Life cycle and population dynamics of the calanoid copepod Pseudocalanus spp. in the Baltic Sea and North Sea

Calanoid copepods of the genus Pseudocalanus constitute important members of the zooplankton in the northern hemisphere and play a major role in the recruitment and stock dynamics of commercially used fish. This study analyses the population dynamics of the calanoid copepod Pseudocalanus spp. in the Baltic and North Sea ecosystems in the framework of the GLOBEC Germany Project. It compares life cycle characteristics, vital rates and secondary production of the two congener species, P. acuspes from the Baltic Sea and P. elongatus from the North Sea. Reproduction, growth and secondary production of these species are interpreted in the light of the nutritional environment and hydrography. The comparison of P. acuspes from the Baltic Sea and P. elongatus from the North Sea revealed strong differences in the population dynamics of this morphologically similar congener species. The highly stratified Baltic Sea makes high demands on the distribution of the glacial relict P. acuspes, which is adapted to a life at high latitudes. To reach optimal temperature conditions, a vertical distribution below the summer thermocline is compellent. This vertical innidation prevents utilisation of food from the euphotic zone by this primarily herbivorous species. The food limitation, the low salinity and the low temperatures in the Baltic Sea lead to diminished growth rates of all stages of P. acuspes, which is indicated by the stage structure and the slow development in May and July. Compared to that, the growth of P. elongatus seemed to be unlimited by food particularly during spring and summer, which is indicated by lower stage durations and higher growth rates and results in a higher max. secondary production. These differences emphasise the importance of careful identification and studies of key species for an understanding of their role in the marine ecosystem.