A first pH annual cycle in the Cretan Sea

The Cretan Sea is a subpart of the ultra-oligotrophic eastern Mediterranean Sea. In this area, major research challenges include gaining a better understanding of the trends and drivers of pH and the air-sea CO2 flux as well as improving our estimates of net primary productivity (a large part of the primary production being at depths out of satellite reach for most of the year).

The JERICO-S3 Cretan Sea Pilot SuperSite (PSS) aims to demonstrate the contribution of a PSS approach to study such challenges via six actions:

The absorption of excess atmospheric CO2 by the oceans causes changes in seawater pH, a phenomenon called ocean acidification. To contribute to ocean acidification observation, since December 2020 a pH sensor deployed in the Cretan Sea PSS is providing pH data, contributing to actions #1, #4, #5. The sensor (SP200-SM, Sensorlab) deployed at the subsurface, on the POSEIDON Heraklion Coastal Buoy (HCB) provides pH data every 3 hours in near real-time (NRT). In parallel, water samples for pH analysis are taken approximately every month to check the sensor’s operation. The samples are analysed using a lab pH instrument (AFT-pH, Sunburst Sensors), which is regularly checked against TRIS buffer as reference material.

It is the first time that an annual pH cycle is obtained in the Cretan Sea and to the best of our knowledge the second time at high frequency (<day) in the eastern Mediterranean (another cycle done in Saronikos Gulf in 2013; González-Dávila et al. 2016). The delayed mode data provided by the pH sensor, after processing, are in good agreement with data from samples (shown in the figure). A preliminary analysis suggests that temperature is the dominant factor controlling the diel to seasonal pH variability. The NRT data quality control procedure is currently under improvement to automatically include a correction from temperature and salinity data obtained by the nearby conductivity-temperature sensor.

The activity organised jointly with the Cretan Sea PSS partners (HCMR, NIVA, SYKE, CNRSMIO), will be pursued until November 2022 (i.e. two annual cycles), after what will be interrupted due to lack of funding.

Data from Cretan PSS

About the JERICO-S3 Pilot SuperSites

JERICO-S3 will provide regionalised innovative monitoring and science strategies at 4 Pilot Supersites in The Gulf of Finland, the North-western Mediterranean, The North Sea and the English Channel and the Cretan Sea.

The Pilot Supersites (PSSs) will be established and tested during a short implementation period (January 2021 to August 2022) to demonstrate how transnationally and trans-institutionally integrated multidisciplinary and multiplatform observations add value to our ability to answer the multiple key scientific and social challenges that the coastal ocean is facing.