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

Despite the midnight sun, herbivore copepods Calanus hyperboreus, C. glacia lis and Pseudocalanus acuspes displayed a normal diel vertical migration (N DVM) under the ice cover of Barrow Strait in spring, ascending into the chl orophyll-rich under-ice surface layer around maximum relative rate of chang e in irradiance (DeltaI/I) at dusk but returning to depth a few hours later , well in advance of the dawn maximum DeltaI/I. Nauplii prey being abundant above 50 m, the upward night-time incursions of the omnivore Metridia long a seldom reached beyond <25 m. In the absence of UV-B radiation or a temper ature gradient, migration out of the euphotic layer was interpreted as a re action to visual predators (e.g. Arctic cod Boreogadus saida and the hyperi id 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, Oi thona similis and Oncaea borealis. Once the feeding migrations developed, t he daytime depth of the centre of mass of the distribution of a copepod was correlated to its size (r(2) = 0.63). Our observations suggest that, under Arctic sea ice, interspecific differences in the pattern and extent of cop epod DVM can be related to the vertical distribution of potential food and to vulnerability to visual predators.