An integrated open-coastal biogeochemistry, ecosystem and biodiversity observatory of the eastern Mediterranean – the Cretan Sea component of the POSEIDON system
Authors: Petihakis George , Perivoliotis Leonidas , Korres Gerasimos , Ballas Dionysios , Frangoulis Constantin , Pagonis Paris , Ntoumas Manolis , Pettas Manos , Chalkiopoulos Antonis , Sotiropoulou Maria , Bekiari Margarita , Kalampokis Alkiviadis , Ravdas Michalis , Bourma Evi , Christodoulaki Sylvia , Zacharioudaki Anna , Kassis Dimitris , Potiris Emmanuel , Triantafyllou George , Tsiaras Kostas , Krasakopoulou Evangelia , Velanas Spyros , Zisis Nikos 
Published: October 2018

Abstract. 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.