The Vertical Distribution and Movements of Marine Plankton Under Midnight-Sun Conditions in Spitsbergen

There is some uncertainty how far marine plankton animals undergo diurnal vertical migrations under the midnight-sun conditions of the arctic summer (Digby 1960). Early reports of vertical migrations of medusae and ctenophores were followed by the observations of Bogorov (1946) describing a constant vertical distribution of copepods during high summer in the Barents Sea. These reports and observations were not accompanied by measurements or estimations of light intensity and transparency. Study of the light intensity in the Sorgat, north-west Spitsbergen, has shown that in high summer there may be sufficiently large changes of illumination to cause considerable vertical movement of the isolumes below the sea surface, although the daily cycle is often confused because in high summer cloud cover and diurnal change of solar altitude affect illumination to a similar degree. It is not, however, a foregone conclusion that vertical migrations should occur in response to the smaller diurnal changes of light during the arctic summer, because with slow change of illumination the greater time available for adaptation may well reduce the response and allow other factors to exert an influence. This paper describes the vertical distribution of the plankton in relation to light intensity on two separate occasions in different localities in Spitsbergen. On the first occasion, in Isfjord in 1948, although weather observations accompanied the plankton hauls no actual measurements of illumination or transparency were made. On the second occasion, in the S6rgat in 1956, measurements of light intensity and transparency allowed subsurface illumination to be calculated fairly closely. Furthermore, the illumination measurements made on this occasion allowed estimates to be formed of the illumination during the earlier collections in Isfjord. These two sets of collections have therefore been considered together.