Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean

Vertical distribution of bioluminescent organisms throughout the water column in the Porcupine Seabight and Porcupine Abyssal plain area of the NE Atlantic Ocean was measured using a free-falling autonomous vehicle equipped with a high-sensitivity video system viewing stimulated flashes of light from impacts on a mesh screen. Data were recorded from 500 m depth down to the sea floor at depths from 740 to 4808 m during spring and autumn of 2001 and 2002. Bioluminescent organisms decreased in abundance from a maximum of 80 m(-3) at 600 m depth to a mean of 0.5 to 1.2 m(-3) at depths greater than 3000 m. During autumn, a seasonally enhanced peak of abundance (mean 27.19 m(-3), compared with 7.52 m(-3) in spring) centred at 1420 m within the depth range of occurrence of Mediterranean intermediate water. It is hypothesised that this bioluminescent layer reflects seasonal increase in deep pelagic biomass fed by downward transport of organic matter following the spring peak of primary production in the surface layers. The bioluminescent layer is a large-scale regional phenomenon extending over 100s of kilometres.