Spring phytoplankton development and zooplankton reproduction in the central Barents Sea in the period 1979-1984

Abstract

Physical and biological conditions have been studied along a fixed transect across the polar front in the central Barents Sea in late spring or early summer in each year from 1979 to 1984, These data are analysed with respeet to the spring bloom development and zooplankton reproduction and the influence of climatic conditions on these processes. Of the six years analysed, 1979 and 1981 were particularly cold years with considerable melting of ice in Atlantic water south of the polar front. High stability due to the meltwater gave rise to spring blooms that culminated as much as 4-6 weeks earlier than in the warmer years when stability in the Atlantic water arose due to the atmospheric warming of the sea surface. Spawning of the dominant herbivore , calanus finmarchicus , evidently oceurred in early spring (late April - early May), as evidenced by dominance of copepodites in stages C1 to CIV in June. The degree of copepodite development in the different years was related to the time of sarnpiirig, water coiumri stability, and temperature, The timing of the copepodite development was less variable than, and showed no clear-cut relationship to, the timing of the spring bloom. This could reflect slower development of Calanus in cold years with early spring blooms. Reproduction of krill and Metridia longa oceurred als0 in early spring. Due to the more rapid development and early culmination of blooms in Atlantic water infiuenced by melting of ice, we expect the conversion efficiency of primary to pelagic secondary production to be generally lower in cold years than in warm years. Such a relationship is not evident, however , in the data for the period 1979-1984. The produced zooplankton biomass and numbers of Calanus copepodites were on the contrary extremely low in 1983 and 1984 which were warm years. Grazing by pelagic fish and advective transport of plankton are factors whieh need to be taken into consideration. The timing of iriflow events of Atlantic water to the Barents Sea in relation to the seasonal vertieal migration of Calanus finmarchieus in the Norwegian Sea could be of partieular importanee in this respect.