Zooplankton reproduction in the Barents Sea vertical distribution of eggs and nauplii of Calanus finmarchicus in relation to spring phytoplankton development

Abstract

Variable influence of ice melting on water column stabilization causes a large variation in timing of the spring phytoplankton bloom in the central Barents Sea. During two cruises in April 1986 and May-June 1987 situations were encountered that ranged from early bloom stage in Atlantic water not influenced by ice to late bloom stage in meltwater regions. In the present paper we examine the reproduction of Ca/anus finmarchirns (Gunnerus) against the background of this large natural variation in the temporal and spatial distribution of phytoplankton food, with emphasis on the vertical distribution of spawning females, eggs, and nauplii. The vertical distribution of the females indicated a shift from shallow spawning at the early bloom stations to deeper spawning in the pycnocline region at the late bloom stations. The females were generally concentrated in the layers with high concentrations of phytoplankton. The vertical distribution of eggs at the bloom and late bloom stations showed the same pattern as chlorophyll, either uniformly distributed in the upper mixed layer or concentrated as a sharp maximum in the region of the pycnocline and the deep chlorophyll maximum. The distribution of nauplii corresponded closely to the distribution of the eggs at these stations, suggesting little or no sinking of the eggs and hatching of the nauplii at depths with a rich food supply. The eggs at the ea rly bloom stations had a deeper distribution, although spawning apparently took place in the shallow part of the water column. This suggests a high rate of sinking of the eggs. Nauplii hatching from them probably encountered low food concentrations at great depths. The success of early spawning is probably limited through this. The indicated high sinking rate at the ea rly bloom stations in contrast to the low rate of sinking at the bloom and late bloom stations could reflect a difference in egg density owing to different feeding conditions. Increased water viscosity caused by mucus from phytoplankton could also have contributed to the lower sinking rate at high phytoplankton concentrations. Wind-induced vertical mixing did not play a major role in governing egg distributions