Arctic zooplankton do not perform diel vertical migration (DVM) during periods of midnight sun

Diel vertical distribution patterns of dominant zooplankton taxa were studied during a period of midnight sun (May 1999) in the Marginal Ice Zone (MIZ) of the Barents Sea along 2 transects across the ice edge. Eight stations were sampled every 6 h over 24 h at 5 depth intervals. Our study confirmed that copepod nauplii (most probablyCalanus hyperboreusandC. glacialis) together withPseudocalanusspp. preferred the surface water layer. The herbivoresC. finmarchicus, C. glacialis, andC. hyperboreusconcentrated in the upper 50 m depth interval, whereas the omnivoresMetridia longaandMicrocalanusspp. were generally found in deeper waters. As a result of vertical distribution patterns of the numerically abundant taxa, a bulk of zooplankton was concentrated within the 0–50 m layer. Vertical distribution patterns of all examined taxa/groups varied in time as a result of habitat changes (with respect to water mass distribution and sea depth) as well as random patchiness. Based on our data and on a comprehensive literature survey on Arctic zooplankton diel vertical migration (DVM), we postulate that common zooplankton taxa in the MIZ of the Barents Sea do not perform DVM under the midnight sun. ArcticC. glacialisin May 1999 occupied deeper layers at stations with more ice cover and less melt water. This distribution pattern was probably related to the species’ reproduction. At the same time, AtlanticC. finmarchicuswas concentrated further away from the ice edge and chlorophyllamaximum, and presumably had not begun its seasonal reproduction.