Effects of a decrease in downwelling irradiance on the daytime vertical distribution patterns of zooplankton and micronekton

The daytime vertical distribution of several species of crustaceans, gelatinous zooplankton, and fish were monitored in situ simultaneously with measurements of downwelling irradiance in Oceanographer Canyon in July 1999. During this submersible-based research cruise, an influx of turbid water significantly decreased downwelling irradiance and had a substantial impact on the depth distributions of a number of organisms. Several species of crustaceans, (Thysanoessa gregaria and Sergestes arcticus) and gelatinous zooplankton (Salpa aspera and Salpa fusiformis) ascended over 100 m in the water column during the influx and returned to their pre-influx depths once the influx had ceased. In situ light measurements demonstrated that each of these species was associated with the same irradiance levels during the influx as they were under pre- and post-influx conditions. By contrast, a statistically significant change in temperature, salinity, and oxygen concentrations measured post-influx had no apparent impact on the depth distributions. These results indicate that these species were adjusting their depth distributions to remain within a range of preferred irradiances. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s002270020788