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 probably Calan Lis hyperboretis and C. glacialis) together with Pseudocalanus spp. preferred the surface water layer. The herbivores C. finmarchicus, C. glacialis, and C. hyperboreus concentrated in the upper 50 in depth interval, whereas the omnivores Metridia longa and Microcalanus spp. 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 in 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, Arctic C. glacialis in 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, Atlantic C. finmarchicus was concentrated further away from the ice edge and chlorophyll a maximum, and presumably had not begun its seasonal reproduction.