Vertical distribution of mesozooplankton in the coastal Canadian Beaufort Sea in summer

This paper contributes to baseline knowledge of lower trophic levels that is necessary to monitor the impact of oil and gas development on the Canadian Beaufort Sea ecosystem. As part of the Nahidik Program, the vertical distribution of mesozooplankton was studied along two transects in the coastal Canadian Beaufort Sea in the summer of 2009. Mesozooplankton was collected with 153 μm conical net in two hydrologically distinct layers – the upper layer which was fresher and warmer due to the Mackenzie River runoff, and the lower layer which was colder and more saline. Two separate mesozooplankton assemblages were distinguished in the individual layers. The average zooplankton abundance in the two layers was 3120 ± 2860 ind. m− 3 and 4200 ± 5550 ind. m− 3 in the upper and lower layer, respectively. The upper layer was largely inhabited by meroplanktonic Polychaeta (752 ± 1038 ind. m− 3) and Bivalvia larvae (228 ± 307 ind. m− 3) as well as by youngest stages of Pseudocalanus spp. (245 ± 499 ind. m− 3). Conversely, the lower layer was mainly occupied by typical marine taxa such as Calanus glacialis (95 ± 76 ind. m− 3), C. hyperboreus (27 ± 12 ind. m− 3) and Triconia borealis (111 ± 81 ind. m− 3). Oithona similis, a widely distributed eurytopic cyclopoid copepod, showed no consistent pattern of vertical distribution (280 and 291 ind. m− 3, in the lower and upper layer, respectively).