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Journal of Plankton Research Vol.23 no.11 pp.1263-1278, 2001
© Oxford University Press 2001

Visual predators and the diel vertical migration of copepods under Arctic sea ice during the midnight sun

Martin Fortier1, Louis Fortier1, Hiroshi Hattori2, Hiroaki Saito3 and Louis Legendre4

1 Giroq, Département De Biologie, Université Laval, Québec, Québec, Canada G1k 7p4; 2 Department Of Marine Sciences And Technology, Hokkaido Tokai University, Minamisawa 5, Minami-Ku, Sapporo 005-8601, Japan; 3 National Fisheries Research Institute, Kushiro 085-0802, Japan; 4 Station Zoologique, Bp 28, 06234 Villefranche-Sur-Mer Cedex, France

Despite the midnight sun, herbivore copepods Calanus hyperboreus, C. glacialis and Pseudocalanus acuspes displayed a normal diel vertical migration (NDVM) under the ice cover of Barrow Strait in spring, ascending into the chlorophyll-rich under-ice surface layer around maximum relative rate of change in irradiance ({Delta}I/I) at dusk but returning to depth a few hours later, well in advance of the dawn maximum {Delta}I/I. Nauplii prey being abundant above 50 m, the upward night-time incursions of the omnivore Metridia longa seldom reached beyond <25 m. In the absence of UV-B radiation or a temperature gradient, migration out of the euphotic layer was interpreted as a reaction to visual predators (e.g. Arctic cod Boreogadus saida and the hyperiid amphipod Themisto libellula). Swarms of T. libellula actively preying on copepods accumulated at the ice–water interface at dusk. Low vulnerability to visual predators and a more uniform vertical distribution of their food explained the limited DVM of the small omnivores Microcalanus pygmaeus, Oithona similis and Oncaea borealis. Once the feeding migrations developed, the daytime depth of the centre of mass of the distribution of a copepod was correlated to its size (r2 = 0.63). Our observations suggest that, under Arctic sea ice, interspecific differences in the pattern and extent of copepod DVM can be related to the vertical distribution of potential food and to vulnerability to visual predators.


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