Moonlit swimming: vertical distributions of macrozooplankton and nekton during the polar night

Macrozooplankton (e.g. krill, amphipods and jellyfish) and nekton(e.g. decapod shrimp, squid and fish) are integral parts of pelagic ecosystems, but knowledge of their vertical distributions and migrations during winter at high latitudes is lacking. This study provides the quantification of macrozooplankton and nektondistributions during the polar nightin a partially ice-covered high Arctic fjord. In January 2012, mid-water trawls and MIK nets were deployed in Rijpfjorden, Svalbard (80° 18′ N, 22° 15′ E) at three depths (20, 75 and 200 m) dayand night. Simultaneously, acoustic volume backscattering strength (a measure of biomass) was recorded using 18-, 38- and 120-kHz echosounders. We observed that the majority of nektonwere below the thermocline(>100 m) dayand night. A diverse fish community (10 species present) dominated the nektonbiomass closely followed by shrimp and squid. Macrozooplankton, mostly large Calanusspp. copepods and gelatinous zooplankton, were found throughout the water column, but the majority were above the thermocline dayand night. A general additive modelwith depth, time and moonlightpredicted biomass to increase with depth for both macrozooplankton (over the top 100 m) and nekton, but revealed no patterns in biomass over time. The model also indicated that full moonpresence increased depth of macrozooplankton backscatter. Our findings suggest a diverse and to some degree active pelagic community during the polar night, and provide some support for the hypothesis that moonlightinduces downward vertical migrations of macrozooplankton.