The correlation of downwelling irradiance and staggered vertical migration patterns of zooplankton

Abstract Field studies on the characteristics of light that influence vertical migrations in the mesopelagic realm are sparse, due to the difficulty in simultaneously monitoring changes in species distributions with changes in downwelling irradiance. Using the Johnson-Sea-Link submersible as a platform, in situ measurements of the changes in downwelling irradiance at sunset were made simul-taneously with observations on changes in animal distribution patterns in Wilkinson Basin, Gun ' of Maine. The results indicate that the vertical migrations for several species of large zooplankton are staggered, with euphausiids (Meganyctiphanes norvegica) migrating first, cydippid ctenophores (Euplokamus) migrating next, and two species of caridean shrimp (Dichelopandalus lepiocerus and Pasiphaea multidentata) migrating last Data collected on daytime dives indicate that the daytime depth distribution is not solely responsible for the migration order, and that different species may be responding to different cues, or have different thresholds for the same cue