Deep-sea pelagic bioluminescence over the Mid-Atlantic Ridge

Measurements were made for the first time of the number of stimulated sources of bioluminescence from the subsurface layer to the seafloor at 14 stations over the northern Mid-Atlantic Ridge, 42°56′N to 53°18′N. Overall, light emissions decreased from typical means of 46 sources m−3 at 300–500 m down to 5 m−3 at 2800 m depth. In the vicinity of the Azores, higher mean abundance (29.4 m−3) occurred at 500–1500 m compared to 13.9 m−3 over the same depth range at a reference station east of the ridge. Compared with the reference station, elevated levels of bioluminescence (25 m−3) were also observed at depths of 700–965 m in the water column over the Faraday Seamount. North of the Subpolar Front there was little variability in the number of bioluminescent sources between the sampling stations. However, in the Frontal Zone high abundance of bioluminescent sources was detected down to 1500 m within an anticyclonic eddy. Smoothing techniques revealed that >90% of the observed variation in the distribution of bioluminescence over the ridge was explained by sample depth in the water column.