature & Chl-a data (green line) using two algorithms, obtained at the HCB buoy (Frangoulis et al. 2024). Τhese first CO2 results from the PFB were presented at the 12th Ferrybox Workshop (1-2 October 2024, Helsinki, Finland). The CO2 PFB data are currently being processed for submission to the SOCAT database, and for improving the algorithms. Unfortunately, due to cuts in the POSEIDON system maintenance funding, the CO2 observations from the Ferrybox and the HCB buoy have been suspended since 2023.