Yearlong moored bioluminescence and current data at KM3NeT neutrino telescope sites in the deep Ionian Sea

Yearlong observations are presented using stand-alone small optical sensors and current meters in the deep Ionian Sea, E-Mediterranean. At two future neutrino telescope sites, off Sicily (I) and off Peloponessos (Gr), we deployed 2500–3000 m long mooring lines with oceanographic instrumentation. At about 150 m above the sea-floor, a glass sphere was mounted to each line holding two 3″-diameter photo-multiplier-tubes ‘PMTs’ in opposing directions for a first deep-sea test. Due to technical problems the background optical count rate could not be well established. Here, the focus is on the variations with time of bioluminescence bursts and their correlation with currents. Spectral analysis demonstrates that the PMT data best resemble those of horizontal currents (kinetic energy), significantly peaking at near-inertial, sub-inertial mesoscale and (Gr only) at tidal frequencies. Out-of-phase differences between signals from opposing PMTs in the same optical unit indicate impacts of bioluminescent organisms as a function of current direction, rather than a bacterial glow constant with time.