Diel vertical migrations and nocturnal feeding of a dense coastal krill scattering layer (Thysanoessa raschi and Meganyctiphanes norvegica) in stratified surface waters

To study the nocturnal feeding of euphausiids during vertical migrations and its impact on the phytoplankton, a phytoplankton-rich water mass (drogue marked) drifting over a dense krill scattering layer (acoustic 104kHz) in the lower St. Lawrence estuary was monitored over 46 h in July 1982. Phytoplankton >20 m was abundant and mostly concentrated at the bottom of the photic layer above the pycnocline. Less than 42% of the particulate carbon was due to phytoplankton. The krill scattering layer was about 2 to 3 km in width, elongated along the 100-m bathymetric contour, and absent when the bottom was shallower than 50 m. Its upper day depth was 50 m. At deeper depths, its vertical distribution frequently changed from unimodal to polymodal shapes and viceversa, often with large concentrations of zooplankton just above the bottom. Typical vertical migrations were observed on both days. At night the scattering layer had a lower scattering strength. Most of it was below the thermocline but net catches showed that large concentrations of euphausiids (up to 57 individuals m^-3) crossed it. Stomach pigment content of Thysanoessa raschi was generally low, but mean stomach fullness was always high. They were more opportunistic than herbivorous. From stomach fullness and the presence of a food bolus in mouth parts, feeding in surface waters appeared to be intensive, but gut content indicated that food was not processed there. It is therefore suggested that individuals underwent vertical interchanges across the thermocline while feeding during the night. Meganyctiphanes norvegica had significant herbivorous activity during the night. The grazing pressure impact of the scattering layer on phytoplankton was negligible.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)